Steel bar and method for manufacturing steel bar

Abstract

The method for manufacturing bar steel according to an embodiment of the present invention comprises the steps of: (a) reheating bar steel containing 0.14 - 0.43 wt% of carbon (C), 0.4 - 1.75 wt% of manganese (Mn), 0.05 - 0.30 wt% of silicon (Si), 0.0 wt% (exclusive) - 0.04 wt% of phosphorus (P), 0.0 wt% (exclusive) - 0.04 wt% of sulfur (S), 0.002 wt% (exclusive) - 1.0 wt% of chromium (Cr), 0.0 wt% (exclusive) - 0.5 wt% of copper (Cu), 0.0 wt% (exclusive) - 0.25 wt% of nickel (Ni), 0.015 wt% (exclusive) - 0.085 wt% of molybdenum (Mo), 0.040 wt% or less of aluminum (Al), 0.002 wt% (exclusive) - 0.15 wt% of vanadium (V), 0.040 wt% or less of nitrogen (N), and the balance of iron (Fe) and inevitable impurities to 1,000 - 1,080℃; (b) hot rolling the steel bar while controlling the rolling to start at a temperature of 950-1050℃ and end at a temperature of 960-1,020℃; and (c) cooling the steel bar.

Description

Steel bars and steel bar manufacturing methods

[0001] The present invention relates to a bar and a method for manufacturing bar.

[0002] Rebar or steel bar is a thin, long steel material used to reinforce concrete. It has strong bonding strength with concrete, and it is widely used in construction and civil engineering sites because it improves the tensile strength of concrete and reduces the width of cracks when concrete cracks.

[0003] Today, it is used as a core material for the construction of buildings, bridges, large offshore structures, underground structures, and storage facilities.

[0004] Recently, structures in redevelopment complexes are becoming larger and longer, and with the mandatory implementation of earthquake-resistant design, securing development and production technology for earthquake-resistant high-strength / ultra-high-strength steel bars is emerging as a hot topic.

[0005] Accordingly, higher-strength rebars are needed than before. For example, while yield strength requirements of up to 500 MPa were previously required, recent demands have increased to 600-700 MPa. Demand for 1.0 GPa-class rebars is also expected in the future.

[0006] As global warming continues, ensuring the safety of buildings and civil engineering structures against natural and man-made disasters such as unexpected earthquakes, fires, strong winds, heavy snow, cold waves, ground subsidence, etc. is essential to protecting the lives and property of citizens.

[0007] In particular, structural safety design to prepare for natural / man-made earthquake disasters is essential, and securing seismic performance and low-temperature toughness is urgently needed.

[0008] In order to achieve high performance in steel, simply increasing the amount of alloying elements added causes problems such as increased cost due to alloy addition, occurrence of cracks and flaws in the product, and decreased toughness and ductility.

[0009] Accordingly, the present invention aims to provide a manufacturing method capable of manufacturing high-strength earthquake-resistant steel bars having stable earthquake-resistant performance at room temperature and in polar regions.

[0010] In order to solve the problems of the prior art described above, the purpose of the present invention is to provide a steel bar and a steel bar manufacturing method according to an optimal alloy element steel grade design that can simultaneously contribute to improving strength and seismic resistance.

[0011] 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.

[0012] According to one embodiment of the present invention, a steel bar comprises carbon (C) 0.14 to 0.43 wt%, manganese (Mn) 0.40 to 1.75 wt%, silicon (Si) 0.05 to 0.30 wt%, phosphorus (P) 0.0 wt% to 0.04 wt%, sulfur (S) 0.0 wt% to 0.04 wt%, chromium (Cr) 0.002 wt% to 1.0 wt%, copper (Cu) 0.0 wt% to 0.5 wt%, nickel (Ni) 0.0 wt% to 0.25 wt%, molybdenum (Mo) 0.015 wt% to 0.085 wt%, aluminum (Al) 0.040 wt% or less, vanadium (V) 0.002 wt% to 0.15 wt%, It contains nitrogen (N) of 0.040 wt% or less and the remainder iron (Fe) and unavoidable impurities, and satisfies a yield strength at room temperature (YS) of 600 MPa or more.

