High-performance steel bar, and method for manufacturing high-performance steel bar

Abstract

A high-performance steel bar according to one embodiment of the present invention comprises 0.03-0.17 wt% of carbon (C), 0.50 wt% or less (excluding 0) of silicon (Si), 0.50-2.50 wt% of manganese (Mn), 0.02 wt% or less (excluding 0) of phosphorus (P), 0.02 wt% or less (excluding 0) of sulfur (S), 0.30-1.00 wt% of nickel (Ni), 0.002-0.500 wt% of chromium (Cr), 0.20 wt% or less (excluding 0) of copper (Cu), 0.003-0.150 wt% of molybdenum (Mo), 0.040 wt% or less (excluding 0) of aluminum (Al), 0.015 wt% or less (excluding 0) of nitrogen (N), and the balance of iron (Fe) and inevitable impurities, wherein the room-temperature yield strength (YS) is at least 600 MPa and the cryogenic yield strength (YS) is at least 820 MPa.

Description

High-performance steel bars and methods for manufacturing high-performance steel bars

[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 materials used to reinforce concrete. They are widely used in construction and civil engineering due to their strong bonding with concrete, their ability to compensate for concrete's vulnerability to tensile stress, and their ability to reduce the width of cracks that may occur. For example, deformed bars, or rebars, are a key component in the construction of bridges, large offshore structures, underground structures, and storage facilities.

[0003] Recently, interest in natural gas has been growing due to environmental concerns and shifts in domestic energy policy. Natural gas is liquefied at temperatures below -170°C and transported as liquefied natural gas (LNG). This liquefied natural gas is then stored and stored in LNG tanks. LNG tanks, designed to store liquefied LNG at temperatures below -170°C, require special structures and materials capable of withstanding temperatures near -170°C.

[0004] LNG tanks are largely structured as an inner and outer tank. The inner tank, which comes into contact with the LNG, is constructed of 9% nickel steel plates capable of withstanding temperatures of -170°C. The outer tank is constructed of reinforced concrete. For the deformed steel bars used in LNG storage tanks, steel bars capable of withstanding temperatures of -170°C are required to maintain the structure without brittle fracture even with rapid temperature drops.

[0005] Therefore, there is a need to develop high-performance steel bars and their manufacturing methods that have excellent strength and do not undergo brittle fracture even in environments with temperatures of -170℃ or lower.

[0006] In order to solve the problems of the prior art described above, the purpose of the present invention is to provide a high-performance steel bar and a steel bar manufacturing method that can secure not only yield strength and tensile strength in a room temperature environment but also yield strength and tensile strength in a low temperature environment.

[0007] 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.

[0008] A high-performance steel bar according to one embodiment of the present invention contains carbon (C) 0.08 to 0.17 wt%, silicon (Si) 0.50 wt% or less (excluding 0), manganese (Mn) 0.50 to 2.50 wt%, phosphorus (P) 0.02 wt% or less (excluding 0), sulfur (S) 0.02 wt% or less (excluding 0), nickel (Ni) 0.30 to 1.00 wt%, chromium (Cr) 0.002 to 0.500 wt%, copper (Cu) 0.20 wt% or less (excluding 0), molybdenum (Mo) 0.003 to 0.150 wt%, aluminum (Al) 0.040 wt% or less (excluding 0), nitrogen (N) 0.015 wt% or less (excluding 0), and the remainder iron (Fe) and unavoidable impurities, and has a room temperature yield strength (YS) of 600 MPa or more, Low temperature yield strength (YS) -170℃ ) is 800MPa or more.

[0009] It may further include at least one of tin (Sn) of 0.1 wt% or less and calcium (Ca) of 0.05 wt% or less.

[0010] The room temperature tensile strength (TS) can be greater than 690 MPa.

[0011] Room temperature elongation (EL) can be greater than 13%.

[0012] Uniform elongation (UE) can be greater than 3%.

[0013] Low temperature notched tensile strength (TS) n ) can be 800MPa or more.

[0014] Notch sensitivity (NSR) can be greater than 1.00.

[0015] It includes a surface layer and a central layer, wherein the surface layer includes at least one of tempered martensite and bainite, and the central layer may include at least one of bainite, acicular ferrite, and pearlite.

[0016] A method for manufacturing a high-performance steel bar according to one embodiment of the present invention comprises the steps of (S1) preparing a steel material, (S2) reheating the steel material, (S3) hot-rolling the steel material to form a hot-rolled product, and (S4) cooling the hot-rolled product to form a final product, wherein the final product comprises carbon (C) 0.08 to 0.17 wt%, silicon (Si) 0.50 wt% or less (excluding 0), manganese (Mn) 0.50 to 2.50 wt%, phosphorus (P) 0.02 wt% or less (excluding 0), sulfur (S) 0.02 wt% or less (excluding 0), nickel (Ni) 0.30 to 1.00 wt%, chromium (Cr) 0.002 to 0.500 wt%, copper (Cu) 0.20 wt% or less (excluding 0), molybdenum (Mo) 0.003 to 0.150 wt%, Contains aluminum (Al) 0.040 wt% or less (excluding 0), nitrogen (N) 0.015 wt% or less (excluding 0) and the remainder iron (Fe) and unavoidable impurities, and the room temperature yield strength (YS) of the final product is 600 MPa or more and ultra-low temperature yield strength (YS -170℃ ) is 800MPa or more.

