High performance steel bar and method for manufacturing the same

Abstract

A high-performance bar steel according to one embodiment of the present invention comprises 0.05 to 0.15 wt% carbon (C), 0.45 wt% or less silicon (Si) (excluding 0), 1 to 2 wt% manganese (Mn), 0.04 wt% or less phosphorus (P) (excluding 0), 0.04 wt% or less sulfur (S) (excluding 0), 0.3 wt% or less copper (Cu) (excluding 0), 0.3 to 0.8 wt% nickel (Ni), 0.25 wt% or less chromium (Cr) (excluding 0), 0.001 to 0.08 wt% molybdenum (Mo), 0.06 wt% or less aluminum (Al) (excluding 0), 0.001 to 0.06 wt% vanadium (V), 0.012 wt% or less nitrogen (N) (excluding 0), and the remainder being iron (Fe) and other unavoidable impurities, and has an abnormal critical diameter (DI) value of 0.29 to It is 1.19 inches, has a room temperature yield strength (YS) of 520 MPa or more, and a room temperature elongation (EL) of 9% or more.

Description

Technology Field

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

[0002] Deformed steel bars are long, slender steel materials used for concrete reinforcement, characterized by their strong adhesion to concrete. In addition, because deformed steel bars have high resistance to tensile force, they combine with concrete—which is excellent at resisting compressive force but weak in tensile strength—to form reinforced concrete structures.

[0003] In particular, reinforced concrete structures are widely used in construction and civil engineering sites because deformed steel bars have the advantage of reducing the width of cracks when they occur in concrete. For example, reinforced concrete structures are used as pivotal materials in construction projects such as bridges, massive offshore structures, underground structures, and storage facilities.

[0004] Recently, interest in natural gas has been increasing as part of environmental concerns and the shift in domestic energy policy. Natural gas is liquefied at temperatures below -170°C and transported in the form of Liquefied Natural Gas (LNG), which is then stored in LNG tanks. LNG tanks designed to store LNG liquefied at temperatures below -170°C require a special structure and materials capable of withstanding temperatures around that level.

[0005] The structure of an LNG tank is largely composed of an inner tank and an outer tank. The inner tank, which comes into contact with LNG, is made of 9% Ni steel plates capable of withstanding an environment of -170°C, while the outer tank is made of reinforced concrete. In the case of the deformed steel bars used in LNG storage tanks, reinforcing bars capable of withstanding -170°C are required to maintain the structure without brittle fracture occurring even with a rapid drop in temperature.

[0006] Therefore, there is a need to develop high-performance steel bars that possess excellent strength even in environments with temperatures of -170℃ or lower and do not undergo brittle fracture, as well as a method for manufacturing the same. The problem to be solved

[0007] According to one embodiment of the present invention, the objective is to provide a high-performance steel bar and a method for manufacturing steel bars that satisfy a room temperature yield strength of 520 MPa or more at 20°C and a uniform elongation of 7% or more at -170°C.

[0008] The problems of the present invention are not limited to those mentioned above, and other unmentioned problems will be clearly understood by those skilled in the art from the description below. means of solving the problem

[0009] A high-performance bar steel according to one embodiment of the present invention comprises 0.05 to 0.15 wt% carbon (C), 0.45 wt% or less silicon (Si) (excluding 0), 1 to 2 wt% manganese (Mn), 0.04 wt% or less phosphorus (P) (excluding 0), 0.04 wt% or less sulfur (S) (excluding 0), 0.3 wt% or less copper (Cu) (excluding 0), 0.3 to 0.8 wt% nickel (Ni), 0.25 wt% or less chromium (Cr) (excluding 0), 0.001 to 0.08 wt% molybdenum (Mo), 0.06 wt% or less aluminum (Al) (excluding 0), 0.001 to 0.06 wt% vanadium (V), 0.012 wt% or less nitrogen (N) (excluding 0), and the remainder being iron (Fe) and other unavoidable impurities, and has an abnormal critical diameter (DI) value of 0.29 to It is 1.19 inches, has a room temperature yield strength (YS) of 520 MPa or more, and a room temperature elongation (EL) of 9% or more.

[0010] According to one embodiment of the present invention, the room temperature tensile strength (TS) may be 580 MPa or higher.

[0011] According to one embodiment of the present invention, the room temperature tensile strength (TS) may be 1.11 times or more of the room temperature yield strength (YS).

[0012] According to one embodiment of the present invention, the yield strength (YS) measured at -170℃ -170℃ ) can be 800MPa or higher.

[0013] According to one embodiment of the present invention, the tensile strength (TS) measured in a -170℃ notched specimen -170℃ ) can be 830MPa or higher.

[0014] According to one embodiment of the present invention, the notch sensitivity (NSR) may be 1 or more.

[0015] According to one embodiment of the present invention, the uniform elongation (UE) measured at -170℃ may be 7% or more.