[0013] In addition, the yield ratio (YR) indicating seismic performance at room temperature can be satisfied as 0.77% or less, the yield ratio (YR) indicating seismic performance at -10℃ can be satisfied as 0.78% or less, and the yield ratio (YR) indicating seismic performance at -40℃ can be satisfied as 0.79% or less.

[0014] Additionally, the elongation at -10°C (EL) may be 9.8% or more and the elongation at -40°C (EL) may be 9.3% or more.

[0015] Additionally, it can form ultra-fine grained ferrite structures.

[0016] In addition, it may further include at least one of niobium (Nb) exceeding 0.001 wt% to 0.15 wt%, tin (Sn) 0.1 wt% or less, calcium (Ca) 0.05 wt% or less, and boron (B) 0.001 wt% to 0.004 wt%.

[0017] In particular, it may preferably further include more than 0.001 wt% to 0.0015 wt% of boron (B).

[0018] And, vanadium (V) may preferably be included in an amount of more than 0.002 wt% to 0.0043 wt%.

[0019] According to one embodiment of the present invention, a method for manufacturing a bar comprises: (a) carbon (C) 0.14 to 0.43 wt%, manganese (Mn) 0.4 to 1.75 wt%, silicon (Si) 0.05 to 0.30 wt%, phosphorus (P) more than 0.0 wt% to 0.04 wt%, sulfur (S) more than 0.0 wt% to 0.04 wt%, chromium (Cr) more than 0.002 wt% to 1.0 wt%, copper (Cu) more than 0.0 wt% to 0.5 wt%, nickel (Ni) more than 0.0 wt% to 0.25 wt%, molybdenum (Mo) more than 0.015 wt% to 0.085 wt%, aluminum (Al) 0.040 wt% or less, vanadium (V) more than 0.002 wt% to 0.15 wt%, A method for producing a steel bar, comprising: (a) a step of reheating a steel bar containing nitrogen (N) of 0.040 wt% or less and the remainder iron (Fe) and unavoidable impurities to 1,000 to 1,080°C; (b) a step of hot-rolling the steel bar, while controlling the rolling start temperature to 950 to 1,050°C and the rolling end temperature to 960 to 1,020°C; and (c) a step of cooling the steel bar.

[0020] In addition, the step (b) above can reheat the steel bar in a billet state, which is a semi-finished product.

[0021] And, the above step (c) can perform cooling using intermediate controlled cooling (ICC).

[0022] In addition, the above step (c) can control the cooling rate to 150 to 400 ℃ / sec.

[0023] In addition, the above step (c) can control the water cooling end temperature to 500 to 650 ℃.

[0024] In addition, the steel bar that has performed the above step (c) can satisfy a yield ratio (YR) indicating seismic performance at room temperature of 0.77% or less, a yield ratio (YR) indicating seismic performance at -10°C of 0.78% or less, and a yield ratio (YR) indicating seismic performance at -40°C of 0.79% or less.

[0025] And, the steel bar that has performed the above step (c) may have an elongation (EL) at -10°C of 9.8% or more.

[0026] In addition, the steel bar that has undergone the above step (c) can satisfy an elongation (EL) of 9.3% or more at -40°C.

[0027] Additionally, the steel bar that has undergone the above step (c) may include ultra-fine grain ferrite in its final microstructure.

[0028] According to one embodiment of the present invention, since it provides suitable mechanical properties, it can contribute to weight reduction in the construction of large / long structures, and it provides the effect of simplifying the manufacturing process of high-functionality steel bars that can exhibit seismic performance (resistance yield ratio, YR < 0.8), thereby reducing manufacturing costs and increasing production efficiency.

[0029] 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.