[0017] The reheating temperature of the above step (S2) may be 1030 to 1200°C.

[0018] The rolling ratio of the above (S3) step can be 14 to 40%.

[0019] The rolling end temperature of the above (S3) step may be 900 to 1000°C.

[0020] The cooling rate of the above (S4) step may be 30 to 400 ℃ / s.

[0021] The cooling end temperature of the above (S4) step may be 540 to 700°C.

[0022] According to one embodiment of the present invention, it is possible to implement a high-performance steel bar and a steel bar manufacturing method that can secure not only yield strength and tensile strength in a room temperature environment but also yield strength and tensile strength in a low temperature environment.

[0023] 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.

[0024] Figure 1 is a flowchart showing a method for manufacturing a high-performance steel bar according to one embodiment of the present invention.

[0025] Figure 2 is a photograph showing the results of observing the surface and center of a high-performance steel bar according to one embodiment of the present invention using an optical microscope.

[0026] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings so that those skilled in the art can easily implement the present invention. However, the present invention is not limited or restricted by the following embodiments.

[0027] Additionally, when a component (or region, layer, portion, etc.) is referred to as being "on," "connected to," or "coupled to" another component, it means that it can be directly placed / connected / coupled to the other component, or that a third component may be placed between them.

[0028] 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.

[0029] In order to clearly explain the present invention, a detailed description of a part that is irrelevant to the description or a related known technology that may unnecessarily obscure the gist of the present invention has been omitted, and when adding reference signs to components of each drawing in this specification, the same or similar reference signs are attached to the same or similar components throughout the specification.

[0030] In addition, terms and words used in this specification and claims should not be interpreted as limited to their usual or dictionary meanings, but should be interpreted as meanings and concepts that conform to the technical idea of ​​the present invention based on the principle that the inventor can appropriately define the concept of the term to explain his or her own invention in the best way.

[0031] 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.

[0032] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0033]

[0034] High-performance steel bar

[0035] A high-performance steel bar according to one embodiment of the present invention is manufactured from a steel material containing carbon (C) 0.08 to 0.17 wt%, silicon (Si) 0.50 wt% or less (excluding 0), manganese (Mn) 0.50 to 2.50 wt%, phosphorus (P) 0.02 wt% or less (excluding 0), sulfur (S) 0.02 wt% or less (excluding 0), nickel (Ni) 0.30 to 1.00 wt%, chromium (Cr) 0.002 to 0.500 wt%, copper (Cu) 0.20 wt% or less (excluding 0), molybdenum (Mo) 0.003 to 0.150 wt%, aluminum (Al) 0.040 wt% or less (excluding 0), nitrogen (N) 0.015 wt% or less (excluding 0), and the remainder iron (Fe) and unavoidable impurities, and may include the same alloy components in the steel bar as a final product.

[0036] In addition, 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) and 0.05 wt% or less of calcium (Ca) instead of the remaining iron (Fe).

[0037] 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.

[0038]

[0039] Carbon (C)

[0040] Carbon (C) is the most effective and important element for increasing the strength of steel.

[0041] Carbon is incorporated into austenite, allowing it to form low-temperature structures with excellent strength, such as martensite and bainite, upon rapid cooling. Furthermore, increasing carbon content can enhance hardness.

[0042] Insufficient carbon content can lead to difficulties in achieving sufficient strength due to the aforementioned effects. Conversely, excessive carbon content can lead to deformation upon cooling, reduced elongation, and reduced low-temperature toughness.

[0043] Accordingly, the high-performance steel bar according to one embodiment of the present invention may contain 0.08 to 0.17 wt% of carbon (C), preferably 0.10 to 0.17 wt%, and more preferably 0.13 to 0.17 wt%.

[0044]

[0045] Silicon (Si)

[0046] Silicon (Si) is added as a deoxidizer to remove oxygen in steel during the steelmaking process together with aluminum, and as a ferrite stabilizing element with a solid solution strengthening effect, it can induce ferrite formation and improve the hardenability and softening resistance of steel.

[0047] When the silicon content exceeds 0.50 wt%, the toughness may be reduced and the plastic workability may be impaired.

[0048] Therefore, the 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 (excluding 0).

[0049]

[0050] manganese (Mn)

[0051] Manganese (Mn) is a solid-solution strengthening element that not only contributes to strength but also enhances the hardenability of steel. Furthermore, as the manganese content increases, pearlite becomes finer and ferrite becomes solid-solution strengthened, thereby improving yield strength.

