High-performance steel bar and manufacturing method therefor

Abstract

A high-performance steel bar, according to one embodiment of the present invention comprises 0.08-0.26 wt% of carbon (C), 0.50 wt% or less (excluding 0) of silicon (Si), 0.6-3.0 wt% of manganese (Mn), 0.025 wt% or less (excluding 0) of phosphorus (P), 0.025 wt% or less (excluding 0) of sulfur (S), 0.002-0.50 wt% of chromium (Cr), 0.50 wt% or less (excluding 0) of copper (Cu), 0.01-3.40 wt% of nickel (Ni), 0.002-0.10 wt% of molybdenum (Mo), 0.005-0.040 wt% of aluminum (Al), 0.10 wt% or less (excluding 0) of vanadium (V), 0.1 wt% or less (excluding 0) of niobium (Nb), 0.001-0.050 wt% of titanium (Ti), 0.015 wt% or less (excluding 0) of nitrogen (N), and the balance of iron (Fe) and inevitable impurities, wherein the room-temperature yield strength (YS) satisfies at least 500 MPa.

Description

High-performance steel bar and its manufacturing method

[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 have strong bonding strength with concrete, and they are widely used in construction and civil engineering sites because they help to reduce the width of cracks in concrete by supplementing concrete that is vulnerable to tensile stress.

[0003] These deformed bars or reinforcing bars are used today as a core material in various civil engineering structures such as bridges, large offshore structures, underground structures, and storage facilities.

[0004] Recently, demand for and interest in liquefied natural gas (LNG) has been increasing globally due to the Russo-Ukrainian War, national nuclear phase-out policies, and the expansion of green energy sources. Furthermore, advances in natural gas extraction technology have led to increased recoverable reserves, leading to a surge in demand for new and renewable energy sources like wind and solar. Specifically, LNG's share of the overall energy mix is ​​projected to increase by 46.1%, from 2.9 billion TOU in 2013 to 4.2 billion TOU by 2040.

[0005] Meanwhile, natural gas undergoes a refining and liquefaction process before being transported to LNG terminals for storage. Natural gas is liquefied at temperatures below -170°C (ultra-low temperature) to become liquefied natural gas (LNG). LNG storage tanks, designed to store this ultra-low temperature LNG, require materials that can withstand ultra-low temperatures.

[0006] LNG tanks are largely composed of an inner shell and an outer shell, each constructed of 9% nickel steel and reinforced concrete, respectively. While LNG tanks were previously designed as a double-protection system with a protective wall built around the outside, the design evolved in the late 1980s to a fully protected system with the inner and outer shells sealed tightly together. This design prevents LNG leakage due to inner shell failure, while the outer shell prevents ground and air leakage.

[0007] Therefore, for the deformed steel bars used in LNG storage tanks, properties that can withstand -170℃ are required to maintain their structure without brittle fracture even when the temperature drops rapidly due to LNG leakage.

[0008] 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 having excellent mechanical properties such as toughness and ductility even in an extremely low-temperature environment and a method for manufacturing the same.

[0009] In addition, the purpose of the present invention is to provide a high-performance steel bar having stable performance in room temperature and cryogenic environments and a method for manufacturing the same.

[0010] 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.

[0011] According to one embodiment of the present invention, a high-performance steel bar comprises carbon (C) 0.08 to 0.26 wt%, silicon (Si) 0.50 wt% or less (excluding 0), manganese (Mn) 0.6 to 3.0 wt%, phosphorus (P) 0.025 wt% or less (excluding 0), sulfur (S) 0.025 wt% or less (excluding 0), chromium (Cr) 0.002 to 0.50 wt%, copper (Cu) 0.50 wt% or less (excluding 0), nickel (Ni) 0.01 to 3.40 wt%, molybdenum (Mo) 0.002 to 0.10 wt%, aluminum (Al) 0.005 to 0.040 wt%, vanadium (V) 0.10 wt% or less (excluding 0), niobium (Nb) 0.1 wt% or less (excluding 0), titanium (Ti) 0.001 to It contains 0.050 wt%, nitrogen (N) 0.015 wt% or less (excluding 0), and the remainder iron (Fe) and unavoidable impurities, and satisfies a room temperature yield strength (YS) of 500 MPa or more.

