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

Abstract

A high-performance steel bar according to an embodiment of the present invention comprises: 0.06-0.32 wt% of carbon (C); 0.1-0.5 wt% of silicon (Si); 0.5-2.0 wt% of manganese (Mn); 0.03 wt % or less (excluding 0) of phosphorus (P), 0.05 wt% or less (excluding 0) of sulfur (S), 0.010 wt% or less (excluding 0) of aluminum (Al), 0.5 wt% or less (excluding 0) of chromium (Cr), 0.3 wt% or less (excluding 0) of copper (Cu), 0.1-1.0 wt% of nickel (Ni), 0.1 wt% or less (excluding 0) of molybdenum (Mo), 0.0100 wt% or less (excluding 0) of nitrogen (N), and the balance being iron (Fe) and inevitable impurities, wherein the steel bar comprises a surface layer portion and a central portion formed to exclude the surface layer portion, the surface layer portion including at least one of bainite, tempered martensite, and martensite, the central portion including at least one of pearlite and ferrite, and has a room-temperature tensile strength (TS) of 580 MPa or more.

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 above-described prior art, the purpose of the present invention is to provide a high-performance steel bar and a steel bar manufacturing method capable of securing tensile 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 comprises carbon (C) 0.06 to 0.32 wt%, silicon (Si) 0.1 to 0.5 wt%, manganese (Mn) 0.5 to 2.0 wt%, phosphorus (P) 0.03 wt% or less (excluding 0), sulfur (S) 0.05 wt% or less (excluding 0), aluminum (Al) 0.010 wt% or less (excluding 0), chromium (Cr) 0.5 wt% or less (excluding 0), copper (Cu) 0.3 wt% or less (excluding 0), nickel (Ni) 0.1 to 1.0 wt%, molybdenum (Mo) 0.1 wt% or less (excluding 0), nitrogen (N) 0.0100 wt% or less (excluding 0), and the remainder iron (Fe) and unavoidable impurities, and comprises a surface portion and a center portion formed to exclude the surface portion, wherein the surface portion comprises bainite, tempered martensite, and It contains at least one of martensite, and the center contains at least one of pearlite and ferrite, and the room temperature tensile strength (TS) is 580 MPa or more.

[0009] According to one embodiment of the present invention, low temperature tensile strength (TS -170℃ ) can be 820MPa or more.

[0010] According to one embodiment of the present invention, the room temperature elongation (EL) may be 13% or more.

[0011] According to one embodiment of the present invention, the ideal critical diameter (DI) value may be 0.3 to 2.1.

[0012] According to one embodiment of the present invention, the surface portion may have an area fraction of 25 to 35% with respect to a cross-section formed in a direction perpendicular to the longitudinal direction.

[0013] According to one embodiment of the present invention, the surface layer may include an area fraction of 72 to 89% tempered martensite and the remainder bainite.

[0014] According to one embodiment of the present invention, the central portion may include an area fraction of 58 to 84% ferrite and the remainder pearlite.

[0015] 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) 0.06 to 0.32 wt%, silicon (Si) 0.1 to 0.5 wt%, manganese (Mn) 0.5 to 2.0 wt%, phosphorus (P) 0.03 wt% or less (excluding 0), sulfur (S) 0.05 wt% or less (excluding 0), aluminum (Al) 0.010 wt% or less (excluding 0), chromium (Cr) 0.5 wt% or less (excluding 0), copper (Cu) 0.3 wt% or less (excluding 0), nickel (Ni) 0.1 to 1.0 wt%, and molybdenum (Mo) 0.1 wt% Below (excluding 0), nitrogen (N) 0.0100 wt% or less (excluding 0) and the remainder iron (Fe) and unavoidable impurities, the final product includes a surface portion and a center portion formed to exclude the surface portion, wherein the surface portion includes at least one of bainite, tempered martensite and martensite, and the center portion includes at least one of pearlite and ferrite, and the room temperature tensile strength (TS) of the final product is 580 MPa or more.

[0016] According to one embodiment of the present invention, the reheating temperature of the step (S2) may be 950 to 1100°C.

[0017] According to one embodiment of the present invention, the reheating time of step (S2) may be 1 to 3 hours.

[0018] According to one embodiment of the present invention, the rolling end temperature of the step (S3) may be 900 to 1050°C.

