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

Abstract

A high-performance steel bar, according to one embodiment of the present invention, comprises 0.04-0.09 wt% of carbon (C), 0.6 wt% or less (excluding 0) of silicon (Si), 1.45-2.5 wt % of manganese (Mn), 0.2-2.0 wt% of nickel (Ni), 0.7 wt% or less (excluding 0) of chromium (Cr), 0.001-0.07 wt% of niobium (Nb), 0.003-0.09 wt% of vanadium (V), 0.02 wt% or less (excluding 0) of phosphorus (P), 0.02 wt% or less (excluding 0) of sulfur (S), 0.5 wt% or less (excluding 0) of copper (Cu), 0.002-0.05 wt% of molybdenum (Mo), 0.001-0.07 wt% of aluminum (Al), 0.01 wt% or less (excluding 0) of nitrogen (N), and the balance of iron (Fe) and inevitable impurities, and satisfies formula 1 below, wherein the yield strength (YSun) of an unnotched specimen at -170°C is 600 MPa or more. [Formula 1] 1668.26×C(wt%) + 1157.54×Si(wt%) + 81.40×Mn(wt%) + 94.51×Ni(wt%) + 202.33×Cr(wt%) + 5228.70×Nb(wt%) + 2256.36×V(wt%) ≥ 595.01

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 yield strength and notch sensitivity in a low-temperature environment.

[0007] The tasks of the present invention are not limited to the tasks mentioned above, and other tasks not mentioned will be clearly understood by those skilled in the art from the description below.

[0008] A high-performance steel bar according to one embodiment of the present invention contains carbon (C) 0.04 to 0.09 wt%, silicon (Si) 0.6 wt% or less (excluding 0), manganese (Mn) 1.45 to 2.5 wt%, nickel (Ni) 0.2 to 2.0 wt%, chromium (Cr) 0.7 wt% or less (excluding 0), niobium (Nb) 0.001 to 0.07 wt%, vanadium (V) 0.003 to 0.09 wt%, phosphorus (P) 0.02 wt% or less (excluding 0), sulfur (S) 0.02 wt% or less (excluding 0), copper (Cu) 0.5 wt% or less (excluding 0), molybdenum (Mo) 0.002 to 0.05 wt%, aluminum (Al) 0.001 to 0.07 wt%, and nitrogen (N) 0.01 wt%. Below (excluding 0) and the remaining iron (Fe) and unavoidable impurities, and satisfying the following equation 1, the yield strength (YS) of the unnotched specimen at -170℃ un ) is 600MPa or more.

[0009] [Formula 1]

[0010] 1668.26×C(wt%) + 1157.54×Si(wt%) + 81.40×Mn(wt%) + 94.51×Ni(wt%) + 202.33×Cr(wt%) + 5228.70×Nb(wt%) + 2256.36×V(wt%) ≥ 595.01

[0011] Tensile strength (TS) of unnotched specimens at -170℃ un ) can be 690MPa or more.

[0012] Tensile strength (TS) of unnotched specimens at -170℃ un ) is the yield strength (YS) of the unnotched specimen at -170℃ un ) can be more than 1.15 times.

[0013] Tensile strength (TS) of notched specimens at -170℃ n ) can be 600 MPa or more.

[0014] Tensile strength (TS) of notched specimens at -170℃ n ) is the yield strength (YS) of the unnotched specimen at -170℃ un ) may be more than that.

[0015] A high-performance steel bar according to one embodiment of the present invention contains carbon (C) 0.04 to 0.09 wt%, silicon (Si) 0.6 wt% or less (excluding 0), manganese (Mn) 1.45 to 2.5 wt%, nickel (Ni) 0.2 to 2.0 wt%, chromium (Cr) 0.7 wt% or less (excluding 0), niobium (Nb) 0.001 to 0.07 wt%, vanadium (V) 0.003 to 0.09 wt%, phosphorus (P) 0.02 wt% or less (excluding 0), sulfur (S) 0.02 wt% or less (excluding 0), copper (Cu) 0.5 wt% or less (excluding 0), molybdenum (Mo) 0.002 to 0.05 wt%, aluminum (Al) 0.001 to 0.07 wt%, and nitrogen (N) 0.01 wt%. Below (excluding 0) and the remaining iron (Fe) and inevitable impurities, but including the yield strength (YS) of the unnotched specimen at -170℃ un ) is 600 MPa or more, and the tensile strength (TS) of the notched specimen at -170℃ n ) is 600MPa or more.

[0016] Tensile strength (TS) of notched specimens at -170℃ n ) is the yield strength (YS) of the unnotched specimen at -170℃ un ) may be more than that.