[0012] In addition, it may further include at least one of tin (Sn) 0.15 wt% or less, magnesium (Mg) 0.10 wt% or less, and calcium (Ca) 0.05 wt% or less.

[0013] And, the average grain size of the ferrite structure can be 5.0 to 13.2 ㎛.

[0014] Additionally, the ductile-brittle transition temperature (DBTT) may be below -41℃.

[0015] And, the final microstructure may include polygonal ferrite, acicular ferrite, pearlite, and bainite.

[0016] Additionally, the room temperature elongation can be greater than 14%.

[0017] Meanwhile, a method for manufacturing a high-performance steel bar according to an embodiment of the present invention comprises: (a) carbon (C) 0.08 to 0.26 wt%, silicon (Si) 0.50 wt% or less (excluding 0), manganese (Mn) 0.6 to 3.0 wt%, phosphorus (P) 0.025 wt% or less (excluding 0), sulfur (S) 0.025 wt% or less (excluding 0), chromium (Cr) 0.002 to 0.50 wt%, copper (Cu) 0.50 wt% or less (excluding 0), nickel (Ni) 0.01 to 3.40 wt%, molybdenum (Mo) 0.002 to 0.10 wt%, aluminum (Al) 0.005 to 0.040 wt%, vanadium (V) 0.10 wt% or less (excluding 0), niobium (Nb) 0.1 wt% or less (excluding 0), A method for producing a steel product, comprising: (a) a step of reheating a steel material containing 0.001 to 0.050 wt% of titanium (Ti), 0.015 wt% or less of nitrogen (N) (excluding 0), and the remainder of iron (Fe) and unavoidable impurities; (b) a step of hot-rolling the steel material by controlling the rolling start temperature to 950 to 1100°C and the rolling end temperature to 800 to 1000°C; and (c) a step of cooling the steel material.

[0018] In addition, the steel may further include at least one of tin (Sn) of 0.15 wt% or less, magnesium (Mg) of 0.10 wt% or less, and calcium (Ca) of 0.05 wt% or less.

[0019] In addition, the above step (b) can be controlled so that the rolling end temperature is below the recrystallization stop temperature of the steel.

[0020] In addition, the step (c) is performed by accelerated control cooling (ACC), and can be cooled at a cooling rate of 30 to 400°C / sec and to a final cooling temperature of 500 to 700°C.

[0021] In addition, the steel bar that has undergone the above step (c) may have an average grain size of the ferrite structure of 5.0 to 13.2 ㎛ or less.

[0022] In addition, the steel bar that has undergone the above step (c) may have a ductile-brittle transition temperature (DBTT) of -41°C or lower.

[0023] The steel bar that has undergone the above step (c) may have a microstructure including polygonal ferrite, bainite, needle-shaped ferrite, and pearlite.

[0024] According to a high-performance steel bar and a manufacturing method thereof according to one embodiment of the present invention, excellent mechanical properties such as room temperature properties, low temperature toughness, ultra-low temperature toughness, and ductility can be secured.

[0025] 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.

[0026] Figure 1 is a flowchart illustrating the process sequence of a method for manufacturing high-performance steel bars according to one embodiment of the present invention.

[0027] Figure 2 shows the final microstructure photographs of the comparative and inventive materials.

[0028] 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.

[0029] 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.

[0030] “And / or” includes any combination of one or more of the associated constructs that can be defined.

[0031] 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.

[0032] 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.

[0033] 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 to have 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.

[0034] 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.

[0035] Unless otherwise specified, the notation 'A to B' for numerical values ​​A and B means 'A or more and B or less'. In such notation, if a unit is attached only to numerical value B, the unit shall be applied to numerical value A as well.

