High-performance steel bar and manufacturing method thereof

Abstract

A high-performance steel bar according to an embodiment of the present invention comprises 0.03 to 0.10 wt% of carbon (C), 0.05 to 0.45 wt% of silicon (Si), 1.45 to 2.00 wt% of manganese (Mn), 0.040 wt% or less of phosphorus (P), 0.040 wt% or less of sulfur (S), 0.060 wt% or less of aluminum (Al), 0.50 to 1.60 wt% of nickel (Ni), 0.05 to 1.20 wt% of chromium (Cr), 0.30 wt% or less of copper (Cu), 0.005 to 0.015 wt% of nitrogen (N), and the balance being Fe and unavoidable impurities. And the balance of iron (Fe) and unavoidable impurities. The yield strength (YS) at the temperature of-170 DEG C reaches 550 MPa or above.

Description

Technical Field

[0001] This invention relates to a high-performance steel bar and its manufacturing method. Background Technology

[0002] Threaded steel bars or reinforcing bars are slender steel materials used to reinforce concrete. They possess the advantages of strong adhesion to concrete and the ability to reduce the width of cracks in concrete that are heavily subjected to tension. They are widely used in construction and civil engineering projects. These threaded steel bars or reinforcing bars are now used as core materials for various civil engineering structures such as bridges, large marine structures, underground buildings, and storage facilities.

[0003] On the other hand, with the strict enforcement of environmental laws, attention to liquefied natural gas (LNG) has been gradually increasing recently. Based on 2013, the total energy of LNG is expected to increase from 2.9 billion TOU to 4.2 billion TOU by 2040, representing a high growth trend of 46.1%, thus indicating a high market demand.

[0004] Natural gas will be transported to liquefied natural gas (LNG) pipelines and stored through a refining-liquefaction process. Natural gas is liquefied to become LNG at temperatures below -170°C (cryogenic), and LNG storage tanks used to store cryogenic LNG need to be made of materials that can withstand extremely low temperatures.

[0005] Liquefied natural gas (LNG) storage tanks typically consist of an inner tank and an outer tank. The inner tank, which comes into contact with the LNG, is made of 9% nickel (Ni) steel plate capable of withstanding temperatures down to -170°C, while the outer tank is constructed of reinforced concrete. For the threaded steel bars used in LNG storage tanks, they must possess the following properties: they must not exhibit brittle fracture under sudden temperature drops and must be able to withstand temperatures down to -170°C to maintain the structure. In particular, there is a recent need for high-performance cryogenic steel bars that can prevent brittle fracture even in the absence of coolant, despite sudden temperature drops caused by LNG leaks.

[0006] In order to prevent brittle failure, which is directly related to liquefied natural gas (LNG) storage tanks, it is necessary to develop high-performance steel bars that give the steel itself high yield strength and crack sensitivity. Summary of the Invention

[0007] Technical issues In order to solve the problems of the prior art as described above, the purpose of this invention is to provide a high-performance steel bar with good low-temperature toughness and crack sensitivity even in ultra-low temperature environments, and a method for manufacturing the same.

[0008] Furthermore, the purpose of this invention is to provide a high-performance steel bar with stable properties in both normal and ultra-low temperature environments, and a method for manufacturing the same.

[0009] The purpose of this invention is not limited to the purposes described above, and other purposes not mentioned can be clearly understood by those skilled in the art based on the following description.

[0010] Technical solution The high-performance steel bar of one embodiment of the present invention comprises 0.03 to 0.10% by weight carbon (C), 0.05 to 0.45% by weight silicon (Si), 1.45 to 2.00% by weight manganese (Mn), less than 0.040% by weight phosphorus (P), less than 0.040% by weight sulfur (S), less than 0.060% by weight aluminum (Al), 0.50 to 1.60% by weight nickel (Ni), 0.05 to 1.20% by weight chromium (Cr), less than 0.30% by weight copper (Cu), 0.005 to 0.015% by weight nitrogen (N), the balance iron (Fe), and unavoidable impurities, and the yield strength (YS) at -170°C reaches 550 MPa or more.

[0011] Furthermore, the present invention also contains less than 0.1% by weight of molybdenum (Mo) and less than 0.080% by weight of vanadium (V).