[0052] If manganese content is insufficient, the aforementioned effects may be minimal. Conversely, if manganese content is excessive, austenite may remain, reducing strength and toughness.

[0053] Therefore, the high-performance steel bar according to one embodiment of the present invention may contain manganese (Mn) in an amount of 0.50 to 2.50 wt%.

[0054]

[0055] Person (P)

[0056] Phosphorus (P) is an element that contributes to strength enhancement. However, excessive phosphorus content can deteriorate the ductility of steel and cause final material deviations due to billet center segregation. When phosphorus content exceeds 0.02 wt%, it can form center segregation and microsegregation, reducing the ductility of the steel. Furthermore, precipitation behavior can reduce impact strength.

[0057] Therefore, the high-performance steel according to one embodiment of the present invention may contain phosphorus (P) in an amount of 0.02 wt% or less (excluding 0).

[0058]

[0059] Yellow (S)

[0060] Sulfur (S) improves the machinability of steel by combining with manganese, zinc, titanium, molybdenum, and other elements. It can also enhance workability by combining with manganese to form fine precipitates (e.g., MnS). However, if the amount of manganese in the steel is insufficient, it can combine with iron to form sulfides (e.g., FeS), which can cause cracks during hot and cold working. Sulfur content exceeding 0.02 wt% can reduce the ductility of the steel and deteriorate its toughness and weldability.

[0061] Therefore, the high-performance steel according to one embodiment of the present invention may contain sulfur (S) in an amount of 0.02 wt% or less (excluding 0).

[0062]

[0063] Nickel (Ni)

[0064] Nickel is an important and common alloying element for low-temperature toughness. Nickel (Ni) refines the structure of steel and is readily dissolved in austenite and ferrite, providing solution strengthening.

[0065] If the nickel content is insufficient, the aforementioned effects may be minimal. Conversely, if the nickel content is excessive, the steel's manufacturing costs increase and its weldability and toughness may deteriorate.

[0066] Therefore, the high-performance steel bar according to one embodiment of the present invention may contain 0.30 to 1.00 wt% of nickel (Ni).

[0067]

[0068] chromium (Cr)

[0069] Chromium (Cr) is a ferrite-stabilizing element. When added to C-Mn steel, it impedes carbon diffusion due to its solute-interfering effect, thereby contributing to grain refinement. Furthermore, chromium enhances the hardenability of steel, improving hardenability. However, excessive chromium content can lead to the formation of coarse carbides at grain boundaries, reducing the steel's room-temperature and low-temperature ductility.

[0070] Therefore, the high-performance steel bar according to one embodiment of the present invention may contain chromium (Cr) in an amount of 0.002 to 0.500 wt%.

[0071]

[0072] copper (Cu)

[0073] Copper (Cu) typically exists in steel at 0.1 to 0.3 wt%. Copper dissolves in ferrite at levels up to 0.35 wt% at room temperature, exhibiting a solid-solution strengthening effect that can improve strength and hardness. It can also enhance corrosion resistance in air and seawater. However, excessive copper content can reduce elongation.

[0074] Therefore, the high-performance steel bar according to one embodiment of the present invention may contain copper (Cu) in an amount of 0.20 wt% or less (excluding 0).

[0075]

[0076] molybdenum (Mo)

[0077] Molybdenum (Mo) can enhance hardenability by up to 10 times that of nickel (Ni), even with small amounts, and it prevents temper embrittlement, providing resistance to tempering. Furthermore, because it forms carbides, molybdenum is an effective alloying element in advanced cutting tools and increases the grain coarsening temperature. Molybdenum is more effective when used in conjunction with chromium than alone to enhance hardenability.

[0078] If the molybdenum content is insufficient, the aforementioned effects may be minimal. Conversely, if the molybdenum content is excessive, the steel manufacturing cost increases and weldability may be reduced.

[0079] Therefore, the high-performance steel bar according to one embodiment of the present invention may contain molybdenum (Mo) in an amount of 0.003 to 0.150 wt%.

[0080]

[0081] Aluminum (Al)

[0082] Aluminum (Al) is added to the steelmaking process as a deoxidizer to remove oxygen from steel. It can also contribute to grain refinement through the formation of precipitates (e.g., AlN).

[0083] If the aluminum content is insufficient, the deoxidation effect may be inadequate. Conversely, if the aluminum content is excessive, ductility and toughness may be reduced.

[0084] Therefore, the high-performance steel bar according to one embodiment of the present invention may contain aluminum (Al) in an amount of 0.040 wt% or less (excluding 0).

[0085]

[0086] Nitrogen (N)

[0087] Even trace amounts of nitrogen (N) significantly affect the mechanical properties of steel. As nitrogen content increases, tensile strength and yield strength increase, while elongation decreases. In particular, the decrease in impact strength and the increase in transition temperature are notable.