[0030] Figure 1 is a flow chart illustrating a method for manufacturing a bar according to the present invention.

[0031] In this specification, when it is said that a component (or region, layer, portion, etc.) is “on,” “connected to,” or “coupled to” another component, it means that it can be directly disposed / connected / coupled to the other component, or a third component may be disposed between them.

[0032] Identical drawing numbers indicate identical components. Furthermore, in the drawings, the thicknesses, proportions, and dimensions of components are exaggerated for the purpose of effectively illustrating the technical content.

[0033] “And / or” includes any combination of one or more of the associated constructs that can be defined.

[0034] While terms such as "first" and "second" may be used to describe various components, these components should not be limited by these terms. These terms are used solely to distinguish one component from another. For example, without departing from the scope of the present invention, a first component may be referred to as a "second component," and similarly, a second component may also be referred to as a "first component." Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0035] Additionally, terms such as "below," "lower," "above," and "upper" are used to describe the relationships between components depicted in the drawings. These terms are relative concepts and are described based on the directions indicated in the drawings.

[0036] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by those skilled in the art to which this invention pertains. Furthermore, terms defined in commonly used dictionaries should be interpreted as having a meaning consistent with their meaning in the relevant technical context, and unless interpreted in an idealized or overly formal sense, they are explicitly defined herein.

[0037] 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.

[0038] 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 also be applied to numerical value A.

[0039] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0040] Bonggang

[0041] According to one embodiment of the present invention, a steel bar comprises carbon (C) 0.14 to 0.43 wt%, manganese (Mn) 0.40 to 1.75 wt%, silicon (Si) 0.05 to 0.30 wt%, phosphorus (P) 0.0 wt% to 0.04 wt%, sulfur (S) 0.0 wt% to 0.04 wt%, chromium (Cr) 0.002 wt% to 1.0 wt%, copper (Cu) 0.0 wt% to 0.5 wt%, nickel (Ni) 0.0 wt% to 0.25 wt%, molybdenum (Mo) 0.015 wt% to 0.085 wt%, aluminum (Al) 0.040 wt% or less, vanadium (V) 0.002 wt% to 0.15 wt%, and nitrogen (N). Contains 0.040 wt% or less and the remainder iron (Fe) and unavoidable impurities.

[0042] In particular, in the case of boron (B), it may preferably contain more than 0.001 wt% to 0.0015 wt%.

[0043] In addition, vanadium (V), an expensive alloying element, may preferably be included in an amount of more than 0.002 wt% to 0.043 wt% to replace manganese (Mn), chromium (Cr), and boron (B).

[0044] In addition, the steel bar according to one embodiment of the present invention may further include at least one of niobium (Nb) exceeding 0.001 wt% to 0.15 wt%, tin (Sn) 0.1 wt% or less, calcium (Ca) 0.05 wt% or less, and boron (B) exceeding 0.001 wt% to 0.004 wt%.

[0045] Although not limited thereto, the bar according to one embodiment of the present invention may serve as a reinforcing bar to reinforce a reinforced concrete structure and prevent collapse.

[0046] According to one embodiment of the present invention, the steel bar having the above-described alloy composition may have a yield strength (YS) of 600 MPa or more measured at room temperature and low temperature (-10°C and -40°C).

[0047] In addition, the steel bar having the above-described alloy composition has an elongation (EL) measured at -10°C of 9.8% or more and an elongation (EL) measured at -40°C of 9.3% or more.

[0048] In addition, according to one embodiment of the present invention, the steel bar having the above-described alloy composition satisfies the mechanical properties of steel bars for building structures, such as a yield ratio (YR) indicating seismic performance at room temperature of 0.77% or less, a yield ratio (YR) indicating seismic performance at -10°C of 0.78% or less, and a yield ratio (YR) indicating seismic performance at -40°C of 0.79% or less.

[0049] In particular, the steel bar according to one embodiment of the present invention has the effect of preventing brittleness that may occur suddenly while significantly increasing impact toughness even at low temperatures of -10°C and -40°C as described above.