[0019] According to one embodiment of the present invention, the rolling ratio of the step (S3) may be 3 or more.

[0020] According to one embodiment of the present invention, the cooling rate of the step (S4) may be 30 to 400°C / sec.

[0021] According to one embodiment of the present invention, in the step (S4), the hot-rolled material is cooled and then reheated and air-cooled, and the reheating temperature may be 500 to 700°C.

[0022] According to one embodiment of the present invention, the low temperature tensile strength (TS) of the final product -170℃ ) can be 820MPa or more.

[0023] According to one embodiment of the present invention, the room temperature elongation (EL) of the final product may be 13% or more.

[0024] According to one embodiment of the present invention, the surface portion may have an area fraction of 25 to 35% with respect to a cross-section formed in a direction perpendicular to the longitudinal direction.

[0025] According to one embodiment of the present invention, the surface layer may include an area fraction of 72 to 89% tempered martensite and the remainder bainite.

[0026] According to one embodiment of the present invention, the central portion may include an area fraction of 58 to 84% ferrite and the remainder pearlite.

[0027] According to one embodiment of the present invention, a high-performance steel bar and a steel bar manufacturing method capable of securing tensile strength and tensile strength in a low-temperature environment can be implemented.

[0028] 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.

[0029] FIG. 1 is a flowchart showing a method for manufacturing a high-performance steel bar according to one embodiment of the present invention.

[0030] 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 practice the present invention. However, the present invention is not limited or restricted by the following embodiments.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0037]

[0038] High-performance steel bar

[0039] A high-performance steel bar according to one embodiment of the present invention is manufactured from a steel material containing carbon (C) 0.06 to 0.32 wt%, silicon (Si) 0.1 to 0.5 wt%, manganese (Mn) 0.5 to 2.0 wt%, phosphorus (P) 0.03 wt% or less (excluding 0), sulfur (S) 0.05 wt% or less (excluding 0), aluminum (Al) 0.010 wt% or less (excluding 0), chromium (Cr) 0.5 wt% or less (excluding 0), copper (Cu) 0.3 wt% or less (excluding 0), nickel (Ni) 0.1 to 1.0 wt%, molybdenum (Mo) 0.1 wt% or less (excluding 0), nitrogen (N) 0.0100 wt% or less (excluding 0), and the remainder iron (Fe) and inevitable impurities, and may include the same alloy components in the steel bar as a final product.

[0040] 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.

[0041]

[0042] carbon (C)

[0043] Carbon (C) is the most effective and important element for increasing the strength of steel.

[0044] Carbon is incorporated into austenite, which can form martensite during quenching. Furthermore, increasing carbon content can improve hardness.

[0045] Insufficient carbon content can lead to difficulties in achieving sufficient strength due to the aforementioned effects. Conversely, excessive carbon content can lead to deformation during quenching and reduced elongation and low-temperature toughness of the steel.

[0046] Therefore, the high-performance steel bar according to one embodiment of the present invention may contain 0.06 to 0.32 wt% of carbon, and preferably 0.06 to 0.29 wt%.

[0047]

[0048] Silicon (Si)

[0049] Silicon (Si) is added to steelmaking processes along with aluminum as a deoxidizer to remove oxygen from the steel, reducing the amount of oxides present within the steel. Furthermore, the addition of silicon can improve the strength of the steel.

[0050] If the silicon content is insufficient, the aforementioned effects may be minimal. Conversely, if the silicon content is excessive, toughness and ductility may be reduced.

[0051] Therefore, the high-performance steel bar according to one embodiment of the present invention may contain 0.1 to 0.5 wt% of silicon (Si), and preferably 0.13 to 0.30 wt%.

[0052]

[0053] manganese (Mn)

[0054] Manganese (Mn) can act as a deoxidizer and, as a solid-solution strengthening element, contribute to increased strength and improved toughness. Manganese combines with sulfur in steel to form fine inclusions, such as MnS, between grains, thereby improving the workability of the steel.

[0055] Additionally, manganese, as an austenite stabilizing element, can induce the formation of needle-like ferrite and bainite when added to steel.

[0056] If manganese content is insufficient, the aforementioned effects may be minimal. Conversely, if manganese content is excessive, austenite may remain, reducing strength and toughness.

[0057] Accordingly, the high-performance steel bar according to one embodiment of the present invention may contain manganese (Mn) in an amount of 0.5 to 2.0 wt%, and preferably 0.51 to 1.76 wt%.