[0088] When nitrogen is added, the austenite grains become finer, making it possible to manufacture fine-grained steel, and the grains are refined by forming nitrides with titanium, zirconium, vanadium, niobium, etc., but if the nitrogen content is excessive, high-temperature toughness may decrease, and grain boundary embrittlement may occur due to nitride precipitation at the austenite grain boundaries.

[0089] Therefore, the 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 (excluding 0).

[0090]

[0091] Sn

[0092] Tin (Sn) can improve the corrosion resistance and strength of steel.

[0093] When tin is added in amounts exceeding 0.1 wt%, it can cause numerous defects, such as blister-like scales swelling or bursting on the steel surface, rather than contributing to improved corrosion resistance. Furthermore, excessive tin content can reduce elongation and low-temperature impact toughness.

[0094] Therefore, 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 (excluding 0).

[0095]

[0096] Calcium (Ca)

[0097] Calcium (Ca) is added to the steelmaking process as a deoxidizer to remove oxygen from steel. It also inhibits the formation of non-metallic inclusions such as manganese (MnS), preventing a decrease in elongation and improving low-temperature toughness.

[0098] If the calcium content is excessive, it may react with the oxygen contained in the steel to form coarse oxide (CaO), which may reduce the elongation.

[0099] Therefore, 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 (excluding 0).

[0100]

[0101] In addition to the aforementioned steel components, the remainder may contain iron and unavoidable impurities. Unavoidable impurities are impurities introduced during the steelmaking process and the steel bar manufacturing process. Since these impurities are widely known in the field, a detailed description will be omitted.

[0102] In one embodiment of the present invention, the addition of elements other than the aforementioned alloy components is not excluded, and various elements may be included within a range that does not impair the technical spirit of the present invention. When additional elements are included, they may be included to replace the remaining iron (Fe).

[0103] A high-performance steel bar according to one embodiment of the present invention may have a room temperature yield strength (YS) of 600 MPa or more and a room temperature tensile strength (TS) of 690 MPa or more.

[0104] A high-performance steel bar according to one embodiment of the present invention may have a room temperature elongation (EL) of 13% or more.

[0105] High-performance steel bars according to one embodiment of the present invention have low-temperature yield strength (YS -170℃ ) can be 800 MPa or more. Here, the low-temperature yield strength (YS -170℃ ) means the yield strength measured in a specimen at -170℃.

[0106] Low temperature yield strength (YS) -170℃ ) is 800 MPa or more, it is possible to suppress or prevent deformation of the steel bar even when an external force is applied in a low-temperature (-170℃) environment due to low-temperature brittleness.

[0107] Accordingly, the high-performance steel bar according to one embodiment of the present invention has a low-temperature yield strength (YS -170℃ ) may be 800 MPa or more, and preferably the low-temperature yield strength (YS -170℃) may be 820 MPa or more, and more preferably, the low-temperature yield strength (YS -170℃ ) can be 823 MPa or more.

[0108] High-performance steel bars according to one embodiment of the present invention have low-temperature notch tensile strength (TS n ) can be 800 MPa or more. Here, the low-temperature notched tensile strength (TS n ) refers to the tensile strength measured in a notched specimen at -170℃.

[0109] Low temperature notched tensile strength (TS) n ) is 800 MPa or more, it is possible to suppress or prevent fatigue failure caused by notches in a low temperature (-170℃) environment.

[0110] A high-performance steel bar according to one embodiment of the present invention may have a uniform elongation (UE) of 3% or more. Here, the uniform elongation (UE) refers to the elongation measured in a specimen at -170°C.

[0111] When the uniform elongation is 3% or more, the occurrence of fracture in the steel bar can be suppressed or prevented even when external force is applied in a low-temperature (-170℃) environment due to low-temperature brittleness.

[0112] Accordingly, the high-performance steel according to one embodiment of the present invention may have a uniform elongation (UE) of 3% or more, and preferably 6% or more.

[0113] A high-performance steel bar according to one embodiment of the present invention may have a notch sensitivity (NSR) of 1.00 or more. Here, the notch sensitivity (NSR) is a ratio of the low-temperature notched tensile strength (TS n ) and low-temperature yield strength (YS -170℃ ) means rain.

[0114] By improving the low-temperature notch sensitivity (NSR), fatigue failure caused by notches in a low-temperature (-170°C) environment can be suppressed or prevented even when notches occur in the steel bar. Accordingly, the low-temperature notch sensitivity (NSR) can be 1.0 or higher.

[0115] Hereinafter, a method for manufacturing a high-performance steel bar according to one embodiment of the present invention will be described in detail.

[0116]

[0117] Method for manufacturing high-performance steel bars

[0118] Hereinafter, a method for manufacturing a high-performance steel bar according to one embodiment of the present invention will be described with reference to FIG. 1.

[0119] Figure 1 is a flowchart showing a method for manufacturing a high-performance steel bar according to one embodiment of the present invention.