[0050] Accordingly, the steel bar according to one embodiment of the present invention is guaranteed to have ultra-high strength with a yield strength (YS) of 600 MPa or more at room temperature and low temperature (-10°C and -40°C), and also has seismic performance with a yield ratio (YR) of 0.79% or less.

[0051] Below, the role and content of each alloy element included in the steel bar according to one embodiment of the present invention will be described in detail.

[0052] carbon (C)

[0053] Carbon (C) is the most effective and important element for increasing the strength of steel. Carbon (C) is dissolved in austenite and forms martensite during quenching.

[0054] Carbon (C) improves quenching hardness as the carbon content increases, but it also increases the possibility of deformation during quenching. Carbon (C) combines with elements such as iron, chromium, molybdenum, and vanadium to form carbides, thereby improving strength and hardness. To this end, the steel bar according to one embodiment of the present invention may preferably contain 0.14 to 0.43 wt% of carbon (C).

[0055] Silicon (Si)

[0056] Silicon (Si) is a residual substance from pig iron and deoxidizers and is dissolved in ferrite unless it forms compounds such as SiO2, so it does not significantly affect the mechanical properties of steel.

[0057] Additionally, silicon (Si) is a powerful deoxidizer. While adding up to 4.5 wt% improves strength, adding more than 2% reduces toughness and impairs plastic workability, so its addition amount is limited. It also has the effect of increasing softening resistance during tempering.

[0058] At this time, the steel bar according to one embodiment of the present invention may preferably contain 0.05 to 0.3 wt% of silicon (Si).

[0059] manganese (Mn)

[0060] Manganese (Mn) is usually contained in steel at 0.35 to 1.0 wt%.

[0061] Some manganese (Mn) is dissolved in steel, and some can combine with sulfur (S) contained in the steel to form the non-metallic inclusion MnS. MnS is ductile and elongates in the processing direction during plastic working. However, the formation of MnS reduces the sulfur content in the steel, weakening the grains and inhibiting the formation of FeS, a low-melting-point compound.

[0062] Manganese (Mn) impairs the acid and oxidation resistance of steel, but improves the yield strength (YS) by fine-tuning pearlite and solid-solution strengthening ferrite. Manganese (Mn) increases the hardening depth during quenching, but when contained in large amounts, it can cause quench cracking or deformation.

[0063] To this end, the steel bar according to one embodiment of the present invention may preferably contain 0.40 to 1.75 wt% of manganese (Mn).

[0064] Person (P)

[0065] Phosphorus (P) is not a problem if it is distributed uniformly throughout the steel, but it usually forms harmful compounds of Fe3P. This Fe3P is extremely brittle and segregates, so it does not become homogenized even after annealing and becomes elongated during processing such as forging and rolling.

[0066] Phosphorus (P) reduces impact resistance, promotes brittleness, and improves machinability in free-cutting steels, but is generally considered a harmful element to steel.

[0067] At this time, the steel bar according to one embodiment of the present invention may preferably contain phosphorus (P) in an amount of more than 0.0 wt% to 0.04 wt%.

[0068] Yellow (S)

[0069] Sulfur (S) typically combines with manganese, zinc, titanium, and molybdenum to improve the machinability of steel. It also combines with manganese (Mn) to form MnS inclusions. When manganese (Mn) is insufficient in steel, sulfur (S) combines with iron to form FeS. This FeS is highly brittle and has a low melting point, causing cracking during hot and cold working.

[0070] Therefore, to avoid the formation of these FeS inclusions, the ratio of manganese to sulfur is set at 5 to 1.

[0071] To this end, the steel bar according to one embodiment of the present invention may preferably contain sulfur (S) in an amount of more than 0.0 wt% to 0.04 wt%.