[0058]

[0059] Person (P)

[0060] Phosphorus (P) is an element that contributes to the enhancement of strength. Phosphorus is a grain boundary segregation element, and when added to steel, grain boundaries become embrittled, which can improve the machinability of the steel.

[0061] However, if it is included excessively, it can deteriorate the ductility of the steel, causing surface cracks during the manufacturing process and reducing impact toughness. In particular, if the phosphorus content exceeds 0.03 wt%, it can combine with the iron in the base material to form Fe3P inclusions, which can reduce strength.

[0062] Therefore, the high-performance steel according to one embodiment of the present invention may contain phosphorus (P) in an amount of 0.03 wt% or less (excluding 0), and preferably 0.028 wt% or less.

[0063]

[0064] Yellow (S)

[0065] Sulfur (S) can combine with manganese to form inclusions such as MnS. While these MnS inclusions can improve workability, they tend to elongate during rolling, and excessive sulfur content can reduce the ductility and impact toughness of the steel.

[0066] Therefore, the high-performance steel according to one embodiment of the present invention may contain sulfur (S) in an amount of 0.05 wt% or less (excluding 0), and preferably 0.035 wt% or less.

[0067]

[0068] Aluminum (Al)

[0069] 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).

[0070] 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, and coarse aluminum precipitates may cause manufacturing problems, such as nozzle clogging.

[0071] Therefore, the high-performance steel bar according to one embodiment of the present invention may contain aluminum (Al) in an amount of 0.010 wt% or less (excluding 0), and preferably 0.002 to 0.008 wt%.

[0072]

[0073] chromium (Cr)

[0074] 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 its hardenability. It also forms a Cr2O3 film on the steel's surface, enhancing its corrosion resistance.

[0075] However, if the chromium content is excessive, coarse carbides may form at grain boundaries, which may reduce the room temperature and low temperature ductility of the steel.

[0076] Therefore, the high-performance steel bar according to one embodiment of the present invention may contain chromium (Cr) in an amount of 0.5 wt% or less (excluding 0), and preferably 0.05 to 0.36 wt%.

[0077]

[0078] copper (Cu)

[0079] Copper (Cu) typically exists in steel at approximately 0.1 to 0.3 wt%. Copper dissolves in ferrite at up to 0.35 wt% at room temperature, exhibiting a solid-solution strengthening effect that can improve strength and hardness. It also enhances corrosion resistance in air and seawater.

[0080] However, if the copper content is excessive, the toughness and elongation may be reduced and it may cause red-hot embrittlement.

[0081] Accordingly, the high-performance steel bar according to one embodiment of the present invention may contain copper (Cu) in an amount of 0.3 wt% or less (excluding 0), and preferably 0.06 to 0.29 wt%.

[0082]

[0083] Nickel (Ni)

[0084] 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.

[0085] 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.

[0086] Therefore, the high-performance steel bar according to one embodiment of the present invention may contain 0.1 to 1.0 wt% of nickel (Ni), and preferably 0.13 to 0.65 wt%.

[0087]

[0088] molybdenum (Mo)

[0089] Molybdenum (Mo) can enhance hardenability by up to 10 times that of nickel (Ni), even with small additions. It also prevents tempering embrittlement, providing resistance to tempering. Because it forms carbides, it is also effective as an alloying element in advanced cutting tools and increases the grain coarsening temperature. Its use in conjunction with chromium to enhance hardenability is even more effective than its use alone.

[0090] 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.

[0091] Therefore, the high-performance steel bar according to one embodiment of the present invention may contain molybdenum (Mo) in an amount of 0.1 wt% or less (excluding 0), and preferably 0.02 to 0.05 wt%.

[0092]

[0093] Nitrogen (N)

[0094] 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.

[0095] 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.

[0096] Therefore, the high-performance steel bar according to one embodiment of the present invention may contain nitrogen (N) in an amount of 0.0100 wt% or less (excluding 0), and preferably 0.0052 to 0.0090 wt%.

[0097]

[0098] 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.

[0099] 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).

[0100] A high-performance steel bar according to one embodiment of the present invention may have a room temperature tensile strength (TS) of 580 MPa or more. Here, room temperature means 20°C, and the room temperature tensile strength (TS) means the tensile strength measured on a specimen at 20°C.