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

[0037]

[0038] High-performance steel bar

[0039] According to one embodiment of the present invention, a high-performance steel bar comprises carbon (C) 0.08 to 0.26 wt%, silicon (Si) 0.50 wt% or less (excluding 0), manganese (Mn) 0.6 to 3.0 wt%, phosphorus (P) 0.025 wt% or less (excluding 0), sulfur (S) 0.025 wt% or less (excluding 0), chromium (Cr) 0.002 to 0.50 wt%, copper (Cu) 0.50 wt% or less (excluding 0), nickel (Ni) 0.01 to 3.40 wt%, molybdenum (Mo) 0.002 to 0.10 wt%, aluminum (Al) 0.005 to 0.040 wt%, vanadium (V) 0.10 wt% or less (excluding 0), niobium (Nb) 0.1 wt% or less (excluding 0), titanium (Ti) 0.001 to Contains 0.050 wt% of iron, 0.015 wt% or less of nitrogen (N) (excluding 0), and the remainder of iron (Fe) and unavoidable impurities.

[0040] A high-performance steel bar according to one embodiment of the present invention including the above-described alloy composition satisfies a room temperature yield strength (YS) of 500 MPa or more.

[0041] The high-performance steel bar according to the present invention can secure excellent properties at room temperature, toughness at low temperatures, and both toughness and ductility in ultra-low temperature environments such as -170°C. More specifically, the high-performance steel bar according to the present invention exhibits improved low-temperature toughness along with strength in ultra-low temperature environments, thereby preventing brittle fracture even in the event of rapid temperature drops when applied to ultra-low temperature structures, thereby exhibiting excellent product characteristics.

[0042] Therefore, the high-performance steel bar according to the present invention can be applied not only to the outer shell of an LNG storage tank, but also to civil engineering and architectural structures in polar regions, and can be utilized in various ways as a safety steel material.

[0043] Meanwhile, a high-performance steel bar according to one embodiment of the present invention may further include at least one of 0.15 wt% or less of tin (Sn), 0.10 wt% or less of magnesium (Mg), and 0.05 wt% or less of calcium (Ca).

[0044] 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.

[0045]

[0046] carbon (C)

[0047] Carbon (C) is the most effective and important element for increasing the strength of steel. It is dissolved in austenite and forms martensite during quenching. While increasing carbon content improves quench hardness, it can also cause deformation during quenching, reducing the steel's elongation and low-temperature toughness. It combines with elements such as iron (Fe), chromium (Cr), molybdenum (Mo), and vanadium (V) to form carbides, enhancing strength and hardness.

[0048] In consideration of this, the high-performance steel bar according to one embodiment of the present invention may contain carbon (C) in an amount of 0.08 to 0.26 wt%, and preferably 0.09 to 0.17 wt%.

[0049]

[0050] Silicon (Si)

[0051] Silicon (Si), a residue from pig iron and deoxidizers, is dissolved in ferrite and has little effect on the mechanical properties of steel unless it forms compounds such as SiO2. As a powerful deoxidizer, adding up to 4.5 wt% improves strength. However, adding more than 2 wt% reduces toughness and impairs plastic workability, so its use is limited. It also increases softening resistance during tempering.

[0052] At this time, the high-performance steel bar according to one embodiment of the present invention may contain silicon (Si) in an amount of 0.50 wt% or less (excluding 0), but preferably 0.151 to 0.50 wt%.

[0053]

[0054] manganese (Mn)

[0055] Some of the manganese (Mn) contained in steel is dissolved in the steel, and some combines with sulfur contained in the steel to form MnS, a non-metallic inclusion. This MnS is ductile and elongates in the working direction during plastic working. However, the formation of MnS reduces the sulfur (S) content in the steel, weakening the grains and suppressing the formation of FeS, a low-melting-point compound. In addition, when added at 1.4 to 2.0 wt% as an austenite stabilizing element, it is advantageous for the formation of acicular ferrite and bainite.

[0056] And, steel with 1.0 to 1.5% manganese added is called strong steel because it imparts viscosity to the steel.