[0016] A method for manufacturing a high-performance bar steel according to an embodiment of the present invention comprises the steps of preparing a steel material (S1), reheating the steel material (S2), hot rolling the steel material to form a hot-rolled material (S3), and cooling and reheating the hot-rolled material to form a structural steel shape, which is a final product (S4), wherein the final product contains 0.05 to 0.15 wt% carbon (C), 0.45 wt% or less silicon (Si) (excluding 0), 1 to 2 wt% manganese (Mn), 0.04 wt% or less phosphorus (P) (excluding 0), 0.04 wt% or less sulfur (S) (excluding 0), 0.3 wt% or less copper (Cu) (excluding 0), 0.3 to 0.8 wt% nickel (Ni), 0.25 wt% or less chromium (Cr) (excluding 0), 0.001 to 0.08 wt% molybdenum (Mo), and 0.06 wt% aluminum (Al). It contains less than or equal to weight% (excluding 0), 0.001 to 0.06 weight% of vanadium (V), less than or equal to 0.012 weight% of nitrogen (N) (excluding 0), and the remainder being iron (Fe) and other unavoidable impurities, and has an abnormal critical diameter (DI) value of 0.29 to 1.19 inches, and as a result of measuring the physical properties of the final product, the room temperature yield strength (YS) is 520 MPa or higher and the room temperature elongation (EL) is 9% or higher.

[0017] According to one embodiment of the present invention, the reheating temperature of step (S2) may be 900 to 1200°C.

[0018] According to one embodiment of the present invention, the rolling end temperature of step (S3) may be 800 to 1000℃.

[0019] According to one embodiment of the present invention, the cooling rate of step (S4) may be 20 to 400℃ / sec.

[0020] According to one embodiment of the present invention, the heat temperature of step (S4) may be 500 to 600°C.

[0021] According to one embodiment of the present invention, the room temperature tensile strength (TS) of the final product may be 580 MPa or higher.

[0022] According to one embodiment of the present invention, the yield strength (YS) measured at a -170℃ unnotched specimen of the final product -170℃ ) is 800 MPa or higher, and tensile strength (TS) measured on a -170℃ notched specimen -170℃ ) can be 830MPa or higher.

[0023] According to one embodiment of the present invention, the notch sensitivity (NSR) of the final product may be 1 or more.

[0024] According to one embodiment of the present invention, the uniform elongation (UE) of the final product measured at -170°C may be 7% or more. Effects of the invention

[0025] It is possible to provide a high-performance steel bar and a method for manufacturing steel bar that not only secures physical properties in a room temperature environment, but also satisfies a room temperature yield strength of 520 MPa or more at 20°C and a uniform elongation of 7% or more at -170°C.

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

[0027] FIG. 1 is a flowchart illustrating a method for manufacturing a high-performance steel bar according to one embodiment of the present invention. Specific details for implementing the invention

[0028] Hereinafter, preferred embodiments of the present invention are 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.

[0029] Additionally, where it is stated that a component (or area, layer, part, etc.) is "on," "connected," or "combined" with another component, it means that it may be directly placed / connected / combined with the other component, or that a third component may be placed between them.

[0030] Terms such as "include" or "have" are intended to indicate the existence of the features, numbers, steps, actions, components, parts, or combinations thereof described in the specification, and should be understood as not precluding the existence or addition of one or more other features, numbers, steps, actions, components, parts, or combinations thereof.

[0031] In order to clearly explain the present invention, detailed descriptions of related prior art that are irrelevant to the explanation or that may unnecessarily obscure the essence of the invention have been omitted. Furthermore, when assigning reference numerals to the components of each drawing in this specification, identical or similar reference numerals are assigned to identical or similar components throughout the entire specification.

[0032] Furthermore, terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted in a meaning and concept consistent with the technical spirit of the invention, based on the principle that the inventor can appropriately define the concept of the terms to best describe his invention.

[0033] Unless otherwise specified, the notation 'A ~ B' for numerical values ​​A and B shall mean 'A or greater, B or less'. In such notation, if a unit is attached only to numerical value B, that unit shall also apply to numerical value A.

[0034] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0036] High-performance steel bars

[0037] A high-performance bar steel according to one embodiment of the present invention is manufactured from a steel material comprising 0.05 to 0.15 wt% carbon (C), 0.45 wt% or less silicon (Si) (excluding 0), 1 to 2 wt% manganese (Mn), 0.04 wt% or less phosphorus (P) (excluding 0), 0.04 wt% or less sulfur (S) (excluding 0), 0.3 wt% or less copper (Cu) (excluding 0), 0.3 to 0.8 wt% nickel (Ni), 0.25 wt% or less chromium (Cr) (excluding 0), 0.001 to 0.08 wt% molybdenum (Mo), 0.06 wt% or less aluminum (Al) (excluding 0), 0.001 to 0.06 wt% vanadium (V), 0.012 wt% or less nitrogen (N) (excluding 0), and the remainder being iron (Fe) and other unavoidable impurities, wherein the same alloy composition is present in the bar steel which is the final product. It can be included.

[0038] Hereinafter, the roles and contents of each alloy element included in the high-performance steel bar according to one embodiment of the present invention will be described in detail.

[0040] Carbon (C)

[0041] Carbon (C) is the most effective and important element for increasing the strength of steel.

[0042] Carbon is dissolved in austenite, stabilizing the austenite and increasing yield strength and tensile strength through solid solution strengthening.

[0043] In addition, the increase in tensile strength improved by carbon is greater than the increase in yield strength, so plastic deformation ability can be improved.

[0044] If the carbon content is less than 0.05 weight%, the aforementioned effect is insufficient, making it difficult to secure sufficient strength. Conversely, if the carbon content exceeds 0.15 weight%, it may cause deformation during quenching or a decrease in the elongation and low-temperature toughness of the steel.