[0101] High-performance steel bars according to one embodiment of the present invention have low-temperature tensile strength (TS) -170℃ ) can be 820MPa or more. Here, the low-temperature tensile strength (TS -170℃ ) refers to the tensile strength measured in a specimen at -170℃.

[0102] Low temperature tensile strength (TS) -170℃ ) is 820 MPa or higher, it is possible to suppress or prevent fracture of the steel bar even when external force is applied in a low-temperature (-170℃) environment due to low-temperature brittleness.

[0103] A high-performance steel bar according to one embodiment of the present invention may have a room temperature elongation (EL) of 13% or more.

[0104] A high-performance steel bar according to one embodiment of the present invention includes a surface portion and a central portion formed to exclude the surface portion, wherein the surface portion may include a hard tissue and the central portion may include a soft tissue.

[0105] When the surface portion forms a cross-section in a direction perpendicular to the longitudinal direction of the steel bar, the surface portion may have an area fraction of 25 to 35% with respect to the cross-section. In other words, the surface portion may have an area fraction of 25 to 35% with respect to the cross-section, and the central portion may have an area fraction of 65 to 75% with respect to the cross-section.

[0106] According to one embodiment of the present invention, the hard tissue may refer to a high-hardness tissue that can be formed through cooling or the like. In other words, the hard tissue may be at least one of bainite, tempered martensite, and martensite.

[0107] For example, the surface layer may include an area fraction of 72 to 89% tempered martensite and a remainder of bainite, and preferably may include 74.5 to 88.5% tempered martensite and a remainder of bainite.

[0108] According to one embodiment of the present invention, the soft tissue may refer to a tissue with low hardness that can be formed through air cooling or slow cooling, etc. In other words, the soft tissue may be at least one of ferrite and pearlite.

[0109] For example, the core may comprise an area fraction of 58 to 84% ferrite and a remainder pearlite, preferably 61.1 to 78.8% ferrite and a remainder pearlite.

[0110] A high-performance steel bar according to one embodiment of the present invention may have an ideal critical diameter (DI) value of 0.3 to 2.1.

[0111] In the present invention, the ideal critical diameter (DI) value can be calculated according to ASTM A 255. In other words, the ideal critical diameter (DI) can be calculated through the contents of carbon (C), silicon (Si), manganese (Mn), chromium (Cr), copper (Cu), nickel (Ni), and molybdenum (Mo).

[0112] Referring to ASTM A 255, it can be confirmed that the ideal critical diameter (DI) value is proportional to the carbon (C) content. Accordingly, in the case of a steel bar whose carbon (C) content does not satisfy the range according to one embodiment of the present invention, the ideal critical diameter (DI) value may fall outside the target range.

[0113] For example, if the ideal critical diameter (DI) value is less than 0.3, the hardenability of the steel may be insufficient, resulting in insufficient formation of hard tissue, and thus the surface area fraction may be less than 25%.

[0114] On the other hand, if the ideal critical diameter (DI) value is 2.1 or more, the hardenability of the steel may be excessive, resulting in excessive formation of hard tissue, and thus the surface area fraction may exceed 35%.

[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] High-performance steel bar manufacturing method

[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] FIG. 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 step (S2) of reheating the steel, the steel may be reheated at a temperature range of 950 to 1100°C for 1 to 3 hours. The above-mentioned temperature range refers to the surface temperature of the steel.

[0127] If the reheating temperature is below 950℃, rolling loads may increase. Conversely, if the reheating temperature exceeds 1100℃, austenite grains may coarsen or decarburization may occur, reducing strength. Furthermore, increased heating costs can lead to increased manufacturing costs and reduced productivity.

[0128] Furthermore, if the reheating time is less than one hour, the re-dissolution of formed carbides may be insufficient. If the reheating time exceeds three hours, austenite grains may coarsen or decarburization may occur, reducing strength. Furthermore, the increased heating cost can lead to increased manufacturing costs and reduced productivity.

[0129] Therefore, the reheating temperature can be 950 to 1100°C, and the reheating time can be 1 to 3 hours.

[0130] 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. That is, the step (S3) of forming the hot-rolled steel may perform a hot-rolling process on the reheated steel.

[0131] 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.