[0057] In consideration of this, the high-performance steel bar according to one embodiment of the present invention can induce the formation of a microstructure advantageous for improving ultra-low temperature toughness and reducing the content of expensive nickel (Ni) by including manganese (Mn) in an amount of 0.60 to 3.0 wt%, preferably 0.76 to 1.82 wt%, more preferably 1.01 to 1.62 wt%.

[0058]

[0059] Person (P)

[0060] Phosphorus (P) is not a problem if distributed uniformly throughout the steel, but it usually forms harmful Fe3P compounds. This Fe3P is extremely brittle and segregates, so it does not homogenize even after annealing and elongates during processing such as forging and rolling. Furthermore, while phosphorus (P) reduces impact resistance, promotes temper embrittlement, and improves machinability in free-cutting steels, it is generally considered a harmful element for steel, so its compositional content is controlled.

[0061] In consideration of this, the high-performance steel according to one embodiment of the present invention may contain phosphorus (P) in an amount of 0.025 wt% or less (excluding 0).

[0062]

[0063] Yellow (S)

[0064] Sulfur (S) typically combines with manganese, zinc, titanium, and molybdenum to improve the machinability of steel. It combines with manganese to form MnS inclusions. If manganese is insufficient in steel, it combines with iron to form FeS. This FeS is highly brittle and has a low melting point, causing cracking during hot and cold working.

[0065] The sulfur (S) composition was controlled to improve elongation and secure low-temperature toughness while avoiding the formation of FeS inclusions.

[0066] To this end, the high-performance steel bar according to one embodiment of the present invention may contain sulfur (S) in an amount of 0.025 wt% or less (excluding 0).

[0067]

[0068] chromium (Cr)

[0069] Chromium (Cr) is a ferrite-stabilizing element. When added to C-Mn steel, it inhibits carbon diffusion due to its solute-interfering effect, thereby affecting grain refinement. It also plays a role in improving hardenability. In one embodiment of the present invention, high-performance steel bars may contain 0.002 to 0.5 wt% of chromium (Cr).

[0070]

[0071] copper (Cu)

[0072] Copper (Cu) dissolves in ferrite at room temperature in concentrations of up to 0.35 wt% and exhibits a solid-solution strengthening effect, slightly improving strength and hardness but reducing elongation. Steels containing copper have problems with hot workability, and particularly when copper content exceeds 0.5 wt%, it can cause red-hot embrittlement.

[0073] To this end, 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 (excluding 0).

[0074]

[0075] Nickel (Ni)

[0076] 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.

[0077] In consideration of this, the high-performance steel bar according to one embodiment of the present invention may contain nickel (Ni) in an amount of 0.01 to 3.4 wt%, but preferably 0.03 to 0.58 wt%.

[0078]

[0079] molybdenum (Mo)

[0080] Molybdenum (Mo) can enhance hardenability by up to 10 times that of nickel (Ni), even in small amounts, and it also prevents temper 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. While it is more effective when combined with chromium to enhance hardenability than when used alone, its high price is a drawback.

[0081] At this time, the high-performance steel bar according to one embodiment of the present invention may preferably contain molybdenum (Mo) in an amount of 0.002 to 0.10 wt%.

[0082]

[0083] Aluminum (Al)

[0084] Aluminum (Al) acts as a powerful deoxidizer during the production of steel and controls non-metallic oxide inclusions in the steel. However, excessive addition can cause manufacturing problems such as nozzle clogging, so it is necessary to control the amount added.

[0085] In consideration of this, the high-performance steel bar according to one embodiment of the present invention may contain 0.005 to 0.040 wt% of aluminum (Al).

[0086]

[0087] Vanadium (V)

[0088] Vanadium (V) is a representative microalloying element with a strong carbide-forming ability. It refines the steel structure and helps improve strength and toughness. Small additions also contribute to strength enhancement by forming precipitates. However, vanadium oxide (V2O5), an oxide, has a high vapor pressure and evaporates at high temperatures, so it can be added in amounts of 0.10 wt% or less.

[0089] At this time, the high-performance steel bar according to one embodiment of the present invention may contain vanadium (V) in an amount of 0.10 wt% or less (excluding 0), but preferably 0.05 wt% or less, and more preferably 0.003 to 0.040 wt%.