[0120] A method for manufacturing a high-performance steel bar according to one embodiment of the present invention includes a step (S1) of preparing a semi-finished steel product, a step (S2) of reheating the steel product, a step (S3) of hot-rolling the steel product to form a hot-rolled product, and a step (S4) of cooling the formed hot-rolled product.

[0121] Hereinafter, each step of a high-performance steel bar manufacturing method according to one embodiment of the present invention will be described in detail.

[0122] According to one embodiment of the present invention, the step (S1) of preparing a semi-finished steel product is a step of preparing a steel product having the above-described alloy composition range in order to manufacture a high-performance steel bar as a final product.

[0123] Specifically, the steel preparation step (S1) may be a step of designing alloy components within the aforementioned alloy composition range to manufacture a semi-finished product. The semi-finished product may be a billet or bloom, but is not limited thereto. Furthermore, the manufacture of the semi-finished product may be performed using processes known in the relevant technical field, such as a steelmaking process or a casting process.

[0124] According to one embodiment of the present invention, a step (S2) of reheating the steel may be performed after the step (S1) of preparing the steel.

[0125] The step (S2) of reheating the steel is a pretreatment step of the rolling step, and may be a step of uniformly heating the steel by charging the steel into a heating furnace so that plastic deformation can be easily performed, thereby re-dissolving the components segregated during casting of the steel.

[0126] In the reheating step (S2), the steel may be reheated to 1030 to 1200°C. If the reheating temperature is below 1030°C, the rolling load may increase. Conversely, if the reheating temperature exceeds 1200°C, austenite grains may coarsen or decarburization may occur, reducing strength. Furthermore, increased heating costs and time may lead to increased manufacturing costs and reduced productivity.

[0127] Therefore, in the present invention, steel can be reheated at a temperature of 1030 to 1200°C.

[0128] According to one embodiment of the present invention, a step (S3) of hot rolling the steel to form a hot-rolled steel may be performed after a step (S2) of reheating the steel.

[0129] In the step of forming hot-rolled steel (S3), the temperature of the reheated steel is slightly reduced due to transport, etc., so that hot rolling can begin at a temperature range of 1020 to 1180°C.

[0130] The above hot rolling may include rough rolling and finish rolling processes. Here, the rough rolling process may be to make the steel into a rolled material having an appropriate shape, thickness, and width, and the finish rolling process may be to adjust the steel to a specified size and roll it at a finishing temperature suitable for the intended use to obtain a good surface and shape.

[0131] In the step of forming the hot rolled material (S3), the rolling ratio can be 14 to 40%.

[0132] When the rolling ratio is less than 14%, it is difficult to secure a uniform, fine-grained structure, which can lead to significant variations in strength and impact toughness. Conversely, when the rolling ratio exceeds 40%, cracks can occur in hot-rolled steel during operation.

[0133] In the step (S3) of forming a hot-rolled product, the finishing rolling temperature may be 900 to 1000°C. If the finishing rolling temperature is lower than 900°C, the rolling load increases, which may lower productivity and reduce the heat treatment effect. Conversely, if the temperature exceeds 1000°C, coarse pearlite structures may form, which may rapidly reduce strength.

[0134] Therefore, the finishing rolling temperature can be 900 to 1000°C.

[0135] By maintaining the finishing rolling temperature within the aforementioned range, continuous dynamic recrystallization can occur within the steel. This allows for refinement of the austenite structure, and further refinement of the structure of the final product, the high-performance steel bar, after cooling, can improve low-temperature properties.

[0136] According to one embodiment of the present invention, after performing the step (S3) of forming a hot-rolled material, a step (S4) of cooling the formed hot-rolled material may be performed.

[0137] The step of cooling the hot rolled material (S4) can be performed through cooling equipment.

[0138] The above cooling facility may be a temp-core facility. That is, the hot rolled steel may be rapidly cooled by a refrigerant and then reheated by the residual heat. For example, the refrigerant may be water.

[0139] In other words, the step (S4) of cooling the hot-rolled material can rapidly cool the hot-rolled material at a cooling rate of 30 to 400°C / sec using cooling water. The above-described temperature range refers to the surface temperature of the hot-rolled material.

[0140] After rapid cooling, the surface of the hot rolled steel is reheated to a temperature range of 540 to 700℃ through the internal residual heat, and can be air-cooled to room temperature again.

[0141] After the step of cooling the hot rolled steel (S4) is performed, the final product, high-performance steel bar, can be formed.

[0142] The high-performance steel bar manufactured by the high-performance steel bar manufacturing method according to one embodiment of the present invention has the above-mentioned room temperature yield strength (YS), room temperature tensile strength (TS), and low temperature yield strength (YS). -170℃ ), low temperature notched tensile strength (TS n ), both uniform elongation (UE) and notch sensitivity (NSR) values ​​can be satisfied.