[0045] Accordingly, a high-performance steel bar according to one embodiment of the present invention may contain 0.05 to 0.15 weight% of carbon (C), and preferably 0.05 to 0.10 weight%.

[0047] Silicon (Si)

[0048] Silicon (Si) is added along with aluminum as a deoxidizer to remove oxygen from steel during the steelmaking process, and can reduce oxide inclusions contained in the steel.

[0049] In addition, silicon can improve the strength and hardenability of steel by inducing ferrite formation as a ferrite stabilizing element with a solid solution strengthening effect.

[0050] However, if the silicon content exceeds 0.45 weight%, excessive solid solution in the ferrite phase may reduce ductility.

[0051] Accordingly, a high-performance steel bar according to one embodiment of the present invention may contain silicon (Si) in an amount of 0.45 weight% or less (excluding 0), and preferably in an amount of 0.17 to 0.24 weight%.

[0053] Manganese (Mn)

[0054] Manganese (Mn) is an element effective for deoxidizing steel and improving toughness.

[0055] Manganese is an austenite-stabilizing element, and its addition can improve strength by inducing the formation of acicular ferrite and bainite.

[0056] In addition, manganese combines with sulfur (S) in steel to form fine sulfides (MnS) between the grains, which can improve the workability of the steel.

[0057] If the manganese content is less than 1 weight%, the aforementioned effect may be insufficient. Conversely, if the manganese content exceeds 2 weight%, the austenite structure may remain, which may actually lower the strength and toughness.

[0058] Accordingly, a high-performance steel bar according to one embodiment of the present invention may contain 1 to 2 weight percent of manganese (Mn), and preferably 1.50 to 1.92 weight percent.

[0060] Ph(P)

[0061] Phosphorus (P) is an element that contributes to strength improvement. However, if present in excessive amounts, it may segregate at grain boundaries, which can reduce impact toughness.

[0062] If the phosphorus content exceeds 0.04 weight%, not only is the toughness of the steel reduced due to segregation, but cracks may also occur on the surface during the manufacturing process, and the strength may be reduced due to the formation of Fe3P compounds within the steel.

[0063] Accordingly, a high-performance steel bar according to one embodiment of the present invention may contain phosphorus (P) in an amount of 0.04 weight% or less (excluding 0), and preferably in an amount of 0.007 to 0.012 weight%.

[0065] Yellow (S)

[0066] Sulfur (S) combines with manganese to form fine precipitates such as MnS, which can improve processability.

[0067] However, MnS precipitates have the property of elongating during rolling, which reduces the impact anisotropy of steel and can cause cracks during heat treatment.

[0068] If the sulfur content exceeds 0.04 weight%, MnS is excessively formed, which reduces the hot workability of the steel and may cause tearing of the steel.

[0069] Accordingly, a high-performance steel bar according to one embodiment of the present invention may contain sulfur (S) in an amount of 0.04 weight% or less (excluding 0), and preferably in an amount of 0.004 to 0.017 weight%.

[0071] Copper (Cu)

[0072] Copper (Cu) exhibits a solid solution strengthening effect, so strength, hardness, and corrosion resistance can be improved.

[0073] However, if the copper content exceeds 0.3 weight%, the toughness may be significantly reduced and may cause red-hot brittleness, which is a type of brittleness that occurs during hot working.

[0074] Accordingly, a high-performance steel bar according to one embodiment of the present invention may contain copper (Cu) in an amount of 0.3 weight% or less (excluding 0), and preferably in an amount of 0.08 to 0.18 weight%.

[0076] Nickel (Ni)

[0077] Nickel is an important and common alloying element that improves low-temperature properties. Nickel (Ni) refines the microstructure of steel and dissolves well in austenite and ferrite, making it useful for solid solution strengthening.

[0078] In addition, when the steel contains nickel, it is possible to improve the low-temperature toughness of the steel while preventing a decrease in weldability and workability.

[0079] If the nickel content is less than 0.3 weight%, the aforementioned effect may be insufficient. Conversely, if the nickel content exceeds 0.8 weight%, the elongation may decrease and the manufacturing cost of the steel may increase.

[0080] Accordingly, a high-performance steel bar according to one embodiment of the present invention may contain nickel (Ni) in an amount of 0.3 to 0.8 weight%, and preferably in an amount of 0.57 to 0.60 weight%.

[0082] Chrome (Cr)

[0083] Chromium (Cr) is a ferrite-stabilizing element; when added to C-Mn steel, it delays carbon diffusion through a solute interference effect, thereby affecting grain size refinement.

[0084] In addition, chromium can improve the corrosion resistance of steel and improve its hardenability, thereby improving quenchability. However, if the chromium content exceeds 0.25 weight%, coarse carbides may form at the grain boundaries, which can reduce the room temperature and low temperature ductility of the steel.

[0085] Accordingly, a high-performance steel bar according to one embodiment of the present invention may contain chromium (Cr) in an amount of 0.25 weight% or less (excluding 0), and preferably in an amount of 0.05 to 0.10 weight%.

[0087] Molybdenum (Mo)

[0088] Molybdenum (Mo) can be dissolved into grain boundaries to promote the formation of grain nuclei and inhibit grain growth. Consequently, the grains are refined, which can improve strength and toughness.

[0089] In addition, molybdenum can increase strength and hardness by forming complex carbides and nitrides together with niobium (Nb), vanadium (V), etc., thereby hindering slip in the matrix and suppressing grain movement.