[0012] Furthermore, the ideal critical diameter value (DI) can be between 0.3 and 1.0.

[0013] Furthermore, the nickel-manganese equivalent (NMeq) can be 2.0 to 4.7% by weight, whereby the nickel-manganese equivalent (NMeq) is the sum of the contents of nickel (Ni) and manganese (Mn).

[0014] Furthermore, the crack sensitivity ratio (CSR) at -170℃ can reach over 0.95.

[0015] Furthermore, the final microstructure may contain bainite and ferrite.

[0016] A method for manufacturing a high-performance steel bar according to an embodiment of the present invention includes: step (a) reheating the steel to 1050-1250°C, wherein the steel contains 0.03-0.10% by weight carbon (C), 0.05-0.45% by weight silicon (Si), 1.45-2.00% by weight manganese (Mn), less than 0.040% by weight phosphorus (P), less than 0.040% by weight sulfur (S), less than 0.060% by weight aluminum (Al), 0.50-1.60% by weight nickel (Ni), 0.05-1.20% by weight chromium (Cr), less than 0.30% by weight copper (Cu), 0.005-0.015% by weight nitrogen (N), the balance being iron (Fe) and unavoidable impurities; step (b) hot rolling the steel at a rolling end temperature controlled at 900-1100°C; and step (c) cooling the steel.

[0017] Furthermore, in step (a), the reheating time of the steel can be 1 to 3 hours.

[0018] Furthermore, in step (b), the hot rolling ratio can be 3 or higher.

[0019] Furthermore, in step (c), air cooling can be performed at a cooling rate of 0.5 to 5.0 °C / sec.

[0020] Furthermore, the crack sensitivity ratio (CSR) of the steel processed in step (c) at a temperature of -170°C can reach 0.95 or higher.

[0021] Furthermore, the final microstructure of the steel obtained through step (c) may include bainite and ferrite.

[0022] Furthermore, the steel may also contain less than 0.1% by weight of molybdenum (Mo) and less than 0.080% by weight of vanadium (V).

[0023] Furthermore, the ideal critical diameter (DI) of the steel can be 0.3 to 1.0.

[0024] Furthermore, the nickel-manganese equivalent (NMeq) of the steel can be 2.0 to 4.7% by weight, whereby the nickel-manganese equivalent (NMeq) is the sum of the contents of nickel (Ni) and manganese (Mn).

[0025] The effects of the invention The high-performance steel bar and its manufacturing method according to an embodiment of the present invention can ensure excellent mechanical properties at an ultra-low temperature of -170°C.

[0026] The effects of this invention are not limited to those mentioned above, and other effects not mentioned can be clearly understood by those skilled in the art based on the description in the scope of the invention claims. Attached Figure Description

[0027] Figure 1 The flowchart illustrates a method for manufacturing a high-performance steel bar according to an embodiment of the present invention.

[0028] Figure 2 Photographs of the microstructure of Material 1 and Material 1 of an embodiment of the present invention for comparison. Detailed Implementation

[0029] In this specification, when the relationship between a structural element (or region, layer, part, etc.) and other structural elements is referred to as "above", "connected", or "combined", it means that it is directly set / connected / combined with other structural elements, or it may also mean that a third structural element is set between them.

[0030] The same reference numerals denote the same structural elements. Furthermore, in multiple figures, the thickness, scale, and dimensions of various structural elements are exaggerated in order to effectively illustrate the technical content.

[0031] "And / or" includes all combinations of more than one that can be defined for the relevant structural elements.

[0032] The terms "first," "second," etc., can be used to describe various structural elements, but the structural elements are not limited to those terms. These terms are used only to distinguish one structural element from others. For example, without departing from the scope of this invention, a first structural element may be named a second structural element, and similarly, a second structural element may be named a first structural element. Unless otherwise explicitly stated in the context, singular expressions include plural expressions.

[0033] Furthermore, terms such as "below," "lower side," "above," and "upper side" are used to explain the relationships between the various structural elements shown in the accompanying drawings. These terms are relative concepts and will be explained based on the directions shown in the accompanying drawings.

[0034] Unless otherwise defined, all terms (technical and scientific terms) used in this specification have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Furthermore, it should be interpreted that terms, such as those defined in commonly used dictionaries, have the same meaning as in the relevant technical context, and are explicitly defined herein unless interpreted in an idealized or overly formalized manner.