[0090]

[0091] niobium (Nb)

[0092] Niobium (Nb) segregates or precipitates at the grain boundaries of austenite grains, inducing the solute drag effect and pinning effect. This delays recrystallization during high-temperature rolling, thereby increasing the recrystallization arrest temperature, thereby contributing to grain refinement of the final ferrite. It also forms fine precipitates, which helps improve strength. However, when added in excess of 0.10 wt%, it can cause coarse precipitation and carbon deficiency in general grains, thereby reducing strength.

[0093] In a high-performance steel bar according to one embodiment of the present invention, niobium (Nb) may be included in an amount of 0.10 wt% or less (excluding 0), preferably 0.05 wt% or less (excluding 0), and more preferably 0.003 to 0.040 wt%.

[0094]

[0095] titanium (Ti)

[0096] Titanium (Ti) is an element that contributes to improving strength by combining with carbon (C) and nitrogen (N) contained in steel to form carbides (TiC), nitrides (TN), or carbonitrides (TiCN). However, when added in amounts exceeding 0.1 wt%, coarsening of carbides can cause nozzle clogging during casting, or a decrease in strength and ductility due to carbon deficiency in the steel.

[0097] A high-performance steel bar according to one embodiment of the present invention may contain 0.001 to 0.050 wt% of titanium (Ti).

[0098]

[0099] Nitrogen (N)

[0100] Nitrogen (N), even in extremely small amounts, has a significant effect on the mechanical properties of steel, increasing tensile strength and yield strength while decreasing elongation. In particular, the reduction in impact value and the increase in transition temperature are remarkable. Nitrogen refines austenite grains, enabling the production of fine-grained steel, and forms nitrides with titanium, zirconium, vanadium, niobium, etc. to further refine grains. However, when present in large amounts, it can reduce high-temperature toughness, cause intergranular embrittlement due to AlN precipitation at austenite grain boundaries, and reduce high-temperature creep strength.

[0101] Therefore, in a high-performance steel bar according to one embodiment of the present invention, nitrogen (N) may be included at 0.015 wt% or less (excluding 0).

[0102]

[0103] In addition, in a high-performance steel bar according to one embodiment of the present invention, one or more of tin (Sn) 0.15 wt% or less, magnesium (Mg) 0.10 wt% or less, and calcium (Ca) 0.05 wt% or less may be further included.

[0104]

[0105] Sn

[0106] Tin (Sn) can increase the tensile strength and yield strength of steel and reduce elongation and impact strength. It can also cause red-hot embrittlement, tempering embrittlement, and low-temperature embrittlement during hot working, and improve corrosion resistance.

[0107] To this end, the high-performance steel bar according to one embodiment of the present invention may contain tin (Sn) in an amount of 0.15 wt% or less.

[0108]

[0109] Magnesium (Mg)

[0110] Magnesium (Mg) has a stronger affinity for oxygen (O2) or sulfur (S) than titanium (Ti), and thus easily forms fine inclusions or precipitates, thereby contributing to deoxidation, improved cleanliness, and increased strength.

[0111] In consideration of this, the high-performance steel bar according to one embodiment of the present invention may contain magnesium (Mg) in an amount of 0.10 wt% or less.

[0112]

[0113] Calcium (Ca)

[0114] Calcium (Ca) is a powerful deoxidizer and induces the formation of CaS in the MnS inclusions formed during steelmaking, which causes them to separate from the steel and, even if they remain, do not elongate like MnS during rolling, preventing deterioration of mechanical properties.

[0115] To this end, the high-performance steel according to one embodiment of the present invention may contain calcium (Ca) in an amount of 0.05 wt% or less.

[0116]

[0117] Meanwhile, in a high-performance steel bar according to one embodiment of the present invention, the high-temperature precipitation strengthening effect of a stable nitride of the vanadium (V) series and the solute drag effect and pinning effect of niobium (Nb) delay the grain growth of initial austenite, thereby increasing the recrystallization stop temperature at which recrystallization does not occur, and delaying the recrystallization phenomenon during high-temperature rolling increases the recrystallization stop temperature.