[0090] If the molybdenum content is less than 0.001 weight%, the aforementioned effects may be insufficient. Conversely, if the molybdenum content exceeds 0.08 weight%, the manufacturing cost of the steel may increase, and excessive low-temperature microstructure may be formed, which may reduce toughness, ductility, and weldability.

[0091] Accordingly, a high-performance steel bar according to one embodiment of the present invention may contain molybdenum (Mo) in an amount of 0.001 to 0.08 weight%, and preferably in an amount of 0.01 to 0.02 weight%.

[0093] Aluminum (Al)

[0094] Aluminum (Al) is added as a deoxidizer in the steelmaking process to remove oxygen from the steel, and contributes to improving elongation by cleaning ferrite and stabilizing austenite by increasing the carbon concentration within the austenite.

[0095] However, if the aluminum content exceeds 0.06 weight%, brittleness increases and non-metallic inclusions are excessively formed, which can cause process problems such as nozzle clogging.

[0096] Accordingly, a structural steel according to one embodiment of the present invention may contain aluminum (Al) in an amount of 0.06 weight% or less (excluding 0), and preferably in an amount of 0.002 to 0.011 weight%.

[0098] Vanadium (V)

[0099] Vanadium (V) can inhibit grain growth as a precipitate-forming element. As a result, the grains are refined, and strength and hardness can be improved.

[0100] If the vanadium content is less than 0.001 weight%, the aforementioned effect may be insufficient. Conversely, if the vanadium content exceeds 0.06 weight%, the manufacturing cost of the steel increases, and oxides (e.g., V2O5) may evaporate at high temperatures.

[0101] Accordingly, a high-performance steel bar according to one embodiment of the present invention may contain vanadium (V) in an amount of 0.001 to 0.06 weight%, and preferably in an amount of 0.002 to 0.055 weight%.

[0103] Nitrogen (N)

[0104] Even in extremely small amounts, nitrogen (N) can have a significant effect on the mechanical properties of steel.

[0105] Nitrogen can combine with molybdenum (Mo), vanadium (V), etc. to form precipitates, which can improve the strength of the final product.

[0106] However, if the nitrogen content exceeds 0.012 weight%, the toughness of the steel may decrease.

[0107] Accordingly, a high-performance steel bar according to one embodiment of the present invention may contain nitrogen (N) in an amount of 0.012 weight% or less (excluding 0), and preferably in an amount of 0.0045 to 0.0082 weight%.

[0109] According to one embodiment of the present invention, in order to improve low-temperature yield strength and uniform elongation at -170℃, the content of nickel (Ni), an element related to low-temperature properties, and the content of chromium (Cr) and molybdenum (Mo), elements for grain refinement, may be additionally limited.

[0110] Nickel (Ni), chromium (Cr), and molybdenum (Mo) are elements with excellent grain refinement properties that affect low-temperature properties; they can improve not only the strength and ductility of steel at room temperature but also the yield strength and elongation at cryogenic temperatures.

[0111] Accordingly, the sum of the nickel (Ni), chromium (Cr), and molybdenum (Mo) content can be limited in the present invention.

[0112] Specifically, a high-performance steel bar according to one embodiment of the present invention can satisfy the following Equation 1.

[0113] [Equation 1]

[0114] [Ni]+[Cr]+[Mo] ≥ 0.65

[0115] (Here, [Ni], [Cr], and [Mo] are the values ​​when the content of each component is expressed in weight percent.)

[0116] If the value of [Ni]+[Cr]+[Mo] is less than 0.65, the improvement in low-temperature properties due to the above elements is insufficient, and at least one of the low-temperature yield strength and uniform elongation of the steel may be inferior. Accordingly, in the present invention, the value of [Ni]+[Cr]+[Mo] can be limited to 0.65 or higher.

[0118] In addition to the steel components described above, the remainder may contain Fe and unavoidable impurities. Unavoidable impurities are those introduced during the steelmaking stage and the manufacturing process of steel bars; as this is widely known in the field, a detailed explanation is omitted.

[0119] 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 scope that does not impair the technical spirit of the present invention. If additional elements are included, they may be included to replace the remainder of iron (Fe).

[0120] A high-performance bar according to one embodiment of the present invention may have an ideal diameter (DI) value of 0.29 to 1.19 inches. Here, the ideal diameter (DI) refers to the diameter of a steel material in which the area fraction of the martensite structure at the center of the round bar is 50% when the steel material in the shape of a round bar is quenched to an ideal quenching strength (H=∞) according to the Grossman test.

[0121] For example, in the case of a steel bar with a diameter smaller than the theoretically calculated ideal critical diameter (DI), the area fraction of the central martensite structure is greater than 50%, and in the case of a steel bar with a diameter larger than the ideal critical diameter (DI), the area fraction of the central martensite structure is less than 50%.

[0122] The Difference Intensity (DI) can be used as an indicator of hardenability. In other words, the larger the DI value, the better the hardenability, which can lead to increased formation of low-temperature microstructures such as martensite. Consequently, the surface of the steel becomes more hardened, and the strength of the final product, high-performance steel bars, can be enhanced.

[0123] However, if the critical diameter (DI) value is excessively large, the hardenability increases excessively, which can cause the core of the steel to become over-hardened. Consequently, the ductility and toughness of the final high-performance bar may be inferior.