[0072] copper (Cu)

[0073] Copper (Cu) typically exists in steel at 0.1 to 0.3 wt%. At room temperature, it dissolves in ferrite at concentrations of up to 0.35 wt%, exhibiting a solid-solution strengthening effect. This slightly improves strength and hardness, but reduces elongation. Steels containing copper (Cu) have problems with hot workability, and particularly when Cu is present at concentrations exceeding 0.5 wt%, it can cause red-hot embrittlement.

[0074] Additionally, even a small amount of copper (Cu) can significantly increase corrosion resistance in the atmosphere or seawater. When copper (Cu) is added in amounts exceeding 0.4 wt%, a precipitation hardening effect due to fine precipitation of copper (Cu) can occur.

[0075] At this time, the steel bar according to one embodiment of the present invention may preferably contain copper (Cu) in an amount of more than 0.0 wt% to 0.5 wt%.

[0076] Vanadium (V)

[0077] Vanadium (V) has a small drag force on grain boundaries when in a solid solution state within the austenite temperature range, so it does not interfere with the recrystallization behavior of austenite particles.

[0078] Vanadium (V) is an element with a significant precipitation strengthening effect by precipitating vanadium carbonitride [V(C, N)] within ferrite during the transformation from austenite to ferrite. Furthermore, the precipitation of vanadium carbonitride [V(C, N)] reduces the amount of carbon (C) and nitrogen (N) dissolved in the matrix, thereby increasing impact toughness.

[0079] At this time, the steel bar according to one embodiment of the present invention may preferably contain vanadium (V) in an amount exceeding 0.002 wt% to 0.15 wt%, and more preferably in an amount exceeding 0.002 wt% to 0.043 wt%.

[0080] chromium (Cr)

[0081] Chromium (Cr) is a ferrite stabilizing element. When added to C-Mn steel, it inhibits the diffusion of carbon (C) due to its solute-interfering effect, thereby affecting grain refinement. Even a trace amount of 0.03 to 0.8% chromium (Cr) can produce this effect.

[0082] At this time, the steel bar according to one embodiment of the present invention may preferably contain chromium (Cr) in an amount exceeding 0.002 wt% to 1.0 wt%, and more preferably in an amount exceeding 0.14 wt% to 0.20 wt%.

[0083] molybdenum (Mo)

[0084] Molybdenum (Mo) can improve hardenability up to 10 times that of nickel even when added in amounts of 0.03 to 0.3 wt%, and prevents tempering embrittlement, thereby providing tempering embrittlement resistance.

[0085] Molybdenum (Mo) forms carbides, making it an effective alloying element for advanced cutting tools and increasing grain coarsening temperature. Molybdenum (Mo) is more effective when used with chromium (Cr) than alone to enhance hardenability, but its high price is a drawback.

[0086] At this time, the steel bar according to one embodiment of the present invention may preferably contain molybdenum (Mo) in an amount of more than 0.015 wt% to 0.085 wt%, and more preferably in an amount of 0.03 wt% to 0.05 wt%.

[0087] Nickel (Ni)

[0088] Nickel (Ni), along with chromium, is one of the most important and common alloying elements. It refines the steel structure and is readily incorporated into austenite and ferrite, making it useful for matrix strengthening. When combined with chromium (Cr) or molybdenum (Mo), it exhibits excellent hardenability, facilitating the heat treatment of large steels. It enhances the low-temperature toughness of steel without compromising weldability or malleability. However, because nickel is a high alloying element, its composition must be controlled to minimize cost.

[0089] At this time, the high-strength rebar for cryogenic use according to one embodiment of the present invention may contain nickel (Ni) in an amount of more than 0.0 wt% to 0.25 wt%.

[0090] Sn

[0091] Tin (Sn) exists in a solid solution state within steel, which can improve properties and corrosion resistance. In an embodiment of the present invention, a high-strength rebar for cryogenic use may contain tin (Sn) in an amount of 0.1% by weight or less.

[0092] Aluminum (Al)

[0093] Aluminum (Al) is an element that acts as a strong deoxidizer, improves toughness by refining the austenite grain size, and forms non-metallic inclusions.