[0035] Terms such as “including” or “having” are used to specify the presence of features, figures, steps, actions, structural elements, components or combinations thereof described in the specification, and should be understood as not precluding the presence or additional possibility of one or more other features, figures, steps, actions, structural elements, components or combinations thereof.

[0036] Unless otherwise specified, the expression "A~B" means "above A and below B" for values ​​A and B. In this expression, if the unit is only specified for value B, the corresponding unit also applies to value A.

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

[0038] High-performance steel bars The high-performance steel bar of one embodiment of the present invention comprises 0.03 to 0.10% by weight carbon (C), 0.05 to 0.45% by weight silicon (Si), 1.45 to 2.00% by weight manganese (Mn), less than 0.040% by weight phosphorus (P), less than 0.040% by weight sulfur (S), less than 0.060% by weight aluminum (Al), 0.50 to 1.60% by weight nickel (Ni), 0.05 to 1.20% by weight chromium (Cr), less than 0.30% by weight copper (Cu), 0.005 to 0.015% by weight nitrogen (N), the balance being iron (Fe) and unavoidable impurities.

[0039] In one embodiment of the present invention, the high-performance steel bar can ensure excellent mechanical properties in the cryogenic state of -170°C, which is the environment of a liquefied natural gas (LNG) storage tank, by controlling the range of the alloy composition, the optimal content of the nickel-manganese equivalent (NMeq) as the sum of the contents of nickel (Ni) and manganese (Mn) described later, and the optimal value of the ideal critical diameter (DI) that interferes with the internal structure and compressibility.

[0040] On the other hand, the high-performance steel bar of one embodiment of the present invention may further contain less than 0.1% by weight of molybdenum (Mo) and less than 0.080% by weight of vanadium (V). Wherein, if molybdenum (Mo) and vanadium (V) are also contained, they can replace the remaining iron (Fe) in the corresponding amounts.

[0041] The yield strength (YS) of the high-performance steel bar of an embodiment of the present invention, which contains the alloy described above, can reach 500 MPa or more at a temperature of -170°C, preferably 550 MPa or more, and more preferably 566 MPa or more.

[0042] According to one embodiment of the present invention, the V-notched tensile strength (TS) of the high-performance steel bar at a temperature of -170°C can reach 600 MPa or more, preferably 700 MPa or more, and more preferably 709 MPa or more.

[0043] In one embodiment of the present invention, the notched crack sensitivity ratio (CSR) of the high-performance steel bar at a low temperature of -170°C can reach 0.95, and preferably can reach more than 0.95 and less than 1.2.

[0044] Crack sensitivity ratio (CSR) can be defined as the ratio between the tensile strength (TS) of a V-notched specimen and the yield strength (YS) of an unnotched specimen, as follows.

[0045] CSR (Crack Sensitivity Ratio) = TS (V-notched) / YS (Un-notched) The following is a detailed description of the role and content of each alloying element contained in a high-performance steel bar according to an embodiment of the present invention.

[0046] Carbon (C) Carbon (C) is the main element determining the strength and hardness of steel. Increased content leads to increased strength but decreased toughness. Excessive addition reduces yield strength, potentially compromising required properties. Therefore, to achieve the desired strength, the addition is limited to below 0.10% by weight. Conversely, insufficient addition makes it difficult to ensure the product's inherent properties; thus, a minimum of 0.03% by weight is maintained.

[0047] Therefore, the high-performance steel bar of one embodiment of the present invention may contain 0.03 to 0.10% by weight of carbon (C).

[0048] Silicon (Si) Silicon (Si) is used as a powerful deoxidizer in steelmaking, thereby reducing oxide inclusions in the steel. Furthermore, while it increases the strength of the steel, excessive addition can lead to decreased ductility due to solid solution strengthening on the ferrite; therefore, its content is limited to below 0.45% by weight.

[0049] In this case, the high-performance steel bar of one embodiment of the present invention may contain 0.05 to 0.45 weight percent silicon (Si).

[0050] Manganese (Mn) Manganese (Mn) is an element that is beneficial for deacidification of steel and improves its strength and toughness. Furthermore, it improves the machinability of steel by combining with sulfur (S) within the steel to form fine MnS between crystal grains. Additionally, when added as an austenite stabilizing element, it promotes the formation of acicular ferrite and bainite.