[0124] In other words, the greater the hardenability of the surface layer, the better the strength of the steel, and if the hardenability of the core is excessive, the ductility and toughness become inferior. Based on these characteristics, the present invention can limit the upper and lower limits of the ideal critical diameter (DI) value of the steel bar to ensure both strength and ductility of the high-performance steel bar.

[0125] As a result, the high-performance steel bar according to one embodiment of the present invention can have excellent strength and ductility by limiting the ideal critical diameter (DI) value and controlling the hardness of the surface layer and the center layer to appropriate levels, respectively.

[0126] Specifically, if the critical diameter (DI) value of the high-performance bar is less than 0.29 inches, the hardening of the surface layer is insufficient, so the increase in hardness and strength may be negligible. Conversely, if the critical diameter (DI) value exceeds 1.19 inches, the hardening of the core is excessive, so the elongation of the high-performance bar, which is the final product, may decrease.

[0127] Accordingly, the high-performance steel bar according to one embodiment of the present invention may have an ideal critical diameter (DI) of 0.29 to 1.19 inches.

[0128] In the present invention, the ideal critical diameter (DI) value was calculated using the following Equation 2 with reference to ASTM A255.

[0129] [Equation 2]

[0130] (0.171 + 0.001×[C] + 0.265×[C] 2)×(1.000 + 3.333×[Mn])×(1.000 + 0.700×[Si])×(1.000 + 0.363×[Ni])×(1.000 + 2.160×[Cr])×(1.000 + 3.000×[Mo])×(1.000 + 0.365×[Cu])×(1.000 + 1.730×[V])

[0131] (Note: [C], [Mn], [Si], [Ni], [Cr], [Mo], [Cu], and [V] are values ​​when expressed as weight percent for each component.)

[0132] A high-performance steel bar according to one embodiment of the present invention may have a room temperature yield strength (YS) of 520 MPa or more, preferably 526 MPa or more. Here, room temperature refers to 20°C, and the room temperature yield strength was measured using an unnotched specimen.

[0133] 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 higher, preferably 587 MPa or higher. Here, the room temperature tensile strength was measured using an un-notch specimen.

[0134] In addition, the room temperature tensile strength (TS) may be 1.11 times or more of the room temperature yield strength (YS) mentioned above.

[0135] A high-performance steel bar according to one embodiment of the present invention may have a room temperature elongation (EL) of 9% or more, preferably 12.00% or more. Here, the room temperature elongation was measured using an unnotched specimen.

[0136] In addition, the high-performance steel bar according to one embodiment of the present invention has a low-temperature yield strength (YS -170℃ ) can be 800 MPa or higher. Here, low-temperature yield strength (YS -170℃ ) refers to the yield strength measured in an unnotched specimen at -170℃.

[0137] Low temperature yield strength (YS -170℃If ) is 800 MPa or higher, 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.

[0138] 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 higher, and preferably 810 MPa or higher.

[0139] A high-performance steel bar according to one embodiment of the present invention has low-temperature tensile strength (TS -170℃ ) may be 830 MPa or higher. Here, low-temperature tensile strength (TS -170℃ ) refers to the tensile strength measured in a v-notch specimen at -170℃. For the v-notch machining, the internal angle was 45°, the bottom radius was 0.25 mm, and the depth was machined to 1 mm along the rib bar direction.

[0140] Low temperature tensile strength (TS) -170℃ If ) is 830 MPa or higher, it is possible to suppress or prevent fracture of the steel bar even when an external force is applied in a low-temperature (-170℃) environment due to low-temperature brittleness.

[0141] Accordingly, the high-performance steel bar according to one embodiment of the present invention has low-temperature tensile strength (TS -170℃ ) may be 830 MPa or higher, and preferably 833 MPa or higher.

[0142] In addition, the high-performance steel bar according to one embodiment of the present invention has a low-temperature yield strength (YS -170℃ Low temperature tensile strength (TS) for ) -170℃ The non-notch sensitivity (NSR) value of ) may be 1 or greater.

[0143] If the notch sensitivity (NSR) is 1 or greater, it is possible to suppress or prevent fracture of the steel bar even when an external force is applied in a low-temperature (-170℃) environment due to low-temperature brittleness.

[0144] A high-performance steel bar according to one embodiment of the present invention may have a uniform elongation (UE) of 7% or more. Here, the uniform elongation (UE) refers to the elongation measured in an unnotched specimen at -170°C.

[0145] If the uniform elongation (UE) is 7% or more, it is possible to suppress or prevent fracture of the steel bar even when an external force is applied in a low-temperature (-170℃) environment due to low-temperature brittleness.

[0146] Accordingly, the high-performance steel bar according to one embodiment of the present invention may have a uniform elongation (UE) of 7% or more, and preferably 7.5% or more.

[0147] Hereinafter, a method for manufacturing a high-performance steel bar according to one embodiment of the present invention will be described in detail.

[0149] High-performance steel bar manufacturing method

[0150] Hereinafter, a method for manufacturing a high-performance steel bar according to an embodiment of the present invention will be described with reference to FIG. 1.

[0151] FIG. 1 is a flowchart illustrating a method for manufacturing a high-performance steel bar according to one embodiment of the present invention.

[0152] A method for manufacturing a high-performance steel bar according to one embodiment of the present invention comprises the steps of preparing a steel material that is a semi-finished product (S1), reheating the steel material (S2), hot rolling the steel material to form a hot-rolled material (S3), and cooling and reheating the formed hot-rolled material (S4).