[0017] A high-performance steel bar according to one embodiment of the present invention can satisfy the following equation 1.

[0018] [Formula 1]

[0019] 1668.26×C(wt%) + 1157.54×Si(wt%) + 81.40×Mn(wt%) + 94.51×Ni(wt%) + 202.33×Cr(wt%) + 5228.70×Nb(wt%) + 2256.36×V(wt%) ≥ 595.01

[0020] Tensile strength (TS) of unnotched specimens at -170℃ un ) can be 690MPa or more.

[0021] Tensile strength (TS) of unnotched specimens at -170℃ un ) is the yield strength (YS) of the unnotched specimen at -170℃ un ) can be more than 1.15 times.

[0022] Elongation at -170℃ (UE) un ) can be more than 3%.

[0023] A high-performance steel bar according to one embodiment of the present invention includes a surface portion and a central portion, wherein the microstructure of the surface portion includes at least one of martensite and tempered martensite, and the microstructure of the central portion may include at least two of bainite, ferrite, acicular ferrite, and pearlite.

[0024] 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) hot-rolling the steel material, while controlling the reheating temperature to 1050 to 1250°C and controlling the rolling end temperature to 900 to 1100°C, and (S3) cooling the steel material, wherein the steel bar subjected to the step (S3) contains carbon (C) 0.04 to 0.09 wt%, silicon (Si) 0.6 wt% or less (excluding 0), manganese (Mn) 1.45 to 2.5 wt%, nickel (Ni) 0.2 to 2.0 wt%, chromium (Cr) 0.7 wt% or less (excluding 0), niobium (Nb) 0.001 to 0.07 wt%, vanadium (V) 0.003 to 0.09 wt%, and phosphorus (P) 0.02 wt%. Below (excluding 0), sulfur (S) 0.02 wt% or less (excluding 0), copper (Cu) 0.5 wt% or less (excluding 0), molybdenum (Mo) 0.002 to 0.05 wt%, aluminum (Al) 0.001 to 0.07 wt%, nitrogen (N) 0.01 wt% or less (excluding 0), and the remainder iron (Fe) and unavoidable impurities, and satisfies the following formula 1.

[0025] [Formula 1]

[0026] 1668.26×C(wt%) + 1157.54×Si(wt%) + 81.40×Mn(wt%) + 94.51×Ni(wt%) + 202.33×Cr(wt%) + 5228.70×Nb(wt%) + 2256.36×V(wt%) ≥ 595.01

[0027] In the above step (S2), the reheating time of the steel may be 1 to 3 hours.

[0028] The above step (S2) may have a hot pressing reduction ratio of 75% or more.

[0029] The steel that underwent the above (S3) step had a yield strength (YS) of an unnotched specimen at -170℃ un ) can be 600 MPa or more.

[0030] The steel that underwent the above (S3) step had a tensile strength (TS) of an unnotched specimen at -170℃ un ) is the yield strength (YS) of the unnotched specimen at -170℃ un ) can be more than 1.15 times.

[0031] The steel that underwent the above (S3) step had a tensile strength (TS) of a notched specimen at -170℃ n ) Yield strength (YS) of unnotched specimens at -170℃ un ) may be more than that.

[0032] According to one embodiment of the present invention, a high-performance steel bar and a steel bar manufacturing method capable of securing yield strength and notch sensitivity in a low-temperature environment can be implemented.

[0033] 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.

[0034] Figure 1 is a flowchart showing a method for manufacturing a high-performance steel bar according to one embodiment of the present invention.

[0035] FIG. 2 is a drawing showing the results of observing the surface and center of a high-performance steel bar according to one embodiment of the present invention using an optical microscope.

[0036] Figure 3 is a drawing showing the optical microscope observation results of Comparative Example 1 and Example 1 described in Table 3.

[0037] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings so that those skilled in the art can easily implement the present invention. However, the present invention is not limited or restricted by the following embodiments.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0044] High-performance steel bar

[0045] A high-performance steel bar according to one embodiment of the present invention contains carbon (C) 0.04 to 0.09 wt%, silicon (Si) 0.6 wt% or less (excluding 0), manganese (Mn) 1.45 to 2.5 wt%, nickel (Ni) 0.2 to 2.0 wt%, chromium (Cr) 0.7 wt% or less (excluding 0), niobium (Nb) 0.001 to 0.07 wt%, vanadium (V) 0.003 to 0.09 wt%, phosphorus (P) 0.02 wt% or less (excluding 0), sulfur (S) 0.02 wt% or less (excluding 0), copper (Cu) 0.5 wt% or less (excluding 0), molybdenum (Mo) 0.002 to 0.05 wt%, aluminum (Al) 0.001 to 0.07 wt%, and nitrogen (N) 0.01 wt%. It is manufactured from steel containing the following (excluding 0) and the remaining iron (Fe) and unavoidable impurities, and the same alloying components may be included in the final product, the steel bar.