[0051] Therefore, in one embodiment of the present invention, a high-performance steel bar is formed by adding 1.45 to 2.00 weight percent of manganese (Mn) to create a microstructure that is beneficial to improving the strength and toughness at extremely low temperatures.

[0052] Phosphorus (P) Phosphorus (P) is a grain boundary segregating element that improves the machinability of steel by embrittled grain boundaries, but it also has the disadvantage of reducing impact toughness. Furthermore, it can cause surface cracks during manufacturing, forming Fe3P compounds that reduce the steel's strength; therefore, its content needs to be controlled to a maximum value.

[0053] Therefore, in one embodiment of the present invention, the high-performance steel bar may contain less than 0.04% by weight of phosphorus (P).

[0054] Sulfur (S) Sulfur (S) is an element that improves machinability by forming MnS inclusions when added. However, MnS has the property of being stretched long during rolling, which reduces impact anisotropy and prevents cracking during high-frequency heat treatment. Therefore, if the amount of sulfur (S) added is too large, it may lead to a decrease in hot rolling workability and cause tearing.

[0055] Therefore, the high-performance steel bar of one embodiment of the present invention may contain less than 0.040% by weight of sulfur (S).

[0056] Aluminum (Al) Aluminum (Al) is used as a powerful deoxidizer in steelmaking to control the oxidation of non-metallic inclusions in steel. However, excessive addition can cause manufacturing problems such as nozzle clogging due to the increase of non-metallic inclusions. Therefore, the maximum value can be controlled at 0.060% by weight.

[0057] In a high-performance steel bar according to one embodiment of the present invention, the aluminum (Al) content can be controlled to be below 0.060% by weight.

[0058] Nickel (Ni) Nickel (Ni) is an expensive element that is harmful to the environment and human health. However, it refines the microstructure of steel and dissolves well in austenite and ferrite, thus serving as a matrix strengthening agent. Furthermore, it is one of the elements with high solidification capabilities, facilitating the heat treatment of steel and enhancing its low-temperature toughness without affecting weldability and ductility. In particular, it possesses the ability to inhibit the formation of δ-ferrite during solidification from its liquid state.

[0059] The high-performance steel bar of one embodiment of the present invention may contain 0.50 to 1.60% by weight of nickel (Ni).

[0060] Molybdenum (Mo) Molybdenum (Mo) is an element added to improve the strength and toughness of steel. However, its content is minimized because it is a high-priced element like nickel (Ni), which increases costs.

[0061] The high-performance steel bar of one embodiment of the present invention may contain less than 0.10% by weight of molybdenum (Mo).

[0062] Chromium (Cr) Chromium (Cr) is a ferrite stabilizing element that affects grain size refinement by forming carbides. Furthermore, it is an essential alloying element for ensuring corrosion resistance.

[0063] The high-performance steel bar of one embodiment of the present invention may contain 0.05 to 1.20 weight percent of chromium (Cr).

[0064] Copper (Cu) Although copper (Cu) is an alloying element that can improve strength and corrosion resistance, if the amount added is too large, it will become the main cause of a significant decrease in toughness and surface cracks caused by hot rolling during the manufacturing process. Therefore, its content needs to be limited.

[0065] The high-performance steel bar of one embodiment of the present invention may contain less than 0.30% by weight of copper (Cu).

[0066] Vanadium (V) Vanadium (V), as a micro-alloying element, can be added in small amounts to form precipitates, thereby improving strength. Furthermore, it can combine with carbon or nitrogen during cooling to form VC and VN precipitates, which facilitate precipitation strengthening and inhibit grain growth.

[0067] The high-performance steel bar of one embodiment of the present invention may contain less than 0.080% by weight of vanadium (V).

[0068] Nitrogen (N) Even trace amounts of nitrogen (N) can significantly affect the mechanical properties of steel. In particular, the reduction in impact value and the increase in transformation temperature are very noticeable. Austenite grains become finer, allowing for the manufacture of fine-grained steel. This grain refinement can be achieved by forming denitrifying compounds with titanium, zirconium, vanadium, niobium, etc. However, if present in large quantities, it can lead to decreased high-temperature toughness and grain boundary brittleness due to the precipitation of alkenes (AlN) at the austenite grain boundaries. Therefore, the content of nitrogen in the austenite can be limited.