[0153] Hereinafter, each step of the method for manufacturing high-performance steel bars according to one embodiment of the present invention will be described in detail.

[0154] According to one embodiment of the present invention, the step (S1) of preparing a steel material that is a semi-finished product is a step of preparing a steel material having the alloy composition range described above in order to manufacture a high-performance steel bar that is a final product.

[0155] Specifically, the step of preparing steel (S1) may be a step of manufacturing a semi-finished product by designing the alloy composition within the aforementioned alloy composition range. The semi-finished product may be a billet or a bloom, but is not limited thereto. Furthermore, the manufacturing of the semi-finished product may be carried out by processes known in the relevant technical field, such as a steelmaking process or a continuous casting process.

[0156] According to one embodiment of the present invention, a step of reheating the steel (S2) may be performed after a step of preparing the steel (S1).

[0157] The step of reheating the steel (S2) is a pretreatment process for the rolling step, and may be a step of charging the steel into a heating furnace and heating the steel uniformly to facilitate plastic deformation, thereby allowing segregated components to be re-dissolved during the casting of the steel.

[0158] In the step (S2) of reheating the steel, the steel may be reheated to 900 to 1200°C. If the reheating temperature is below 900°C, the rolling load may increase. Conversely, if the reheating temperature exceeds 1200°C, the austenite grains may coarsen or decarburization may occur, which may impair strength. In addition, increased heating costs and time may lead to increased manufacturing costs and reduced productivity.

[0159] Therefore, in the present invention, steel can be reheated at a temperature of 900 to 1200°C.

[0160] According to one embodiment of the present invention, after the step (S2) of reheating the steel, the step (S3) of hot rolling the steel to form a hot-rolled material may be performed.

[0161] The step (S3) of forming the hot-rolled material can be performed by hot-rolling the reheated steel material to form the hot-rolled material.

[0162] The above hot rolling may include rough rolling and finishing rolling processes. Here, the rough rolling process may be the process of making the steel into a rolled material having a suitable shape, thickness, and width, and the finishing rolling process may be the process of adjusting the steel to a predetermined size and rolling it at a finishing temperature suitable for the application to obtain a good surface and shape.

[0163] In the step (S3) of forming the hot-rolled material, the finishing rolling temperature may be 800 to 1000°C. If the finishing rolling temperature is less than 800°C, the rolling load increases and productivity may decrease. Conversely, if it exceeds 1000°C, the pearlite structure is formed coarsely and the strength may decrease rapidly.

[0164] According to one embodiment of the present invention, after performing the step (S3) of forming a hot-rolled material, the step (S4) of cooling and reheating the formed hot-rolled material may be performed.

[0165] The step (S4) of cooling and reheating the hot-rolled material can be performed by cooling through a cooling facility. Specifically, the hot-rolled material can be cooled at a rate of 20 to 400℃ / sec.

[0166] If the cooling rate of the hot-rolled material is less than 20℃ / sec, the ferrite and pearlite microstructures are formed coarsely, which may reduce the strength of the final product, the bar steel. Conversely, if it exceeds 400℃ / sec, low-temperature microstructures such as bainite or martensite are formed excessively, which may reduce the room temperature and low-temperature elongation of the final product, the bar steel.

[0167] The above cooling equipment may be a Temp-core equipment. In other words, the hot-rolled material can be reheated using residual heat after being rapidly cooled with cooling water. Specifically, the hot-rolled material can be reheated to a temperature of 500 to 600°C.

[0168] After the step (S4) of cooling the hot-rolled material is performed, a high-performance steel bar, which is the final product, can be formed.

[0169] The high-performance steel bar manufactured by the method for manufacturing high-performance steel bar according to one embodiment of the present invention has the aforementioned room temperature yield strength (YS), room temperature tensile strength (TS), room temperature elongation (EL), and low temperature yield strength (YS -170℃ ), low-temperature tensile strength (TS -170℃ It can satisfy all values ​​of uniform elongation (UE) and notch sensitivity (NSR).

[0171] Comparative Examples and Examples

[0172] Preferred comparative examples and embodiments are presented below to aid in understanding the present invention. However, the following comparative examples and embodiments are intended only to aid in understanding the present invention, and the present invention is not limited by the following embodiments.

[0173] Table 1 shows the alloy element composition of the comparative example and the example and the abnormal critical diameter (DI) values ​​calculated therefrom, and Table 2 shows the physical properties of the comparative example and the example.

[0174] The comparative example and the embodiment were each manufactured using semi-finished products having the alloy composition listed in Table 1 below, reflecting the rolling conditions of general reinforcing bar products, and the approximate values ​​are as follows. The semi-finished product, sufficiently heated at a temperature of approximately 1050°C, was immediately hot-rolled to give it a reinforcing bar shape after heating, and the temperature of the rolled material after the shape was given was approximately 900°C. In addition, immediate cooling was carried out by water cooling, and after an initial cooling rate of 400°C / sec, the temperature due to reheating was approximately 570°C, and then air cooling was carried out at approximately 20°C / sec in the cooling zone.

[0175] Except for the conditions described above, the manufacturing processes of the comparative examples and embodiments of the present invention were controlled as control variables under the same conditions within the range described in the method for manufacturing high-performance steel bars according to one embodiment of the present invention.