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

[0047] carbon (C)

[0048] Carbon (C) is the most effective and important element for increasing the strength of steel.

[0049] Carbon is incorporated into austenite, which can form martensite during quenching. Furthermore, increasing carbon content can improve hardness.

[0050] 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.

[0051] Therefore, the high-performance steel bar according to one embodiment of the present invention may contain 0.04 to 0.09 wt% of carbon.

[0052] Silicon (Si)

[0053] Silicon (Si) is added as a deoxidizer to remove oxygen in steel during the steelmaking process together with aluminum, and as a ferrite stabilizing element with a solid solution strengthening effect, it can induce ferrite formation and improve the hardenability and softening resistance of steel.

[0054] If the silicon content is insufficient, the aforementioned effects may be minimal. Conversely, if the silicon content is excessive, toughness may be reduced and plastic workability may be impaired.

[0055] Therefore, the high-performance steel bar according to one embodiment of the present invention may contain silicon (Si) in an amount of 0.6 wt% or less.

[0056] manganese (Mn)

[0057] Manganese (Mn) is a solid-solution strengthening element that not only contributes to strength but also enhances the hardenability of steel. Furthermore, as the manganese content increases, pearlite becomes finer and ferrite becomes solid-solution strengthened, thereby improving yield strength.

[0058] If manganese content is insufficient, the aforementioned effects may be minimal. Conversely, if manganese content is excessive, austenite may remain, reducing strength and toughness.

[0059] Accordingly, the high-performance steel bar according to one embodiment of the present invention may contain 1.45 to 2.5 wt% of manganese (Mn).

[0060] Nickel (Ni)

[0061] 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.

[0062] 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.

[0063] Therefore, the high-performance steel bar according to one embodiment of the present invention may contain 0.2 to 2.0 wt% of nickel (Ni).

[0064] chromium (Cr)

[0065] Chromium (Cr) is a ferrite-stabilizing element. When added to C-Mn steel, it impedes carbon diffusion due to its solute-interfering effect, thereby contributing to grain refinement. Furthermore, chromium enhances the hardenability of steel, improving hardenability. However, excessive chromium content can lead to the formation of coarse carbides at grain boundaries, reducing the steel's room-temperature and low-temperature ductility.

[0066] Therefore, the high-performance steel bar according to one embodiment of the present invention may contain chromium (Cr) in an amount of 0.7 wt% or less.

[0067] niobium (Nb)

[0068] Niobium (Nb) reacts with carbon and nitrogen in steel to form precipitates (e.g., NbC or Nb(C,N)). When niobium is added, the precipitates produce a pinning effect that suppresses grain growth and refines the grain size. Furthermore, the strength of the base metal and weld joint can be improved.

[0069] If the niobium content is insufficient, the aforementioned effects may be minimal. Conversely, if the niobium content is excessive, coarse precipitates may form, reducing the steel's impact properties.

[0070] Therefore, the high-performance steel bar according to one embodiment of the present invention may contain niobium (Nb) in an amount of 0.001 to 0.07 wt%.

[0071] Vanadium (V)

[0072] Vanadium (V) has a stronger carbide-forming ability than chromium (Cr), and because it refines the steel structure, it causes precipitation strengthening when precipitates are formed, which helps improve strength. Furthermore, adding vanadium can improve toughness.

[0073] If the vanadium content is insufficient, the aforementioned effects may be minimal. Conversely, if the vanadium content is excessive, the manufacturing cost of the steel increases, and oxides (e.g., V2O5) may evaporate at high temperatures.

[0074] Therefore, the high-performance steel bar according to one embodiment of the present invention may preferably contain 0.03 to 0.09 wt% of vanadium (V).

[0075] Person (P)

[0076] Phosphorus (P) is an element that contributes to strength enhancement. However, excessive phosphorus content can deteriorate the ductility of steel and cause final material deviations due to billet center segregation. When phosphorus content exceeds 0.02 wt%, it can form center segregation and microsegregation, reducing the ductility of the steel. Furthermore, precipitation behavior can reduce impact strength.

[0077] Therefore, the high-performance steel according to one embodiment of the present invention may contain phosphorus (P) in an amount of 0.02 wt% or less.