[0118] Ultimately, by refining the grain size of the final ferrite, a low ductile-brittle transition temperature (DBTT) can be secured, thereby satisfying stability and economy at ultra-low temperatures.

[0119] At this time, the ductile-brittle transition temperature (DBTT) refers to the temperature at which a metal material with brittle properties at low temperatures rapidly changes into a material with ductile properties as its ductile properties increase due to active dislocation slip phenomenon as atomic diffusion becomes easier as the temperature increases. The lower the DBTT, the better the low-temperature toughness can be judged.

[0120] Therefore, in a high-performance steel bar according to one embodiment of the present invention, the recrystallization stopping temperature at which recrystallization due to hot deformation does not occur can be increased by adding micro-alloying elements, vanadium (V) and niobium (Nb).

[0121] In particular, in a high-performance steel bar according to one embodiment of the present invention, when hot deformation is applied below the recrystallization stop temperature, instead of continuous dynamic recrystallization of austenite, pancake-shaped particles are formed, and the final ferrite grain size becomes finer, increasing the grain boundary fraction per unit area.

[0122] At this time, in the high-performance steel according to one embodiment of the present invention, as the grain boundary fraction per unit area increases, the number of nucleation sites during phase transformation increases, so that fine ferrite can be formed, and ultra-low temperature stability can be maximized through the ultra-fine grain ferrite thus formed.

[0123] Here, the ferrite structure may include polygonal ferrite and needle-shaped ferrite, and the average grain size of the ferrite structure may be 5.0 to 13.2 ㎛, but preferably 5.0 to 8.0 ㎛, and more preferably 5.0 to 7.5 ㎛.

[0124] Meanwhile, low-temperature toughness can be confirmed through the average grain size of ferrite and the ductile-brittle transition temperature (DBTT). According to one embodiment of the present invention, the ductile-brittle transition temperature (DBTT) may be -41°C or lower, and preferably -50 to -75°C.

[0125] In other words, it can be confirmed that the high-performance steel bar according to one embodiment of the present invention can have excellent low-temperature toughness by securing a significantly low ductile-brittle transition temperature (DBTT) along with a fine-grained ferrite structure.

[0126] In addition, the high-performance steel according to one embodiment of the present invention may have a notch sensitivity ratio (NSR) of 1.02 or more at -170°C.

[0127] A high-performance steel bar according to one embodiment of the present invention may have a final microstructure including polygonal ferrite, bainite, needle-shaped ferrite, and pearlite.

[0128]

[0129] Manufacturing method of high-performance steel bar

[0130] Figure 1 is a flowchart illustrating the process sequence of a method for manufacturing high-performance steel bars according to one embodiment of the present invention.

[0131] Figure 2 shows the final microstructure photographs of the comparative and inventive materials.

[0132] Referring to FIG. 1, a method for manufacturing a high-performance steel bar according to one embodiment of the present invention includes (a) a reheating step, (b) a hot rolling step, and (c) a cooling step.

[0133] More specifically, (a) carbon (C) 0.08 to 0.26 wt%, silicon (Si) 0.50 wt% or less (excluding 0), manganese (Mn) 0.6 to 3.0 wt%, phosphorus (P) 0.025 wt% or less (excluding 0), sulfur (S) 0.025 wt% or less (excluding 0), chromium (Cr) 0.002 to 0.50 wt%, copper (Cu) 0.50 wt% or less (excluding 0), nickel (Ni) 0.01 to 3.40 wt%, molybdenum (Mo) 0.002 to 0.10 wt%, aluminum (Al) 0.005 to 0.040 wt%, vanadium (V) 0.10 wt% or less (excluding 0), niobium (Nb) 0.1 wt% or less (excluding 0), titanium (Ti) 0.001 to A method for hot rolling a steel material comprising: a step of reheating a steel material containing 0.050 wt% of iron (Fe), 0.015 wt% or less of nitrogen (N) (excluding 0), and the remainder of iron (Fe) and unavoidable impurities; (b) a step of hot rolling the steel material by controlling the rolling start temperature to 950 to 1100°C and the rolling end temperature to 800 to 1000°C; and (c) a step of cooling the steel material.