[0132] In the step (S3) of forming a hot-rolled steel, the rolling ratio of the steel may be 3 or greater. Here, the rolling ratio may be the ratio of the cross-sectional area of ​​the steel before step (S3) to the cross-sectional area of ​​the hot-rolled steel after step (S3). If the rolling ratio is less than 3, it may be difficult to secure a uniform and fine structure, which may result in significant deviations in strength and impact toughness.

[0133] In the step (S3) of forming a hot-rolled product, the finishing rolling temperature may be 900 to 1050°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 it exceeds 1050°C, coarse pearlite structures may form, which may rapidly reduce strength.

[0134] 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.

[0135] The step of cooling the hot rolled material (S4) can be performed through cooling equipment.

[0136] 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.

[0137] In other words, the step of cooling the hot-rolled material (S4) can rapidly cool the hot-rolled material at a cooling rate of 30 to 400°C / sec through cooling water.

[0138] After the hot rolled material is rapidly cooled, the surface of the hot rolled material is reheated to a temperature range of 500 to 700℃ through the internal residual heat, and can be cooled again to room temperature.

[0139] After the step of cooling the hot rolled steel (S4) is performed, the final product, high-performance steel bar, can be formed.

[0140] 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 tensile strength (TS) and low temperature tensile strength (TS) -170℃ ) and room temperature elongation (EL) values ​​can all be satisfied.

[0141]

[0142] Comparative examples and examples

[0143] 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.

[0144] Table 1 shows the alloy element composition of comparative examples and examples, Table 2 shows the ideal critical diameter (DI), surface and center area fractions, and physical property values ​​of comparative examples and examples, and Table 3 shows the surface and center area fraction values ​​by structure.

[0145] The comparative examples and examples were each manufactured using semi-finished products having the alloy compositions described in Table 1 below, and a reheating step was performed at a temperature of 1050°C for 2 hours, and hot rolling was performed at a rolling end temperature of 970°C with a rolling ratio of 4.8. After hot rolling, cooling was started at a rate of 30°C / sec through accelerated controlled cooling (ACC) and ended at a rate of 400°C / sec, with a cooling end temperature of 550°C. Thereafter, the products were reheated to 600°C using internal residual heat and then air-cooled.

[0146] 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.

[0147] In Table 2, the ideal critical diameter (DI) value was calculated according to ASTM A 255. Specifically, the ideal critical diameter (DI) value was calculated using Equation 1 below. In Equation 1 below, C, Si, Mn, Cr, Cu, Ni, and Mo represent their respective contents (weight %).

[0148] [Formula 1]

[0149] DI = (0.54×C)×(1.00 + 0.7×Si)×MF Mn ×(1.00 + 2.16×Cr)×(1.00 + 0.365×Cu)×(1.00 + 0.363×Ni)×(1.00 + 3.0×Mo)

[0150] Here, MF Mn When Mn is 1.20 wt% or less, it is (3.3333×Mn + 1.00), and when Mn is more than 1.20 wt% but 1.95% or less, it is (5.10×Mn - 1.12).

[0151] In addition, the room temperature tensile strength (TS) and room temperature elongation (EL) of the comparative examples and examples were measured in an environment of 20°C, and the low temperature tensile strength (TS -170℃ ) was measured in an environment of -170℃.

[0152] In Table 2 below, the surface and center represent the area fraction of the surface and the area fraction of the center, respectively, and the sum of the area fractions of the surface and the center is 100%.

[0153] In Table 3 below, tempered martensite and bainite in the surface layer represent the respective microstructure area fractions, and the sum of the area fractions of tempered martensite and bainite structures is 100%. Similarly, ferrite and pearlite in the center represent the respective microstructure area fractions, and the sum of the area fractions of ferrite and pearlite structures is 100%.

[0154] In Table 1 below, the unit of composition of alloy elements is weight%, in Table 2 below, the unit of ideal critical diameter (DI) is inch, the unit of area fraction of surface and center is %, the unit of room temperature elongation (EL) is %, and the unit of room temperature tensile strength (TS) and low temperature tensile strength (TS) -170℃ ) is in MPa.

[0155] Additionally, in Table 3 below, the area fraction units of tempered martensite, bainite, ferrite, and pearlite are %.