[0078] Yellow (S)

[0079] Sulfur (S) improves the machinability of steel by combining with manganese, zinc, titanium, molybdenum, and other elements. It can also enhance workability by combining with manganese to form fine precipitates (e.g., MnS). However, if the amount of manganese in the steel is insufficient, it can combine with iron to form sulfides (e.g., FeS), which can cause cracks during hot and cold working. Sulfur content exceeding 0.02 wt% can reduce the ductility of the steel and deteriorate its toughness and weldability.

[0080] Therefore, the high-performance steel bar according to one embodiment of the present invention may contain sulfur (S) in an amount of 0.02 wt% or less.

[0081] copper (Cu)

[0082] Copper (Cu) typically exists in steel at approximately 0.1 to 0.3 wt%. Copper dissolves in ferrite at concentrations of up to 0.35 wt% at room temperature, exhibiting a solid-solution strengthening effect that can improve strength and hardness. It can also enhance corrosion resistance in air and seawater. However, excessive copper content can reduce elongation, and particularly, exceeding 0.5 wt% can cause red-hot embrittlement.

[0083] Therefore, the high-performance steel bar according to one embodiment of the present invention may contain copper (Cu) in an amount of 0.5 wt% or less.

[0084] molybdenum (Mo)

[0085] Molybdenum (Mo) can improve hardenability by up to 10 times that of nickel (Ni) with just 0.1 to 0.3 wt% 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. Molybdenum is even more effective when used with chromium than alone to enhance hardenability.

[0086] 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.

[0087] Therefore, the high-performance steel bar according to one embodiment of the present invention may contain 0.002 to 0.05 wt% of molybdenum (Mo).

[0088] Aluminum (Al)

[0089] 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).

[0090] 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.

[0091] Therefore, the high-performance steel bar according to one embodiment of the present invention may contain 0.001 to 0.07 wt% of aluminum (Al).

[0092] Nitrogen (N)

[0093] 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.

[0094] 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.

[0095] Therefore, the high-performance steel bar according to one embodiment of the present invention may contain nitrogen (N) in an amount of 0.01 wt% or less.

[0096] 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.

[0097] 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).

[0098] A high-performance steel bar according to one embodiment of the present invention can satisfy Equation 1.

[0099] [Formula 1]

[0100] 1668.26×C(wt%) + 1157.54×Si(wt%) + 81.40×Mn(wt%) + 94.51×Ni(wt%) + 202.33×Cr(wt%) + 5228.70×Nb(wt%) + 2256.36×V(wt%) ≥ 595.01

[0101] In the above formula 1, each element represents the content (wt%) of each element included in the steel bar. For example, in the case of a steel bar with a carbon (C) content of 0.05 wt%, the C (wt%) value in the above formula 1 is 0.05.

[0102] When equation 1 is satisfied, the high-performance steel bar according to one embodiment of the present invention may have a low-temperature yield strength of 600 MPa or more. Here, the low-temperature yield strength (YS un ) means the yield strength measured in an unnotched specimen at -170℃.

[0103] A high-performance steel bar according to one embodiment of the present invention can satisfy Equation 2.

[0104] [Formula 2]

[0105] 12.45×C(wt%) + 1145.44×Si(wt%) + 238.09×Mn(wt%) + 140.14×Ni(wt%) + 280.55×Cr(wt%) + 4367.63×Nb(wt%) + 1749.46×V(wt%) ≥ 722.32

[0106] In the above formula 2, each element represents the content (wt%) of each element included in the steel bar. For example, in the case of a steel bar with a carbon (C) content of 0.05 wt%, the C (wt%) value in the above formula 2 is 0.05.

[0107] When equation 2 is satisfied, the high-performance steel bar according to one embodiment of the present invention has a low-temperature unnotched tensile strength (TS un ) can be 690 MPa or more. Here, the low-temperature unnotched tensile strength (TS un ) refers to the tensile strength measured in an unnotched specimen at -170℃.

[0108] Low temperature unnotched tensile strength (TS) un ) is 690 MPa 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. More preferably, the low-temperature unnotched tensile strength (TS un ) is the low-temperature yield strength (YS) un ) can be more than 1.15 times.

[0109] In addition, the high-performance steel bar according to one embodiment of the present invention has a low-temperature notch tensile strength (TS n ) can be 600 MPa or more. Here, the low-temperature notched tensile strength (TS n ) refers to the tensile strength measured in a notched specimen at -170℃.

[0110] Low temperature notched tensile strength (TS) n ) is 600 MPa or more, the low-temperature notch sensitivity (NSR: Notch Sensitivity Ratio) of the steel bar can be improved. Here, the low-temperature notch sensitivity (NSR) is the tensile strength (TS) measured in a notched specimen at -170°C. n ) and yield strength (YS un ) ratio (TS n / YS un )am.