[0134] Here, the steel may be an ingot or a billet manufactured through a continuous casting process prior to the reheating step. Therefore, the alloying elements and compositional ranges contained in the steel can be understood to be identical to those contained in high-performance steel bars manufactured through ingots and billets.

[0135] At this time, the reason for controlling the rolling end temperature, which is the hot deformation finishing temperature range of the steel, to 800 to 1000℃ is to promote phase transformation through hot deformation below the recrystallization stop temperature and ultimately form a fine ferrite structure.

[0136] That is, due to the high-temperature precipitation strengthening effect of stable nitrides of the vanadium (V) series and the solute drag effect and pinning effect of niobium (Nb), the grain growth of the initial austenite is delayed, and the recrystallization stop temperature at which recrystallization due to hot deformation does not occur is raised. When deformation is applied below the recrystallization stop temperature, the final ferrite grain size becomes finer due to continuous dynamic recrystallization of austenite, and the grain boundary fraction per unit area increases.

[0137] Through this, the number of nucleation sites during phase transformation of steel can be increased, thereby minimizing ferrite, and ultra-low temperature stability can be maximized through the ultra-fine grained ferrite formed in this way.

[0138] Ultimately, by obtaining a fine grain size of ferrite, the ductile-brittle transition temperature (DBTT) can be significantly lowered.

[0139] And, in the above step (a), the recrystallization temperature (RST) of the steel may increase due to the delay in grain growth of the initial austenite.

[0140] At this time, in the step (b), the rolling end temperature, which is the hot deformation finishing temperature range of the steel, can be controlled to be below the recrystallization stop temperature of the steel.

[0141] And, in the step (c), the steel can be cooled by accelerated controlled cooling (ACC), wherein the accelerated controlled cooling (ACC) is a cooling process that induces internal and external structural changes in the steel by spraying a tempcore and water as a cooling medium at high pressure / uniformly in the middle of the final cooling completion stage immediately after hot deformation of the steel.

[0142] In the method for manufacturing a high-performance steel bar according to one embodiment of the present invention, in step (c), the steel material can be cooled at a cooling rate of 30 to 400°C / sec.

[0143] And, in the method for manufacturing high-performance steel bars according to one embodiment of the present invention, in step (c), the steel material can be cooled to a final cooling temperature (recuperation temperature) of 500 to 700°C.

[0144] Ultimately, the method for manufacturing high-performance steel bars according to one embodiment of the present invention can realize steel bars having a structure that satisfies high strength and low-temperature toughness even at ultra-low temperatures by hot-deforming steel and then controlling the reheating and cooling temperature through accelerated controlled cooling (ACC).

[0145] At this time, the steel material that has performed the above step (c) can have the ferrite refined by increasing the number of nucleation sites through phase transformation while increasing the grain boundary fraction per unit area, so that the average grain size of the ferrite structure can be 5.0 to 13.2 ㎛, preferably 5.0 to 8.0 ㎛, and more preferably 5.0 to 7.5 ㎛.

[0146] The steel material that has undergone the above step (c) may have a final microstructure including polygonal ferrite, needle-shaped ferrite, pearlite, and bainite, as shown in Fig. 2.

[0147] And, the steel material that has performed the above step (c) can secure a ductile-brittle transition temperature (DBTT) of -41°C or lower, and preferably -50 to -75°C, by implementing a fine-grained ferrite structure.

[0148] In addition, the high-performance steel bar manufactured by the above-described manufacturing method within the composition range of the above-described alloy components may have a yield strength of 1.15 times or more the yield strength at room temperature, a uniform elongation of 3% or more, and a notch sensitivity (NSR) of 1.0 or more in an ultra-low temperature environment of -165 to -170°C, and the notch sensitivity (NSR) may preferably satisfy 1.02 or more.

[0149]

[0150] Comparative and experimental examples

[0151] 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.