[0176] The room temperature properties of the comparative examples and embodiments were measured at 20°C, and the low temperature properties were measured at -170°C. Specifically, room temperature yield strength (YS), room temperature tensile strength (TS), and room temperature elongation (EL) were measured using unnotched specimens at 20°C, and the low temperature yield strength (YS -170℃ ) and uniform elongation (UE) were measured using unnotched specimens at a temperature of -170℃, and low-temperature tensile strength (TS -170℃ ) was measured through a v-notch specimen at a temperature of -170℃.

[0177] In addition, the aforementioned specimens were subjected to tensile testing based on EN 14620-3, and the measured values ​​are listed in Table 1 below.

[0178] In Table 1 below, the unit of the composition of alloy elements is weight %, and the unit of the ideal critical diameter (DI) is inch. In Table 2 below, room temperature yield strength (YS), room temperature tensile strength (TS), and low temperature yield strength (YS -170℃ ) and low-temperature tensile strength (TS -170℃ The unit of ) is MPa, the ratio of room temperature tensile strength to room temperature yield strength (TS / YS) and notch sensitivity (NSR) are dimensionless units, and the units of room temperature elongation (EL) and uniform elongation (UE) are %.

[0179] division Chemical composition [wt.%] DI C Si Mn P S Cu Ni Cr Mo Al V N Ni+Cr+Mo Comparative Example 1 0.05 0.16 1.52 0.008 0.006 0.06 0.57 0.05 0.02 0.008 0.002 0.0071 0.64 0.28 Comparative Example 2 0.33 0.22 1.33 0.023 0.028 0.25 0.14 0.21 0.06 0.003 0.050 0.0078 0.41 2.49 Example 1 0.06 0.18 1.53 0.007 0.006 0.10 0.57 0.07 0.02 0.007 0.002 0.0073 0.66 0.39 Example 2 0.07 0.19 1.50 0.012 0.017 0.18 0.58 0.10 0.02 0.011 0.002 0.0045 0.70 0.47 Example 3 0.10 0.24 1.92 0.011 0.005 0.09 0.60 0.10 0.02 0.006 0.055 0.0055 0.72 1.19 Example 4 0.05 0.17 1.53 0.008 0.004 0.08 0.59 0.05 0.01 0.002 0.002 0.0082 0.65 0.29

[0181] division Room temperature properties (20 ℃) Low temperature properties (-170 ℃) YS TS TS / YS EL YS -170℃ TS -170℃ (v-notch) NSR UE Comparative Example 1 513 581 1.131 26.00 814 813 0.99 13.9 Comparative Example 2 652 860 1.310 8.80 897 925 1.03 2.5 Example 1 561 624 1.112 19.95 811 833 1.03 12.0 Example 2 557 636 1.141 18.45 835 861 1.03 8.6 Example 3 670 755 1.126 12.00 914 1018 1.04 7.5 Example 4 526 587 1.115 24.00 819 852 1.04 13.1

[0183] Referring to Tables 1 and 2 above, Comparative Example 1 is a comparative example with an abnormal critical diameter (DI) value of 0.28 inches. That is, Comparative Example 1 is a comparative example with insufficient curability.

[0184] Comparative Example 1 did not satisfy the upper critical diameter (DI) value of 0.29 inches or more, which is the range according to one embodiment of the present invention, and it can be confirmed that the room temperature yield strength (YS) and notch sensitivity (NSR) values ​​were 513 MPa and 0.99, respectively.

[0185] In other words, Comparative Example 1 has room temperature yield strength (YS) and low temperature yield strength (YS -170℃ The values ​​of ) did not satisfy the target ranges of 520 MPa or more and 1 or more, respectively, in the present invention.

[0186] In addition, it can be seen that Comparative Example 1 has a value of [Cr] + [Ni] + [Mo], which is [Equation 1], of 0.64, which does not satisfy the range of 0.65 or higher according to one embodiment of the present invention.

[0187] Accordingly, Comparative Example 1 has a low-temperature yield strength (YS -170℃ The value of ) did not satisfy the target range of 520 MPa or higher in the present invention.

[0188] Comparative Example 2 is a comparative example with an abnormal critical diameter (DI) value of 2.49 inches. That is, Comparative Example 2 is a comparative example with excessive hardenability.

[0189] Comparative Example 2 can be seen to have room temperature elongation (EL) and uniform elongation (UE) values ​​of 8.80% and 2.5%, respectively, as the abnormal critical diameter (DI) value does not satisfy the range of 1.19 inches or less according to one embodiment of the present invention.

[0190] In other words, Comparative Example 2 did not satisfy the room temperature elongation (EL) and uniform elongation (UE) values, which are the target ranges of 9% or more and 7% or more, respectively, in the present invention.

[0191] In addition, Comparative Example 2 has a value of [Cr] + [Ni] + [Mo] in [Equation 1] of 0.41, which can be seen as not satisfying the range of 0.65 or higher according to one embodiment of the present invention.

[0192] Accordingly, Comparative Example 2 did not satisfy the uniform elongation (UE) value of 7% or more, which is the target range in the present invention.