[0111] By improving the low-temperature notch sensitivity (NSR), even if a notch occurs in the steel bar, fatigue failure due to the notch can be suppressed or prevented in a low-temperature (-170°C) environment. More preferably, the low-temperature notch sensitivity (NSR) can be 1.0 or greater.

[0112] High-performance steel bars according to one embodiment of the present invention have low-temperature elongation (UE) un ) can be 3% or more. Here, the low-temperature elongation (UE) un ) refers to the elongation measured in the unnotched specimen at -170℃.

[0113] When the low-temperature elongation is 3% or more, the occurrence of fracture in the steel bar can be suppressed or prevented even when external force is applied in a low-temperature (-170℃) environment due to low-temperature brittleness.

[0114] Hereinafter, a method for manufacturing a high-performance steel bar according to one embodiment of the present invention will be described in detail.

[0115] Method for manufacturing high-performance steel bars

[0116] 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.

[0117] Figure 1 is a flowchart showing a method for manufacturing a high-performance steel bar according to one embodiment of the present invention.

[0118] A method for manufacturing a high-performance steel bar according to one embodiment of the present invention includes a first step (S1) of preparing a semi-finished steel product, a second step (S2) of hot-rolling the steel product prepared in the first step (S1) to form a hot-rolled product, and a third step (S3) of cooling the hot-rolled product.

[0119] Hereinafter, each step of a high-performance steel bar manufacturing method according to one embodiment of the present invention will be described in detail.

[0120] The first step (S1) of preparing a semi-finished steel product is a step of preparing a steel product having the above-described alloy composition range for the purpose of manufacturing the final product, a high-performance steel bar. More specifically, this step may be a step of manufacturing a semi-finished product by designing the alloy components within the above-described alloy composition range. The semi-finished product may be a billet or bloom, but is not limited thereto. Furthermore, the manufacturing of the semi-finished product may be performed using a process known in the relevant technical field, such as a steelmaking process or a casting process.

[0121] In a method for manufacturing high-performance steel bars according to one embodiment of the present invention, the second step (S2) may form a hot-rolled steel by hot-rolling the steel material that has undergone the first step (S1). The second step (S2) may include a reheating step and a hot-rolling step.

[0122] First, the reheating step may be performed prior to the hot rolling step, reheating the steel for subsequent processing. Specifically, this step may involve loading the steel into a heating furnace and uniformly heating the steel to facilitate plastic deformation.

[0123] Steel can be reheated at temperatures between 1050 and 1250°C. Reheating temperatures below 1050°C can increase deformation resistance during hot rolling and increase rolling load, potentially reducing rollability. Conversely, reheating temperatures exceeding 1250°C can cause austenite grain coarsening or decarburization, which can reduce strength. Furthermore, increased heating costs and time can lead to increased manufacturing costs and reduced productivity.

[0124] Additionally, the steel can be reheated for 1 to 3 hours. If the reheating time is less than 1 hour, the rolling load may increase. Conversely, if the reheating time exceeds 3 hours, the grain size may become excessively coarsened.

[0125] Therefore, in the present invention, the steel can be reheated at a temperature of 1000 to 1200°C for 1 to 3 hours.

[0126] Next, a hot rolling step may be performed. The hot rolling step may include rough rolling and finish rolling. The rough rolling step may involve forming the steel into a rolled material having an appropriate shape, thickness, and width, while the finish rolling step may involve adjusting the steel to a specified size and rolling it at a finishing temperature appropriate for the intended use to achieve a desirable surface and shape.

[0127] At this time, the finishing rolling temperature may be 900 to 1100°C. If the finishing rolling temperature is lower than 900°C, the rolling load may increase, which may lower productivity and reduce the heat treatment effect. Conversely, if it exceeds 1100°C, the pearlite structure may form coarsely, which may rapidly reduce strength. Therefore, the finishing rolling temperature is preferably 900 to 1100°C, and more preferably 950 to 1050°C.

[0128] The above hot rolling step can be performed at a reduction ratio of 75% or more, and more preferably at a reduction ratio of 78% or more.

[0129] Thereafter, the method for manufacturing a high-performance steel bar according to one embodiment of the present invention may perform a third step (S3) of cooling the hot-rolled material that has undergone the second step (S2). The third step (S3) may cool the hot-rolled material through a temp-core process.

[0130] Specifically, the third stage (S3) can rapidly cool the hot-rolled steel at a temperature range of 500 to 610°C and a cooling rate of 100 to 400°C / sec. The above-described temperature range refers to the surface temperature of the hot-rolled steel.