[0156] Classification Chemical composition [wt.%] CSiMnPSAlCrCuNiMoN Example 10.080.131.760.0090.0100.0050.060.080.560.020.0067 Example 20.060.161.520.0080.0060.0040.050.060.570.020.0052 Example 30.290.160.510.0280.0350.0020.360.290.130.050.0079 Example 40.170.251.300.0200.0200.0080.150.250.600.030.0089 Comparative example 10.050.161.520.0090.0060.0050.050.080.580.020.0072 Comparative example 20.330.301.350.0200.0180.0020.100.100.650.030.0090

[0157] DivisionDI Surface Center ELTSTS -170℃Example 10.5528.271.824646851 Example 20.3525.374.722591830 Example 31.1132.167.913691880 Example 41.1434.665.415585820 Comparative Example 10.2919.480.628580816 Comparative Example 22.1143.756.311832947

[0158] Surface layer, center, tempered martensite, bainite, ferrite, pearlite, Example 178.421.678.821.2, Example 274.525.577.422.6, Example 388.511.561.138.9, Example 482.717.367.532.5, Comparative example 171.128.984.915.1, Comparative example 289.810.257.342.7

[0159] Referring to Tables 1 to 3 above, Comparative Example 1 is a comparative example in which the carbon (C) content is 0.05 wt%. It can be confirmed that Comparative Example 1 has an insufficient carbon (C) content, and thus does not satisfy the range of 0.06 to 0.29 wt% according to the above-described embodiment of the present invention.

[0160] Accordingly, in the case of Comparative Example 1, it can be confirmed that the ideal critical diameter (DI) value is 0.29, which does not satisfy the range of 0.3 to 2.1 targeted by the present invention, and further, the surface area fraction is 19.4%, which does not satisfy the range of 25 to 35% targeted by the present invention.

[0161] Specifically, it can be confirmed that the area fraction of tempered martensite in the surface layer is 71.1%, which does not satisfy the range of tempered martensite of 72 to 89% targeted by the present invention, and the area fraction of ferrite in the center is 84.9%, which does not satisfy the range of ferrite of 58 to 84% targeted by the present invention.

[0162] As a result, in the case of Comparative Example 1, the carbon (C) content was insufficient, so the ideal critical diameter (DI) value was insufficient, and the formation of hard tissue corresponding to the surface layer was insufficient. Accordingly, the low-temperature tensile strength (TS) -170℃ ) It can be confirmed that the value is 816 MPa, which does not satisfy the range of 820 MPa or more targeted by the present invention.

[0163] Comparative Example 2 is a comparative example in which the carbon (C) content is 0.33 wt%.

[0164] In the case of Comparative Example 2, it can be confirmed that the carbon (C) content is excessive and does not satisfy the range of 0.06 to 0.29 wt% according to the above-described embodiment of the present invention.

[0165] Accordingly, in the case of Comparative Example 2, it can be confirmed that the ideal critical diameter (DI) value is 2.11, which does not satisfy the range of 0.3 to 2.1 targeted by the present invention, and further, the surface area fraction is 43.7%, which does not satisfy the range of 25 to 35% targeted by the present invention.

[0166] Specifically, it can be confirmed that the area fraction of tempered martensite in the surface layer is 89.8%, which does not satisfy the range of tempered martensite of 72 to 89% targeted by the present invention, and the area fraction of ferrite in the center is 57.3%, which does not satisfy the range of ferrite of 58 to 84% targeted by the present invention.

[0167] As a result, in the case of Comparative Example 1, the carbon (C) content was excessive, resulting in an excessive critical diameter (DI) value and excessive formation of hard tissue corresponding to the surface layer. Accordingly, it can be confirmed that the room temperature elongation (EL) value was 11%, which does not satisfy the target range of 13% or more of the present invention.

[0168] On the other hand, in the case of Examples 1 to 4 according to one embodiment of the present invention, it can be confirmed that the alloy composition range satisfies all of the embodiments of the present invention, and the ideal critical diameter (DI) value also satisfies the range targeted by the present invention, which is 0.3 to 2.1. Room temperature tensile strength (TS), low temperature tensile strength (TS -170℃ ) and room temperature elongation (EL) values ​​all satisfy the range targeted in the present invention.

[0169] 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.