[0069] Therefore, the high-performance steel bar of one embodiment of the present invention may contain 0.005 to 0.015% by weight of nitrogen (N).

[0070] Furthermore, the ideal critical diameter (DI) of the high-performance steel bar in one embodiment of the present invention can be 0.3 to 1.0, preferably 0.45 to 1.0, and more preferably 0.55 to 1.0.

[0071] The ideal critical diameter (DI) is the maximum diameter from the surface of the steel to the core where 50% martensite is formed when the steel is quenched.

[0072] The ideal critical diameter (DI) is also related to the mechanical properties of steel in ultra-low temperature environments, and has become a benchmark for representing the solidification ability of steel.

[0073] The ideal critical diameter (DI) value can be calculated using the following formula, which is formed by the product of the content (weight percentage) and coefficients (α, β, γ, δ, ε, ζ, η, θ) of the necessary elements constituting the steel, such as carbon (C), silicon (Si), manganese (Mn), etc.

[0074] Ideal critical diameter (D1) = α[c] × β[Si] × γ[Mn] × δ[Cu] × ε[Ni] × ζ[Cr] × η[Mo] × θ[V] (α, β, γ, δ, ε, ζ, η, θ are coefficients, and the corresponding detailed coefficients are specified in ASTM A 255.) In particular, a higher ideal critical diameter (DI) value leads to the formation of a large amount of bainite in the steel bar's microstructure, resulting in a hardened structure. Ultimately, the formation of this microstructure is the main reason for the significant improvement in the mechanical properties of high-performance steel bars under ultra-low temperature conditions.

[0075] Therefore, the high-performance steel bar of one embodiment of the present invention is manufactured with an ideal critical diameter (DI) of 0.3 to 1.0, thereby ensuring excellent mechanical properties in ultra-low temperature environments.

[0076] Furthermore, in one embodiment of the present invention, the high-performance steel bar can control the nickel-manganese equivalent (NMeq), which is the sum of the contents of nickel (Ni) and manganese (Mn), to be 2.0 to 4.7 by weight, and preferably the NMeq to be 2.11 to 4.7 by weight.

[0077] If the amount added is too small, resulting in a low NMeq, it may be impossible to achieve the required mechanical properties under normal temperature and ultra-low temperature conditions. If the NMeq is too large, it may lead to increased manufacturing costs.

[0078] Nickel (Ni) is well dissolved in the lattice structure of austenite and ferrite, while manganese (Mn) is dissolved in ferrite and provides an effect of increasing the strength of steel. However, nickel (Ni) is a high-priced alloying component and a substance that can be fatal to humans. If the content of manganese (Mn) increases, it will cause severe cracking and distortion of the steel when it is rapidly cooled. Therefore, it is necessary to properly control the nickel-manganese equivalent (NMeq), which is the sum of the contents of nickel (Ni) and manganese (Mn).

[0079] In existing steel bars, the microstructure consists of polygonal ferrite and pearlite formed in some of the grains.

[0080] However, in a high-performance steel bar according to an embodiment of the present invention, the final microstructure may include ferrite and bainite at low temperature.

[0081] In particular, the formation of a large amount of bainite can be confirmed, which means that the higher the ideal critical diameter (DI) value, the more hard tissue can be formed.

[0082] Manufacturing method of high-performance steel bars Reference Figure 1 The manufacturing method of high-performance steel bars according to an embodiment of the present invention includes (a) a reheating step, (b) a hot rolling step and (c) a cooling step.

[0083] More specifically, the present invention may include: step (a) reheating steel to 1050–1250°C, said steel comprising 0.03–0.10% by weight carbon (C), 0.05–0.45% by weight silicon (Si), 1.45–2.00% by weight manganese (Mn), less than 0.040% by weight phosphorus (P), less than 0.040% by weight sulfur (S), less than 0.060% by weight aluminum (Al), 0.50–1.60% by weight nickel (Ni), 0.05–1.20% by weight chromium (Cr), less than 0.30% by weight copper (Cu), 0.005–0.015% by weight nitrogen (N), the balance being iron (Fe) and unavoidable impurities; step (b) hot rolling said steel at a rolling end temperature controlled at 900–1100°C; and step (c) cooling said steel.