[0143] In addition, a high-performance steel bar manufactured by a high-performance steel bar manufacturing method according to one embodiment of the present invention may include a surface portion and a central portion.

[0144] In the cross-section of a bar cut in a direction perpendicular to the rolling direction, the center is defined as the area that is 65 to 75% of the total area, and the surface is defined as the area that is the remaining area surrounding the center (i.e., 25 to 35% of the total area).

[0145] The surface layer may be formed of a structure including at least one of tempered martensite and bainite, and the central portion may be formed of a structure including at least one of bainite, acicular ferrite, and pearlite.

[0146] For further details, see Figure 2.

[0147] Figure 2 is a photograph showing the results of observing the surface and center of a high-performance steel bar according to one embodiment of the present invention using an optical microscope. In Figure 2, (a) is a photograph observing the microstructure of the surface of the steel bar, and (b) is a photograph observing the microstructure of the center of the steel bar.

[0148] Referring to Fig. 2(a), it can be confirmed that tempered martensite and bainite structures are formed in a complex manner on the surface of the steel bar, and referring to Fig. 2(b), it can be confirmed that bainite, needle-shaped ferrite, and pearlite structures are formed in a complex manner on the center of the steel bar.

[0149] According to one embodiment of the present invention, the room temperature and low temperature strength can be improved through the tempered martensite and bainite structures formed on the surface layer of the steel bar.

[0150] In addition, the room temperature elongation, uniform elongation, and notch sensitivity of the bar can be improved through the bainite, needle ferrite, and pearlite structures formed in the center of the bar.

[0151]

[0152] Comparative examples and examples

[0153] Below, preferred comparative examples and examples are presented to aid in understanding the present invention. However, the following comparative examples and examples are provided solely to aid in understanding the present invention, and the present invention is not limited to the examples below.

[0154] Table 1 shows the alloy element compositions of comparative examples and examples, and Table 2 shows the operating conditions and property values ​​of comparative examples and examples.

[0155] The comparative examples and examples were each manufactured using semi-finished products having the alloy compositions described in Table 1 below, and hot rolling and cooling were performed using the operating conditions described in Table 2.

[0156] In terms of cooling rate, cooling of the bar was performed through accelerated controlled cooling, with cooling starting at a cooling rate of 30°C / sec and ending at a cooling rate of 400°C / sec.

[0157] Other than the conditions described above, the manufacturing process of the comparative examples and examples of the present invention was controlled under the same conditions within the range described in the high-performance steel bar manufacturing method according to one embodiment of the present invention described above, as a control variable.

[0158] In addition, the room temperature properties of the comparative examples and examples were measured in an environment of 20°C, and the low temperature properties were measured in an environment of -170°C.

[0159] In Table 1 below, the unit of composition of alloy elements is weight%, and in Table 2 below, the unit of rolling start temperature, rolling end temperature, and reheat temperature is ℃, and the yield strength at room temperature (YS), tensile strength at room temperature (TS), and yield strength at low temperature (YS) are -170℃ ) and low-temperature notched tensile strength (TS n ) is in MPa, and the units of room temperature elongation (EL) and uniform elongation (UE) are in %.

[0160] Classification CSiMnPSCuCrMoNiSnAlCaNComparative Example 10.0700.121.550.0100.0120.230.110.020.590.0070.0150.0020.009Comparative Example 20.0700.121.550.0100.0120.230.110.020.590.0070.0150.0020.009Comparative Example 30.0350.121.620.0100.0100.130.110.060.560.0070.0150.0020.009Comparative Example 40.0460.171.530.0080.0100.150.050.020.530.0080.0300.0020.010Comparative Example 50.0700.151.550.0100.0080.110.080.120.470.0070.0200.0100.008Comparative Example 60.0700.151.550.0100.0080.110.080.120.470.0070.0200.0100.008Example 10.1340.121.780.0100.0100.200.110.040.590.0070.0150.0020.009Example 20.1620.251.830.0090.0080.130.080.100.580.0100.0250.0020.009

[0161]

[0162] ClassificationOperating conditionsRoom temperature properties (20℃)Low temperature properties (-170℃)Rolling start temperature (℃)Rolling end temperature (℃)Cooling speed (℃ / sec)Recuperation temperature (℃)YS (MPa)TS (MPa)EL (%)YS -170℃ (MPa)UE(%)TS n (MPa)NSRComparative Example 11,1801,00030~40067057368016.67628.57701.01Comparative Example 21,02091058060770612.97828.37981.02Comparative Example 31,10096053555367713.47839.48061.03Comparative Example 41,10096053558968315.978111.08121.04Comparative Example 51,180100567056369120.574810.57781.04Comparative Example 61,02092058061670616.57749.68131.05 Example 11,10096057360469913.88236.28261.00 Example 21,10099557763671416.08299.08461.02

[0163]

[0164] Referring to Tables 1 and 2 above, Comparative Example 1 is a comparative example in which the carbon (C) content is 0.070 wt%.