[0094] In an embodiment of the present invention, a cryogenic high-strength rebar may contain 0.050 wt% or less of aluminum (Al).

[0095] Nitrogen (N)

[0096] Even trace amounts of nitrogen (N) significantly affect the mechanical properties of steel, increasing tensile strength and yield strength while decreasing elongation. In particular, a significant decrease in impact strength and an increase in transition temperature are observed.

[0097] Nitrogen (N) is an interstitial element with a rapid diffusion rate and a continuous solubility in ferrite, ranging from a maximum of 0.1 wt% to 0.003 wt%. Furthermore, nitrogen (N) forms nitrides with titanium, zirconium, vanadium, niobium, and other elements, thereby refining grain sizes.

[0098] At this time, the steel bar according to one embodiment of the present invention may preferably contain nitrogen (N) in an amount of 0.040 wt% or less.

[0099] Boron (B)

[0100] Boron (B) stabilizes grain boundaries within steel, significantly improving hardenability with small additions. However, excessive additions can cause segregation at grain boundaries, lowering the steel's toughness and other physical properties.

[0101] At this time, the steel bar according to one embodiment of the present invention may contain boron (B) in an amount of more than 0.001 wt% to 0.004 wt%.

[0102] Steel bar manufacturing method

[0103] A method for manufacturing a steel bar according to one embodiment of the present invention may include, as illustrated in FIG. 1, (a) a reheating step (S10), (b) a hot rolling step (S20), and (c) a water cooling step (S30).

[0104] First, the steel contains carbon (C) 0.14 to 0.43 wt%, manganese (Mn) 0.4 to 1.75 wt%, silicon (Si) 0.05 to 0.30 wt%, phosphorus (P) 0.0 wt% to 0.04 wt%, sulfur (S) 0.0 wt% to 0.04 wt%, chromium (Cr) 0.002 wt% to 1.0 wt%, copper (Cu) 0.0 wt% to 0.5 wt%, nickel (Ni) 0.0 wt% to 0.25 wt%, molybdenum (Mo) 0.015 wt% to 0.085 wt%, aluminum (Al) 0.040 wt% or less, vanadium (V) 0.002 wt% to 0.15 wt%, and nitrogen (N). It may contain less than 0.040 wt% and the remainder iron (Fe) and unavoidable impurities.

[0105] In addition, the steel according to one embodiment of the present invention may further include at least one of niobium (Nb) exceeding 0.001 wt% to 0.15 wt%, tin (Sn) 0.1 wt% or less, calcium (Ca) 0.05 wt% or less, and boron (B) exceeding 0.001 wt% to 0.004 wt%.

[0106] (a) In the reheating step (S10), the steel material can be reheated from a semi-finished product, a billet, to 1,000 to 1,080°C.

[0107] And, (b) in the hot rolling step (S20), the steel material reheated in (a) the reheating step (S10) can be hot rolled. It can be configured to start hot rolling at a rolling start temperature of 950 to 1050°C, perform continuous rolling, and perform final finishing rolling at a rolling end temperature of 960 to 1,020°C.

[0108] At this time, 960 ℃ at the rolling end temperature is the temperature at which the Gibbs free energy of the two phases is the same, and the reason for completing the final hot deformation at 960 to 1,020 ℃ is to promote phase transformation and ultimately form an austenite structure.

[0109] To explain more specifically, when the strain is increased to the critical strain at the critical temperature, fine ferrite can be formed through fine austenite having high-angle grain boundaries by continuous dynamic recrystallization of austenite, and the plastic deformation capacity and low-temperature stability can be maximized through the ultra-fine-grained ferrite formed in this way.

[0110] And, (c) in the water cooling stage (S30), cooling can be performed by intermediate control cooling (ICC). Intermediate control cooling (ICC) is a cooling method that induces internal structural changes in the steel bar by spraying water as a cooling medium at high pressure / uniformly in the middle of the final cooling completion stage immediately after hot deformation.