[0131] After rapid cooling, the surface of the hot rolled steel is reheated to a temperature range of 535 to 630℃ through the internal residual heat, and can be air-cooled to room temperature again.

[0132] A high-performance steel bar manufactured by a high-performance steel bar manufacturing method according to one embodiment of the present invention can satisfy all of the low-temperature yield strength (YSun), low-temperature unnotched tensile strength (TSun), low-temperature notched tensile strength (TSn), and low-temperature elongation (UEun) values ​​described above.

[0133] In addition, the high-performance steel bar manufactured by the high-performance steel bar manufacturing method according to one embodiment of the present invention can satisfy the ratio of low-temperature tensile strength to low-temperature yield strength (TSun / YSun) and low-temperature notch sensitivity (NSR) values ​​described above.

[0134] A high-performance steel bar manufactured using a method for manufacturing high-performance steel bars according to one embodiment of the present invention may have a surface layer comprising a microstructure of at least one of martensite and tempered martensite, and a central layer comprising a microstructure of at least two of bainite, ferrite, acicular ferrite, and pearlite. For a detailed description thereof, reference is further made to FIG. 2.

[0135] Figure 2 is a drawing showing the results of observing the surface and center portion of a high-performance steel bar according to one embodiment of the present invention using an optical microscope. In Figure 2, (a) is the result of observing the center portion of a high-performance steel bar according to the present invention, and (b) is the result of observing the surface portion of a high-performance steel bar according to the present invention.

[0136] Referring further to FIG. 2, the central portion of the high-performance steel bar according to one embodiment of the present invention may be composed of a composite structure including at least two microstructures of bainite, ferrite, acicular ferrite, and pearlite. In FIG. 2, it can be confirmed that the central portion (a) of the high-performance steel bar is composed of a microstructure of bainite, pearlite, and acicular ferrite.

[0137] In addition, the surface layer of the high-performance steel bar according to one embodiment of the present invention may be composed of at least one microstructure of martensite and tempered martensite. In Fig. 2, it can be confirmed that the surface layer (b) of the high-performance steel bar is composed of a microstructure of martensite and tempered martensite.

[0138] By having a core and a surface layer and being composed of the above-described microstructure, the present invention can provide a high-performance core and a method for manufacturing the high-performance core that can secure yield strength and notch sensitivity in a low-temperature environment.

[0139] Comparative examples and examples

[0140] 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.

[0141] Table 1 shows the alloy element compositions of comparative examples and examples, and Table 2 shows whether Equations 1 and 2 are satisfied and the low-temperature physical properties of comparative examples and examples. Table 3 shows the results of measuring the crystal grain size and microstructure fraction of comparative example 1 and example 1.

[0142] Fig. 3 is a drawing showing the optical microscope observation results of Comparative Example 1 and Example 1. In Fig. 3, (c) is the observation result of the central microstructure of Comparative Example 1, and (d) is the observation result of the central microstructure of Example 1.

[0143] 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 1200°C for 2 hours. Thereafter, hot rolling was performed at a rolling end temperature of 1000°C with a reduction ratio of 78%, thereby manufacturing specimens with a diameter of 20 mm.

[0144] Additionally, it was cooled to 550℃ at a cooling rate of 300℃ / sec, and then cooled to 600℃ with internal residual heat.

[0145] 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.

[0146] In Table 3 below, the crystal grain size and microstructure fraction of Comparative Example 1 and Experimental Example 1 were analyzed using an optical microscope.

[0147] In addition, the low-temperature properties of the comparative examples and examples were measured in an environment of -170°C.

[0148] In Table 1 below, the composition unit of alloy elements is weight%, and in Table 2 below, the low-temperature yield strength (YS un ), low temperature unnotched tensile strength (TS un ) and low-temperature notched tensile strength (TS n ) is in MPa, and the low-temperature elongation (UE) un ) is in %.

[0149]

[0150] [Correction pursuant to Rule 91, October 17, 2024]

[0151]

[0152] Referring to Tables 1 and 2 above, Comparative Example 1 is a comparative example in which all other alloy composition ranges satisfy the ranges according to one embodiment of the present invention, but the manganese (Mn) content is 1.44 wt% and the left-hand side value of Equation 1 is 578.38.

[0153] In the case of Comparative Example 1, the content of manganese (Mn) did not satisfy the range of 1.45 to 2.5 wt% according to the above-described embodiment of the present invention, and the left-hand side value of Equation 1 did not satisfy the range of 595.01 or more according to the above-described embodiment of the present invention, so the low-temperature yield strength (YS un ) value and low temperature notch sensitivity (NSR) do not satisfy the target range of the present invention, which is 600 MPa or more and 1.0 or more, respectively.