[0165] In the case of Comparative Example 1, the room temperature elongation (EL), uniform elongation (UE) and notch sensitivity (NSR) satisfy the range according to one embodiment of the present invention, but the room temperature yield strength (YS) is 573 MPa, the room temperature tensile strength (TS) is 680 MPa, and the low temperature yield strength (YS) is 680 MPa. -170℃ ) 762MPa and low temperature notched tensile strength (TS) n ) It can be confirmed that 770MPa was achieved.

[0166] In other words, in the case of Comparative Example 1, the aforementioned properties are within the range according to one embodiment of the present invention: room temperature yield strength (YS) 600 MPa or more, room temperature tensile strength (TS) 690 MPa or more, low temperature yield strength (YS -170℃ ) 800MPa or more and low temperature notched tensile strength (TS)n ) It can be confirmed that it does not satisfy 800MPa.

[0167] Comparative Example 2 is a comparative example with the same alloy element composition as Comparative Example 1, but with different operating conditions. Specifically, it can be confirmed that the rolling start temperature is 1020°C, the rolling end temperature is 910°C, and the reheat temperature is 580°C, which are lower than those of Comparative Example 1.

[0168] In the case of Comparative Example 2, the room temperature yield strength (YS), room temperature tensile strength (TS), room temperature elongation (EL), uniform elongation (UE), and notch sensitivity (NSR) satisfy the range according to one embodiment of the present invention, and the low temperature yield strength (YS -170℃ ) 782MPa and low temperature notched tensile strength (TS) n ) It can be confirmed that 798MPa was achieved.

[0169] In the case of Comparative Example 2, the strengths were improved compared to Comparative Example 1, but the low-temperature yield strength (YS -170℃ ) and low-temperature notched tensile strength (TS n ) is the low temperature yield strength (YS) which is in the range according to one embodiment of the present invention. -170℃ ) 800MPa or more and low temperature notched tensile strength (TS) n ) It can be confirmed that it does not satisfy 800MPa.

[0170] Comparative Example 3 is a comparative example in which the carbon (C) content is 0.035 wt%.

[0171] In the case of Comparative Example 3, the room temperature elongation (EL), uniform elongation (UE) and notch sensitivity (NSR) satisfy the range according to one embodiment of the present invention, but the room temperature yield strength (YS) is 553 MPa, the room temperature tensile strength (TS) is 677 MPa and the low temperature yield strength (YS) is 677 MPa. -170℃ ) It can be confirmed that 783MPa was obtained.

[0172] In other words, in the case of Comparative Example 3, the aforementioned properties are each in the range according to one embodiment of the present invention: room temperature yield strength (YS) 600 MPa or more, room temperature tensile strength (TS) 690 MPa or more, and low temperature yield strength (YS -170℃ ) It can be confirmed that it does not satisfy more than 800MPa.

[0173] Comparative Example 4 is a comparative example in which the carbon (C) content is 0.046 wt%.

[0174] In the case of Comparative Example 4, the room temperature elongation (EL), uniform elongation (UE) and notch sensitivity (NSR) satisfy the range according to one embodiment of the present invention, but the room temperature yield strength (YS) is 589 MPa, the room temperature tensile strength (TS) is 683 MPa and the low temperature yield strength (YS) is 683 MPa. -170℃ ) It can be confirmed that 781MPa was achieved.

[0175] In other words, in the case of Comparative Example 4, the aforementioned properties are each in the range according to one embodiment of the present invention: room temperature yield strength (YS) 600 MPa or more, room temperature tensile strength (TS) 690 MPa or more, and low temperature yield strength (YS -170℃ ) It can be confirmed that it does not satisfy more than 800MPa.

[0176] Comparative Example 5 is a comparative example having a carbon (C) content of 0.070 wt%, but having a different alloy composition from Comparative Examples 1 and 2.

[0177] In the case of Comparative Example 5, the room temperature tensile strength (TS), room temperature elongation (EL), uniform elongation (UE) and notch sensitivity (NSR) satisfy the range according to one embodiment of the present invention, but the room temperature yield strength (YS) is 563 MPa, the low temperature yield strength (YS) is 563 MPa, and the low temperature yield strength (YS) is 563 MPa. -170℃ ) 748MPa and low temperature notched tensile strength (TS) n ) It can be confirmed that 778MPa was achieved.

[0178] In other words, in the case of Comparative Example 5, the aforementioned properties are in the range according to one embodiment of the present invention, such as room temperature yield strength (YS) of 600 MPa or more and low temperature yield strength (YS -170℃ ) 800MPa or more and low temperature notched tensile strength (TS) n ) It can be confirmed that it does not satisfy 800MPa.

[0179] Comparative Example 6 is a comparative example that has the same alloy element composition as Comparative Example 5, but different operating conditions. Specifically, it can be confirmed that the rolling start temperature is 1020°C, the rolling end temperature is 920°C, and the reheat temperature is 580°C, which are lower than those of Comparative Example 5.