[0111] At this time, intermediate control cooling (ICC) controls the cooling rate to 150 to 400 ℃ / sec, and the water cooling end temperature can be controlled to 500 to 650 ℃. By controlling the cooling temperature after hot deformation, the steel bar is manufactured to have a structure that satisfies seismic performance and low-temperature toughness even in the deep part.

[0112] Accordingly, the steel bar that has undergone the step (c) above can satisfy a yield ratio (YR) indicating seismic performance at room temperature of 0.77% or less, a yield ratio (YR) indicating seismic performance at -10°C of 0.78% or less, and a yield ratio (YR) indicating seismic performance at -40°C of 0.79% or less. In addition, the steel bar that has undergone the step (c) above can satisfy an elongation (EL) at -10°C of 9.8% or more and an elongation (EL) at -40°C of 9.3% or more.

[0113] Accordingly, the method for manufacturing a bar according to one embodiment of the present invention can satisfy the alloy composition standard of KS D 3866, which is a domestic hot-rolled bar standard for architectural structures. At the same time, the method for manufacturing a bar according to one embodiment of the present invention can manufacture a high-performance bar having both seismic performance and ultra-high strength performance, with a yield ratio (YR) of less than 0.8% and a yield strength (YS) of 600 MPa or more.

[0114] Comparative and experimental examples

[0115] Hereinafter, preferred comparative examples and experimental examples are presented to aid understanding of the present invention. However, the following experimental examples are provided solely to aid understanding of the present invention, and the present invention is not limited to the following experimental examples.

[0116] Tables 1 to 3 below show the major alloy compositions (unit: wt%) of the experimental examples and comparative examples. Table 4 below shows the process conditions for manufacturing the specimens of the experimental examples and comparative examples. Tables 5 and 6 below show the results of measuring the mechanical properties of the specimens manufactured according to the process conditions of Table 4.

[0117]

[0118]

[0119]

[0120]

[0121]

[0122]

[0123] Comparative goods and invention goods

[0124] Let us examine in detail comparative materials 1 and 2 of the comparative examples and inventive materials 1, 2, 3, 4, and 5 of the experimental examples.

[0125] In comparison with comparative materials 1 and 2, inventive materials 1, 2, 3, 4, and 5 (reducing expensive vanadium (V), replacing Mo, Cr, and B) have a similar yield strength of 600 MPa.

[0126] However, in the case of invention materials 1 to 5, the tensile strength can be increased, thereby realizing a low yield ratio (YR) (less than 0.8%) indicating earthquake resistance. In addition, in the case of invention materials 1 to 5, stable yield ratio (YR) and elongation (EL) can be satisfied in low-temperature environments of -10℃ and -40℃ similar to polar regions.

[0127] Therefore, in the case of invention materials 1 to 5, it is possible to secure economic feasibility with a small amount of alloy input and reduce the production cost of high-performance structural steel bars for building structures.

[0128] Furthermore, the weld crack susceptibility evaluation results confirmed that excellent weldability was also secured. Ultimately, through the development of deep structures in the steel bar, which were developed through the vanadium (V)-substituted low-cost component system and the intermediate controlled cooling (ICC) process applied after the final rolling, ultra-high strength, excellent seismic performance at both room and low temperature environments, and high-performance weldability were secured.

[0129] 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 can 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) 0.14 to 0.43 wt%, manganese (Mn) 0.40 to 1.75 wt%, silicon (Si) 0.05 to 0.30 wt%, phosphorus (P) 0.0 wt% to 0.04 wt%, sulfur (S) 0.0 wt% to 0.04 wt%, chromium (Cr) 0.002 wt% to 1.0 wt%, copper (Cu) 0.0 wt% to 0.5 wt%, nickel (Ni) 0.0 wt% to 0.25 wt%, molybdenum (Mo) 0.015 wt% to 0.085 wt%, aluminum (Al) 0.040 wt% or less, vanadium (V) 0.002 wt% to 0.15 wt%, nitrogen (N) Contains 0.040 wt% or less and the remainder iron (Fe) and unavoidable impurities, Satisfying the yield strength (YS) of 600 MPa or more at room temperature, Bonggang.