[0084] The steel can be an ingot or a billet produced by a continuous casting process prior to the reheating step. Therefore, the alloying elements contained in the steel and the range of their composition should be understood to apply equally to high-performance steel bars produced from ingots and billets.

[0085] Thus, according to one embodiment of the present invention, high-performance steel bars with excellent mechanical properties in cryogenic environments can be manufactured. More specifically, the high-performance steel bars manufactured by the method of the present invention ensure low-temperature toughness and improve crack sensitivity in cryogenic environments. When applied to cryogenic structures, they can prevent brittle fracture even under sudden temperature drops, thereby exhibiting excellent product characteristics.

[0086] On the other hand, the steel may also contain less than 0.1% by weight of molybdenum (Mo) and less than 0.080% by weight of vanadium (V), and the nickel-manganese equivalent (NMeq) as the sum of the contents of nickel (Ni) and manganese (Mn) may reach 2.0 to 4.7% by weight, and the NMeq may reach 2.11 to 4.7% by weight.

[0087] The following details each step of the high-performance steel bar manufacturing method.

[0088] First, in the reheating step (a) of the steel, the steel having the alloy composition range is reheated at a temperature of 1050–1250°C for 1–3 hours. If the reheating temperature and time are less than 1050°C or less than 1 hour, insufficient resolution of the precipitates may occur due to insufficient heating temperature and time, leading to increased rolling load. Conversely, if the reheating temperature is greater than 1250°C or exceeds the reheating time range, austenite grain coarsening or decarburization may occur, affecting strength. Furthermore, increased heating costs and time may lead to higher manufacturing costs and decreased productivity.

[0089] Next, in the rolling step (b), the reheated steel is hot-rolled, and the rolling end temperature is controlled at 900–1100°C. If the rolling end temperature is greater than 1100°C, it may be difficult to ensure strength due to the formation of coarse pearlite. Conversely, if the rolling end temperature is less than 900°C, it may lead to a decrease in productivity and a reduction in the heat treatment effect due to the rolling load. On the other hand, the rolling end temperature is preferably 950–1050°C.

[0090] According to an embodiment of the present invention, in the method for manufacturing high-performance steel bars, the hot rolling ratio of the steel can reach 3 or more in step (b).

[0091] Finally, in the cooling step (c), the steel can be cooled by air cooling after hot rolling deformation, and the cooling rate can be implemented at a rate of 0.5 to 5.0 °C / sec. According to the cooling step (c) of the present invention, even without using expensive self-tempering heat treatment equipment to perform the quenching process to improve the quality of the rolled product, the microstructure and physical properties corresponding to the steel or steel bar that has undergone self-tempering heat treatment can be ensured, and the mechanical properties under ultra-low temperature environments can be sufficiently guaranteed. Ultimately, the present invention achieves the required physical properties and significantly reduces the production and equipment costs required for manufacturing high-performance steel bars, even without using self-tempering heat treatment equipment, which is a common process in steel bar manufacturing.

[0092] like Figure 2 As shown, the steel processed in step (c) can have a final microstructure comprising bainite and ferrite at low temperatures. That is, with... Figure 2 Compared to the existing steel bar shown (comparative material 1), the present invention (inventive material 1) forms a large amount of bainite and a hard structure due to the high ideal critical diameter (DI), resulting in significantly improved mechanical properties compared to the comparative material.

[0093] Comparative and experimental examples Hereinafter, preferred comparative examples and experimental examples are presented to aid in understanding the present invention. However, the following experimental examples are only for the purpose of aiding in understanding the present invention, and the present invention is not limited to the following experimental examples.

[0094] Tables 1 and 2 below show the main alloy composition (unit: weight percentage) of the inventive material constituting this experimental example and the comparative material of the comparative example. Table 3 below shows the operating conditions for manufacturing each test piece of this experimental example and the comparative example and the measurement results of the mechanical properties of the test pieces based on the operating conditions for manufacturing each test piece.

[0095] The item "YS" means yield strength, the item "TS" means tensile strength, the item "CSR" means crack sensitivity ratio, which is a value calculated as the ratio between the tensile strength of the notched specimen and the yield strength of the unnotched specimen. The symbol "un" means unnotched specimen, and the symbol "n" means notched specimen.