[0180] In the case of comparative example 6, room temperature yield strength (YS), room temperature tensile strength (TS), room temperature elongation (EL), and low temperature notched tensile strength (TS) n ), uniform elongation (UE) and notch sensitivity (NSR) satisfy the range according to one embodiment of the present invention, and low temperature yield strength (YS -170℃ ) It can be confirmed that the value is 774 MPa.

[0181] In the case of Comparative Example 6, the strengths were improved compared to Comparative Example 5, but the low-temperature yield strength (YS -170℃ ) can be confirmed to not satisfy the range of 800 MPa or more according to one embodiment of the present invention.

[0182] As a result, in the case of Comparative Examples 1 to 6, the carbon (C) content did not satisfy the range of 0.08 wt% or more according to one embodiment of the present invention, and it could be confirmed that at least one of the room temperature properties and the low temperature properties was not satisfied.

[0183] On the other hand, in the case of Examples 1 and 2, it can be confirmed that the alloy composition range satisfies all of the embodiments of the present invention.

[0184] Through this, yield strength at room temperature (YS), tensile strength at room temperature (TS), elongation at room temperature (EL), and yield strength at low temperature (YS) -170℃), low temperature notched tensile strength (TS n ), it can be confirmed that the values ​​of uniform elongation (UE) and notch sensitivity (NSR) all satisfy the ranges targeted in the present invention.

[0185] 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 may 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. Contains carbon (C) 0.08 to 0.17 wt%, silicon (Si) 0.50 wt% or less (excluding 0), manganese (Mn) 0.50 to 2.50 wt%, phosphorus (P) 0.02 wt% or less (excluding 0), sulfur (S) 0.02 wt% or less (excluding 0), nickel (Ni) 0.30 to 1.00 wt%, chromium (Cr) 0.002 to 0.500 wt%, copper (Cu) 0.20 wt% or less (excluding 0), molybdenum (Mo) 0.003 to 0.150 wt%, aluminum (Al) 0.040 wt% or less (excluding 0), nitrogen (N) 0.015 wt% or less (excluding 0), and the remainder iron (Fe) and unavoidable impurities. The room temperature yield strength (YS) is 600 MPa or more, Low temperature yield strength (YS) -170℃ ) is 800MPa or more, High performance steel.

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

3. In paragraph 1, The room temperature tensile strength (TS) is 690 MPa or more. High performance steel.

4. In paragraph 1, The room temperature elongation (EL) is 13% or more. High performance steel.

5. In paragraph 1, Uniform elongation (UE) is 3% or more, High performance steel.

6. In paragraph 1, Low temperature notched tensile strength (TS) n ) is 800MPa or more, High performance steel.

7. In paragraph 1, Notch sensitivity (NSR) is 1.00 or greater, High performance steel.

8. In paragraph 1, Including the surface and the center, The above surface layer comprises at least one of tempered martensite and bainite, The above central portion comprises at least one of bainite, needle ferrite and pearlite. High performance steel. 9.(S1) Step of preparing steel; (S2) A step of reheating the above steel material; (S3) A step of hot rolling the above steel to form a hot-rolled steel; (S4) A step of cooling the hot rolled material to form a final product; Including, but not limited to, The above final product contains carbon (C) 0.08 to 0.17 wt%, silicon (Si) 0.50 wt% or less (excluding 0), manganese (Mn) 0.50 to 2.50 wt%, phosphorus (P) 0.02 wt% or less (excluding 0), sulfur (S) 0.02 wt% or less (excluding 0), nickel (Ni) 0.30 to 1.00 wt%, chromium (Cr) 0.002 to 0.500 wt%, copper (Cu) 0.20 wt% or less (excluding 0), molybdenum (Mo) 0.003 to 0.150 wt%, aluminum (Al) 0.040 wt% or less (excluding 0), nitrogen (N) 0.015 wt% or less (excluding 0), and the remainder iron (Fe) and unavoidable impurities. The room temperature yield strength (YS) of the above final product is 600 MPa or more, Ultra-low temperature yield strength (YS) -170℃ ) is 800MPa or more, Method for manufacturing high-performance steel bars.

10. In paragraph 9, The reheating temperature of the above step (S2) is 1030 to 1200℃. Method for manufacturing high-performance steel bars.

11. In paragraph 9, The rolling ratio of the above (S3) step is 14 to 40%. Method for manufacturing high-performance steel bars.

12. In paragraph 9, The rolling end temperature of the above (S3) step is 900 to 1000℃. Method for manufacturing high-performance steel bars.

13. In paragraph 9, The cooling rate of the above step (S4) is 30 to 400℃ / s. Method for manufacturing high-performance steel bars.

14. In paragraph 9, The cooling end temperature of the above (S4) step is 540 to 700℃. Method for manufacturing high-performance steel bars.

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