2. In paragraph 1, The yield ratio (YR) indicating seismic performance at room temperature is 0.77% or less, the yield ratio (YR) indicating seismic performance at -10℃ is 0.78% or less, and the yield ratio (YR) indicating seismic performance at -40℃ is 0.79% or less. Bonggang.

3. In paragraph 1, The elongation (EL) at -10℃ is 9.8% or more and the elongation (EL) at -40℃ is 9.3% or more. Bonggang.

4. In paragraph 1, Containing ultrafine-grained ferrite in the final microstructure, Bonggang.

5. In paragraph 1, Further comprising at least one of niobium (Nb) exceeding 0.001 wt% to 0.15 wt%, tin (Sn) 0.1 wt% or less, calcium (Ca) 0.05 wt% or less, and boron (B) exceeding 0.001 wt% to 0.004 wt%. Bonggang.

6. In paragraph 1, Boron (B) containing more than 0.001 wt% to 0.0015 wt%, Bonggang.

7. In paragraph 1, In the first paragraph, Vanadium (V) preferably comprises more than 0.002 wt% to 0.043 wt%, Bonggang. 8.(a) Carbon (C) 0.14 to 0.43 wt%, manganese (Mn) 0.4 to 1.75 wt%, silicon (Si) 0.05 to 0.30 wt%, phosphorus (P) 0.0 wt% to 0.04 wt%, sulfur (S) 0.0 wt% to 0.04 wt%, chromium (Cr) 0.002 wt% to 1.0 wt%, copper (Cu) 0.0 wt% to 0.5 wt%, nickel (Ni) 0.0 wt% to 0.25 wt%, molybdenum (Mo) 0.015 wt% to 0.085 wt%, aluminum (Al) 0.040 wt% or less, vanadium (V) 0.002 wt% to 0.15 wt%, nitrogen (N) A step of reheating a steel bar containing 0.040 wt% or less and the remainder iron (Fe) and unavoidable impurities to 1,000 to 1,080°C; (b) a step of hot rolling the above-mentioned steel bar, controlling the rolling start temperature to 950 to 1050 ℃ and the rolling end temperature to 960 to 1,020 ℃; and (c) a step of cooling the above-mentioned steel bar; including, Method for manufacturing steel bars.

9. In paragraph 8, Step (b) above, Reheating the steel in a semi-finished billet state. Method for manufacturing steel bars.

10. In paragraph 8, Step (c) above, Cooling is performed using intermediate controlled cooling (ICC). Method for manufacturing steel bars.

11. In paragraph 8, Step (c) above, Controlling the cooling speed to 150 to 400 ℃ / sec, Method for manufacturing steel bars.

12. In paragraph 8, Step (c) above, Controlling the water cooling end temperature to 500 to 650 ℃, Method for manufacturing steel bars.

13. In paragraph 8, The steel bar that performed the above step (c) is The yield ratio (YR) indicating seismic performance at room temperature is 0.77% or less, the yield ratio (YR) indicating seismic performance at -10℃ is 0.78% or less, and the yield ratio (YR) indicating seismic performance at -40℃ is 0.79% or less. Method for manufacturing steel bars.

14. In paragraph 8, The steel bar that performed the above step (c) is - Elongation at break (EL) of 9.8% or more at -10℃, Method for manufacturing steel bars.

15. In paragraph 8, The steel bar that performed the above step (c) is - Elongation at break (EL) of 9.3% or more at -40℃, Method for manufacturing steel bars.

16. In paragraph 8, The steel bar that performed the above step (c) is Containing ultrafine-grained ferrite in the final microstructure, Method for manufacturing steel bars.

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