[0154] Comparative Example 2 is a comparative example in which Equation 1 and other alloy composition ranges satisfy all the ranges according to one embodiment of the present invention, but the content of chromium (Cr) is 0.98 wt%.

[0155] In the case of Comparative Example 2, the content of chromium (Cr) did not satisfy the range of 0.7 wt% or less according to the above-described embodiment of the present invention, and thus it was confirmed that the low-temperature notch sensitivity (NSR) was 0.95, which did not satisfy the range of 1.0 or more targeted by the present invention.

[0156] Comparative Example 3 is a comparative example in which the alloy composition range of Formula 1 and another alloy composition range are all satisfied according to one embodiment of the present invention, but the content of vanadium (V) is 0.1 wt%.

[0157] In the case of Comparative Example 3, the content of vanadium (V) did not satisfy the range of 0.03 to 0.09 wt% according to the above-described embodiment of the present invention, so the low-temperature unnotched tensile strength (TS un ) is the low-temperature yield strength (YS) un ) does not satisfy a value of 1.15 times or more, and it can be confirmed that the low-temperature notch sensitivity (NSR) does not satisfy a value of 1.0 or more, which is the range targeted by the present invention.

[0158] 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 one embodiment of the present invention and also satisfies Equation 1. Through this, the low-temperature yield strength (YS un ), low temperature unnotched tensile strength (TS un ), low temperature notched tensile strength (TS n ) and low temperature elongation (UE) un ) can be confirmed to satisfy all the ranges targeted in the present invention.

[0159] In addition, referring to Table 3 and Figure 3, it can be confirmed that in the case of Comparative Example 1, the average grain size is 11.8 ㎛, the standard deviation is 9.7 ㎛, and the ferrite area fraction is 89.6%. In comparison, in the case of Example 1, the average grain size is 9.6 ㎛, the standard deviation is 6.5 ㎛, and the ferrite area fraction is 83.5%.

[0160] Through this, it can be confirmed that in the case of Example 1, the crystal grains are finer than in the case of Comparative Example 1, and bainite is formed instead of ferrite, thereby improving the low-temperature yield strength and notch sensitivity.

[0161] As described above, preferred embodiments of the present invention have been described. It will be apparent to those skilled in the art that the present invention may be embodied in other specific forms, in addition to the embodiments described above, without departing from the spirit or scope thereof. Therefore, the above-described embodiments should be considered illustrative rather than restrictive, and accordingly, the present invention is not limited to the above description, but may be modified within the scope of the appended claims and their equivalents.

Claims

1. Carbon (C) 0.04 to 0.09 wt%, silicon (Si) 0.6 wt% or less (excluding 0), manganese (Mn) 1.45 to 2.5 wt%, nickel (Ni) 0.2 to 2.0 wt%, chromium (Cr) 0.7 wt% or less (excluding 0), niobium (Nb) 0.001 to 0.07 wt%, vanadium (V) 0.003 to 0.09 wt%, phosphorus (P) 0.02 wt% or less (excluding 0), sulfur (S) 0.02 wt% or less (excluding 0), copper (Cu) 0.5 wt% or less (excluding 0), molybdenum (Mo) 0.002 to 0.05 wt%, aluminum (Al) 0.001 to 0.07 wt%, nitrogen (N) 0.01 wt% or less (excluding 0) and the remainder Contains iron (Fe) and inevitable impurities, and satisfies the following equation 1: The yield strength (YSun) of the unnotched specimen at -170℃ is 600MPa or more. High performance steel bar. [Formula 1] 1668.26×C(wt%) + 1157.54×Si(wt%) + 81.40×Mn(wt%) + 94.51×Ni(wt%) + 202.33×Cr(wt%) + 5228.70×Nb(wt%) + 2256.36×V(wt%) ≥ 595.01 2. In paragraph 1, - The tensile strength (TSun) of the unnotched specimen at 170℃ is 690 MPa or more. High performance steel bar.

3. In paragraph 2, The tensile strength (TSun) of the unnotched specimen at -170℃ is at least 1.15 times the yield strength (YSun) of the unnotched specimen at -170℃. High performance steel bar.

4. In paragraph 1, The tensile strength (TSn) of the notched specimen at -170℃ is 600MPa or more. High performance steel bar.

5. In paragraph 4, The tensile strength (TSn) of the notched specimen at -170℃ is greater than or equal to the yield strength (YSun) of the unnotched specimen at -170℃. High performance steel bar.