[0096] Table 1 Table 2 Referring to Tables 1 and 2, in Comparative Materials 1 and 2 and Inventive Materials 1 to 8, the nickel-manganese equivalent (NMeq) as the sum of the contents of nickel (Ni) and manganese (Mn) is 2.10 wt%, 2.05 wt%, 2.11 wt%, 2.43 wt%, 3.42 wt%, 2.34 wt%, 2.59 wt%, 3.42 wt%, 4.63 wt%, and 3.35 wt%, respectively.

[0097] Furthermore, by adding 0.02% by weight of molybdenum (Mo), a high-priced alloying element, to the inventive material 5, the strength and toughness of the steel can be increased, and it has been confirmed that the yield strength (YS) and tensile strength (TS) in low-temperature environments can be ensured.

[0098] In particular, comparative material 1 and inventive materials 7 and 8 contain 0.003 wt percent, 0.003 wt percent, and 0.079 wt percent of vanadium (V), respectively.

[0099] Specifically, as shown in Invention Materials 1 to 8, the high-performance steel bar of one embodiment of the present invention, by controlling the alloy composition and NMeq, as shown in Table 3, exhibits mechanical properties that are corresponding to or superior to those of the comparative materials, even in ultra-low temperature environments.

[0100] Table 3 Referring to Table 3, specifically to Comparative Materials 1 and 2 and Inventive Materials 1 to 8. The air cooling in Table 3 is performed at a cooling rate of 0.5 to 5.0 °C / sec.

[0101] Furthermore, the ideal critical diameter (DI) values ​​in comparative materials 1 and 2 are 0.44 and 0.36, respectively, while in inventive materials 1 to 8 they are 0.97, 0.55, 0.72, 0.58, 0.73, 0.76, and 0.78, respectively.

[0102] Ultimately, the yield strength (YS_un) of comparative materials 1 and 2 at a low temperature of -170℃ was 509.0 MPa and 500.4 MPa, respectively, while the yield strength (YS_un) of invention materials 1 to 8 were 653.0 MPa, 566.2 MPa, 655.3 MPa, 724.7 MPa, 710.3 MPa, 679.0 MPa, 725.3 MPa, and 748.0 MPa, respectively.

[0103] Furthermore, the tensile strength (TS_n) of comparative materials 1 and 2 at a low temperature of -170°C is 620.2 MPa and 650.2 MPa, respectively, while the tensile strength (TS_n) of the invention materials 1 to 8 are 825.9 MPa, 709.7 MPa, 817.0 MPa, 824.3 MPa, 849.5 MPa, 857.5 MPa, 976.6 MPa, and 904.6 MPa, respectively.

[0104] Regarding the crack sensitivity rate (CSR) of the notch-test specimens, the comparative materials 1 and 2 are 1.23 and 1.25, respectively, while the inventive materials 1 to 8 are 0.95, 1.05, 1.00, 1.01, 1.04, 1.01, 1.01, and 1.00, respectively.

[0105] As described above, the ideal critical diameter value (DI) related to the internal structure and compressibility of the steel is preferably controlled at 0.55 to 0.97 in Inventive Materials 1 to 8. In order to ensure the strength and toughness of the steel in low temperature environments, the nickel-manganese equivalent (NMeq), which is the sum of the contents of nickel (Ni) and manganese (Mn), is preferably controlled at 2.11 to 4.63 weight percentages.

[0106] Furthermore, after hot rolling deformation, the invention materials 1 to 8 were subjected to an air cooling process instead of an accelerated cooling process using their own tempering heat treatment equipment.

[0107] Specifically, even when a cooling process using air cooling, which may lead to a deterioration in the mechanical properties of steel compared to a cooling process using its own tempering heat treatment equipment, is implemented, Invention Materials 1 to 8 can still control the nickel-manganese equivalent (NMeq), which is the sum of the contents of nickel (Ni) and manganese (Mn), as well as the ideal critical diameter value (DI). By controlling the operating conditions during air cooling, excellent strength and toughness can be ensured in a low-temperature environment of -170°C.

[0108] In particular, such as Figure 2 As shown, the microstructure of comparative material 1 consists of polygonal ferrite and pearlite formed in some of the crystal grains.