[0193] On the other hand, in the case of Examples 1 to 4 according to an embodiment of the present invention, it can be confirmed that the alloy composition range and the ideal critical diameter (DI) value calculated therefrom satisfy all embodiments of the present invention. Through this, not only room temperature properties such as room temperature yield strength (YS), room temperature tensile strength (TS), the ratio of room temperature tensile strength to room temperature yield strength (TS / YS), and room temperature elongation (EL), but also low temperature yield strength (YS -170℃ ), low-temperature tensile strength (TS -170℃ It can be confirmed that low-temperature properties such as notch sensitivity (NSR) and uniform elongation (UE) also satisfy all the target ranges in the present invention.

[0194] As described above, preferred embodiments according to the present invention have been examined. It is obvious to those skilled in the art that, in addition to the embodiments described above, the present invention may be embodied in other specific forms without departing from its spirit or scope. Therefore, the embodiments described above should be regarded as illustrative rather than restrictive, and accordingly, the present invention is not limited to the description above but may be modified within the scope of the appended claims and their equivalents. Explanation of the symbols

[0195] S1: Steel preparation stage S2: Steel reheating step S3: Hot-rolled material formation stage S4: Hot-rolled material cooling stage

Claims

Claim 1 It contains 0.05 to 0.15 wt% carbon (C), 0.45 wt% or less silicon (Si) (excluding 0), 1 to 2 wt% manganese (Mn), 0.04 wt% or less phosphorus (P) (excluding 0), 0.04 wt% or less sulfur (S) (excluding 0), 0.3 wt% or less copper (Cu) (excluding 0), 0.3 to 0.8 wt% nickel (Ni), 0.25 wt% or less chromium (Cr) (excluding 0), 0.001 to 0.08 wt% molybdenum (Mo), 0.06 wt% or less aluminum (Al) (excluding 0), 0.001 to 0.06 wt% vanadium (V), 0.012 wt% or less nitrogen (N) (excluding 0), and the remainder being iron (Fe) and other unavoidable impurities, with an abnormal critical diameter (DI) value of 0.29 to 1.19 High-performance bar steel having an inch length, a room temperature yield strength (YS) of 520 MPa or more, and a room temperature elongation (EL) of 9% or more. Claim 2 In claim 1, a high-performance steel bar having a room temperature tensile strength (TS) of 580 MPa or higher. Claim 3 In paragraph 2, a high-performance steel bar having a room temperature tensile strength (TS) of at least 1.11 times the room temperature yield strength (YS). Claim 4 In claim 1, the yield strength (YS) measured at -170℃ -170℃ High-performance steel bar with a ) of 800 MPa or higher. Claim 5 In claim 1, the tensile strength (TS) measured in a -170℃ notched specimen -170℃ High-performance steel bar with a ) of 830 MPa or higher. Claim 6 In paragraph 5, a high-performance steel bar having a notch sensitivity (NSR) of 1 or greater. Claim 7 A high-performance steel bar according to claim 1, having a uniform elongation (UE) of 7% or more measured at -170℃. Claim 8 (S1) a step of preparing steel; (S2) a step of reheating the steel; (S3) a step of hot-rolling the steel to form a hot-rolled material; and (S4) a step of cooling and reheating the hot-rolled material to form a final product, a structural steel; The above-mentioned final product comprises 0.05 to 0.15 wt% carbon (C), 0.45 wt% or less silicon (Si) (excluding 0), 1 to 2 wt% manganese (Mn), 0.04 wt% or less phosphorus (P) (excluding 0), 0.04 wt% or less sulfur (S) (excluding 0), 0.3 wt% or less copper (Cu) (excluding 0), 0.3 to 0.8 wt% nickel (Ni), 0.25 wt% or less chromium (Cr) (excluding 0), 0.001 to 0.08 wt% molybdenum (Mo), 0.06 wt% or less aluminum (Al) (excluding 0), 0.001 to 0.06 wt% vanadium (V), 0.012 wt% or less nitrogen (N) (excluding 0), and the remainder being iron (Fe) and other unavoidable impurities, with an abnormal critical diameter (DI) value of 0.29 to A method for manufacturing high-performance steel bars, wherein the length is 1.19 inches, and the results of measuring the physical properties of the final product show that the room temperature yield strength (YS) is 520 MPa or higher and the room temperature elongation (EL) is 9% or higher. Claim 9 A method for manufacturing high-performance steel bars according to claim 8, wherein the reheating temperature of step (S2) is 900 to 1200℃. Claim 10 A method for manufacturing high-performance steel bars according to claim 8, wherein the rolling end temperature of step (S3) is 800 to 1000℃. Claim 11 A method for manufacturing high-performance steel bars according to claim 8, wherein the cooling rate of step (S4) is 20 to 400℃ / sec. Claim 12 A method for manufacturing high-performance steel bars according to claim 8, wherein the double heating temperature of step (S4) is 500 to 600℃. Claim 13 A method for manufacturing a high-performance steel bar according to claim 8, wherein the room temperature tensile strength (TS) of the final product is 580 MPa or higher. Claim 14 In claim 8, the yield strength (YS) measured in the -170℃ unnotched specimen of the final product -170℃ ) is 800 MPa or higher, and tensile strength (TS) measured on a notched specimen at -170℃ -170℃ A method for manufacturing high-performance steel bars having a ) of 830 MPa or higher. Claim 15 A method for manufacturing a high-performance steel bar according to claim 14, wherein the notch sensitivity (NSR) of the final product is 1 or greater. Claim 16 A method for manufacturing high-performance steel bars according to claim 8, wherein the uniform elongation (UE) of the final product measured at -170℃ is 7% or more.

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