[0170] [Explanation of symbols]

[0171] S1: Steel preparation stage

[0172] S2: Steel reheating stage

[0173] S3: Hot-rolled material formation stage

[0174] S4: Hot-rolled sheet cooling stage

Claims

1. Contains 0.06 to 0.32 wt% of carbon (C), 0.1 to 0.5 wt% of silicon (Si), 0.5 to 2.0 wt% of manganese (Mn), 0.03 wt% or less of phosphorus (P) (excluding 0), 0.05 wt% or less of sulfur (S) (excluding 0), 0.010 wt% or less of aluminum (Al) (excluding 0), 0.5 wt% or less of chromium (Cr) (excluding 0), 0.3 wt% or less of copper (Cu) (excluding 0), 0.1 to 1.0 wt% of nickel (Ni), 0.1 wt% or less of molybdenum (Mo) (excluding 0), 0.0100 wt% or less of nitrogen (N) (excluding 0), and the remainder of iron (Fe) and unavoidable impurities. Including a surface portion and a center portion formed to exclude the surface portion, The above surface layer comprises at least one of bainite, tempered martensite and martensite, The above central portion comprises at least one of pearlite and ferrite, The room temperature tensile strength (TS) is 580 MPa or more, High performance steel.

2. In paragraph 1, Low temperature tensile strength (TS) -170℃ ) is 820MPa or higher, High performance steel.

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

4. In paragraph 1, The ideal critical diameter (DI) value is 0.3 to 2.1, High performance steel.

5. In paragraph 1, The above surface portion is formed in a cross section perpendicular to the longitudinal direction, Having an area fraction of 25 to 35%, High performance steel.

6. In paragraph 1, The above surface layer comprises 72 to 89% of tempered martensite in area fraction and the remainder of bainite. High performance steel.

7. In paragraph 1, The above central portion contains 58 to 84% ferrite in area fraction and the remainder pearlite. High performance steel. 8.(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; and (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) 0.06 to 0.32 wt%, silicon (Si) 0.1 to 0.5 wt%, manganese (Mn) 0.5 to 2.0 wt%, phosphorus (P) 0.03 wt% or less (excluding 0), sulfur (S) 0.05 wt% or less (excluding 0), aluminum (Al) 0.010 wt% or less (excluding 0), chromium (Cr) 0.5 wt% or less (excluding 0), copper (Cu) 0.3 wt% or less (excluding 0), nickel (Ni) 0.1 to 1.0 wt%, molybdenum (Mo) 0.1 wt% or less (excluding 0), nitrogen (N) 0.0100 wt% or less (excluding 0), and the remainder iron (Fe) and unavoidable impurities. The above final product comprises a surface portion and a central portion formed to exclude the surface portion, The above surface layer comprises at least one of bainite, tempered martensite and martensite, The above central portion comprises at least one of pearlite and ferrite, The final product above has a room temperature tensile strength (TS) of 580 MPa or more. Method for manufacturing high-performance steel bars.

9. In paragraph 8, The reheating temperature of the above step (S2) is 950 to 1100℃. Method for manufacturing high-performance steel bars.

10. In paragraph 8, The reheating time of the above step (S2) is 1 to 3 hours. Method for manufacturing high-performance steel bars.

11. In paragraph 8, The rolling end temperature of the above (S3) step is 900 to 1050℃. Method for manufacturing high-performance steel bars.

12. In paragraph 8, The rolling ratio of the above step (S3) is 3 or more, Method for manufacturing high-performance steel bars.

13. In paragraph 8, The cooling rate of the above step (S4) is 30 to 400℃ / sec. Method for manufacturing high-performance steel bars.

14. In paragraph 8, In the above step (S4), the hot rolled material is cooled, then reheated and air-cooled. The above-mentioned reheating temperature is 500 to 700℃. Method for manufacturing high-performance steel bars.

15. In paragraph 8, Low temperature tensile strength (TS) of the above final product -170℃ ) is 820MPa or higher, Method for manufacturing high-performance steel bars.

16. In paragraph 8, The room temperature elongation (EL) of the above final product is 13% or more. Method for manufacturing high-performance steel bars.

17. In paragraph 8, The above surface portion is formed in a cross section perpendicular to the longitudinal direction, Having an area fraction of 25 to 35%, Method for manufacturing high-performance steel bars.

18. In paragraph 8, The above surface layer comprises 72 to 89% of tempered martensite in area fraction and the remainder of bainite. Method for manufacturing high-performance steel bars.

19. In paragraph 8, The above central portion contains 58 to 84% ferrite in area fraction and the remainder pearlite. Method for manufacturing high-performance steel bars.

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