[0109] However, it can be confirmed that the inventive material 1, with an ideal critical diameter (DI) of 0.97, formed a large amount of bainite. Thus, it can be confirmed that the higher the ideal critical diameter (DI) of steel, the more hard structure can be formed. Finally, it can be confirmed that due to the formation of this structure, the mechanical properties of the inventive material are significantly improved compared to the comparative material.

[0110] As described above, preferred embodiments of the present invention have been presented. Besides the embodiments described above, the present invention can be implemented in other specific ways without departing from its spirit or scope, which is self-evident to those skilled in the art. Therefore, the embodiments described should be considered exemplary rather than limiting. Thus, the present invention is not limited to the description, and various modifications can be made within the scope of the appended claims and their equivalents.

Claims

1. A high-performance steel bar, characterized in that, It contains 0.03–0.10% carbon (C), 0.05–0.45% silicon (Si), 1.45–2.00% manganese (Mn), less than 0.040% phosphorus (P), less than 0.040% sulfur (S), less than 0.060% aluminum (Al), 0.50–1.60% nickel (Ni), 0.05–1.20% chromium (Cr), less than 0.30% copper (Cu), 0.005–0.015% nitrogen (N), the balance being iron (Fe) and unavoidable impurities. The yield strength at -170℃ reaches over 550MPa.

2. The high-performance steel bar according to claim 1, characterized in that, It also contains less than 0.1% by weight of molybdenum (Mo) and less than 0.080% by weight of vanadium (V).

3. The high-performance steel bar according to claim 1, characterized in that, The ideal critical diameter value DI is 0.3 to 1.

0.

4. The high-performance steel bar according to claim 1, characterized in that, The nickel-manganese equivalent (NMeq) is 2.0 to 4.7% by weight, and the nickel-manganese equivalent (NMeq) is the sum of the contents of nickel (Ni) and manganese (Mn).

5. The high-performance steel bar according to claim 1, characterized in that, The crack sensitivity rate at -170℃ reaches over 0.

95.

6. The high-performance steel bar according to claim 1, characterized in that, The final microstructure consists of bainite and ferrite.

7. A method for manufacturing a high-performance steel bar, characterized in that, include: Step (a) involves reheating the steel to 1050–1250°C, wherein the steel comprises 0.03–0.10% by weight carbon (C), 0.05–0.45% by weight silicon (Si), 1.45–2.00% by weight manganese (Mn), less than 0.040% by weight phosphorus (P), less than 0.040% by weight sulfur (S), less than 0.060% by weight aluminum (Al), 0.50–1.60% by weight nickel (Ni), 0.05–1.20% by weight chromium (Cr), less than 0.30% by weight copper (Cu), 0.005–0.015% by weight nitrogen (N), the balance being iron (Fe) and unavoidable impurities; Step (b) involves hot rolling the steel at a rolling end temperature of 900–1100°C. as well as Step (c) involves cooling the steel.

8. The method for manufacturing high-performance steel bars according to claim 7, characterized in that, In step (a), the reheating time of the steel is 1 to 3 hours.

9. The method for manufacturing high-performance steel bars according to claim 7, characterized in that, In step (b), the hot rolling ratio is 3 or more.

10. The method for manufacturing high-performance steel bars according to claim 7, characterized in that, In step (c), Air cooling is performed at a cooling rate of 0.5–5.0 °C / sec.

11. The method for manufacturing high-performance steel bars according to claim 7, characterized in that, The steel processed in step (c) exhibits a crack sensitivity rate of over 0.95 at a temperature of -170°C.

12. The method for manufacturing high-performance steel bars according to claim 7, characterized in that, The final microstructure of the steel obtained through step (c) comprises bainite and ferrite.

13. The method for manufacturing high-performance steel bars according to claim 7, characterized in that, The steel also contains less than 0.1% by weight of molybdenum (Mo) and less than 0.080% by weight of vanadium (V).

14. The method for manufacturing high-performance steel bars according to claim 7, characterized in that, The ideal critical diameter value DI of the steel is 0.3 to 1.

0.

15. The method for manufacturing high-performance steel bars according to claim 7, characterized in that, The nickel-manganese equivalent (NMeq) of the steel is 2.0 to 4.7% by weight, where NMeq is the sum of the contents of nickel (Ni) and manganese (Mn).

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