6. Carbon (C) 0.04 to 0.09 wt%, silicon (Si) 0.6 wt% or less (excluding 0), manganese (Mn) 1.45 to 2.5 wt%, nickel (Ni) 0.2 to 2.0 wt%, chromium (Cr) 0.7 wt% or less (excluding 0), niobium (Nb) 0.001 to 0.07 wt%, vanadium (V) 0.003 to 0.09 wt%, phosphorus (P) 0.02 wt% or less (excluding 0), sulfur (S) 0.02 wt% or less (excluding 0), copper (Cu) 0.5 wt% or less (excluding 0), molybdenum (Mo) 0.002 to 0.05 wt%, aluminum (Al) 0.001 to 0.07 wt%, nitrogen (N) 0.01 wt% or less (excluding 0) and the remainder Contains iron (Fe) and inevitable impurities, The yield strength (YSun) of the unnotched specimen at -170℃ is 600MPa or more, The tensile strength (TSn) of the notched specimen at -170℃ is 600MPa or more. High performance steel bar.

7. In paragraph 6, The tensile strength (TSn) of the notched specimen at -170℃ is greater than or equal to the yield strength (YSun) of the unnotched specimen at -170℃. High performance steel bar.

8. In paragraph 6, Satisfying the following equation 1, High performance steel bar. [Formula 1] 1668.26×C(wt%) + 1157.54×Si(wt%) + 81.40×Mn(wt%) + 94.51×Ni(wt%) + 202.33×Cr(wt%) + 5228.70×Nb(wt%) + 2256.36×V(wt%) ≥ 595.01 9. In paragraph 6, - The tensile strength (TSun) of the unnotched specimen at 170℃ is 690 MPa or more. High performance steel bar.

10. In paragraph 9, The tensile strength (TSun) of the unnotched specimen at -170℃ is at least 1.15 times the yield strength (YSun) of the unnotched specimen at -170℃. High performance steel bar.

11. In paragraph 6, At -170℃, the elongation (UEun) is 3% or more. High performance steel bar.

12. In paragraph 6, Including the surface and the center, The microstructure of the above surface layer includes at least one of martensite and tempered martensite, The microstructure of the above central portion includes at least two of bainite, ferrite, needle ferrite and pearlite. High performance steel bar. 13.(S1) Step of preparing steel; (S2) A step of hot rolling the above steel material, controlling the reheating temperature to 1050 to 1250°C and controlling the rolling end temperature to 900 to 1100°C; and (S3) A step of cooling the above steel material; Including, The steel bar that has undergone the above (S3) step contains 0.04 to 0.09 wt% of carbon (C), 0.6 wt% or less of silicon (Si) (excluding 0), 1.45 to 2.5 wt% of manganese (Mn), 0.2 to 2.0 wt% of nickel (Ni), 0.7 wt% or less of chromium (Cr) (excluding 0), 0.001 to 0.07 wt% of niobium (Nb), 0.003 to 0.09 wt% of vanadium (V), 0.02 wt% or less of phosphorus (P) (excluding 0), 0.02 wt% or less of sulfur (S) (excluding 0), 0.5 wt% or less of copper (Cu) (excluding 0), 0.002 to 0.05 wt% of molybdenum (Mo), 0.001 to 0.07 wt% of aluminum (Al), and 0.01 wt% of nitrogen (N). Contains the following (excluding 0) and the remaining iron (Fe) and inevitable impurities, and satisfies the following formula 1. Method for manufacturing high-performance steel bars. [Formula 1] 1668.26×C(wt%) + 1157.54×Si(wt%) + 81.40×Mn(wt%) + 94.51×Ni(wt%) + 202.33×Cr(wt%) + 5228.70×Nb(wt%) + 2256.36×V(wt%) ≥ 595.01 14. In paragraph 13, In the above step (S2), The reheating time of steel is 1 to 3 hours. Method for manufacturing high-performance steel bars.

15. In paragraph 13, The above step (S2) is, With a hot rolling reduction of 75% or more, Method for manufacturing high-performance steel bars.

16. In paragraph 13, The steel material that has undergone the above step (S3) is The yield strength (YSun) of the unnotched specimen at -170℃ is 600MPa or more. Method for manufacturing high-performance steel bars.

17. In paragraph 16, The steel material that has undergone the above step (S3) is The tensile strength (TSun) of the unnotched specimen at -170℃ is at least 1.15 times the yield strength (YSun) of the unnotched specimen at -170℃. Method for manufacturing high-performance steel bars.

18. In paragraph 16, The steel material that has undergone the above step (S3) is The tensile strength (TSn) of the notched specimen at -170℃ is greater than the yield strength (YSun) of the unnotched specimen at -170℃. Method for manufacturing high-performance steel bars.

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