[0152] Tables 1 and 2 below show the main alloy compositions (unit: weight%) that make up the invention material of this experimental example and the comparative material of the comparative example, Table 3 below shows the operating conditions for manufacturing each specimen of this experimental example and the comparative example, and Tables 4 and 5 show the results of measuring the mechanical properties of specimens implemented according to the operating conditions for manufacturing each specimen of this experimental example and the comparative example.

[0153] The mechanical properties of this comparative material and the inventive material were measured according to the KS D 3504 and ISO 15630-1 standards.

[0154] 'YS' stands for yield strength, 'TS' stands for tensile strength, 'EL' stands for elongation, 'UE' stands for uniform elongation, and 'NSR' stands for notch sensitivity ratio, which is calculated as the ratio of tensile strength of notched specimen to yield strength of unnotched specimen. 'un' stands for unnotched specimen, and 'n' stands for notched specimen. Room temperature corresponds to 25℃.

[0155] Classification Chemical composition (weight%) CSiMnPSCuCrMoNiComparative material 10.070.151.630.0100.0100.150.200.020.57Comparative material 20.270.121.000.0260.0240.230.110.020.02Comparative material 30.070.120.750.0100.0100.230.100.050.60Inventive material 10.130.221.370.0150.0100.2 00.100.020.16Inventive 20.080.271.440.0090.0050.100.100.010.17Inventive 30.080.301.480.0090.0020.130.150.030.17Inventive 40.070.301.490.0080.0020.200.080.020.18Inventive 50.100.331.480.0090.0010.160.110.010.04

[0156] Classification Chemical composition (weight%) CEQRSTVNbAlCaN Comparative material 10.0020.0030.0020.0020.0090.43843 Comparative material 20.0030.0020.0150.0010.0100.48950 Comparative material 30.0020.0010.0150.0010.0100.28861 Inventive material 10.0020.0180.0250.00 10.0040.41901Inventive Material 20.0010.0130.0200.0010.0030.36841Inventive Material 30.0010.0170.0240.0020.0060.38847Inventive Material 40.0010.0140.0290.0030.0050.36833Inventive Material 50.0370.0380.0370.0020.0080.39947

[0157] Looking at Tables 1 and 2 above, Comparative Materials 1 to 3 contain niobium (Nb) in amounts of 0.003 wt%, 0.002 wt%, and 0.001 wt%, respectively, and Inventive Materials 1 to 5 contain niobium (Nb) in amounts of 0.018 wt%, 0.013 wt%, 0.017 wt%, 0.014 wt%, and 0.038 wt%, respectively.

[0158] In addition, while comparative materials 1 to 3 contain 0.002 wt%, 0.003 wt%, and 0.002 wt% of vanadium (V), inventive materials 1 to 5 contain 0.002 wt%, 0.001 wt%, 0.001 wt%, 0.001 wt%, and 0.037 wt% of vanadium (V), respectively.

[0159] This difference in the content of niobium (Nb) affects the recrystallization stop temperature (RST), where the grain growth of the initial austenite is delayed by the solute drag effect and pinning effect of niobium (Nb), thereby increasing the recrystallization stop temperature at which recrystallization due to hot deformation does not occur. In this way, the inventive material can achieve refinement of the average grain size of ferrite and improvement of the room temperature elongation and low temperature toughness by adding vanadium (V) and niobium (Nb) and increasing their content compared to the comparative material, as shown in Table 4 below.

[0160] More specifically, the inventive material has a high content of vanadium (V) and niobium (Nb) added compared to the comparative material, which enhances the high-temperature precipitation of stable V-based nitrides and increases the recrystallization arrest temperature at which recrystallization does not occur due to the delay in initial austenite grain growth caused by the solute attraction effect of Nb. When deformation is applied below the recrystallization arrest temperature, the final ferrite grain size is refined through continuous dynamic recrystallization of austenite, thereby increasing the grain boundary fraction per unit area. Through this, the ferrite structure can be refined by increasing the nucleation sites during phase transformation, and the ultra-fine-grained ferrite thus formed can maximize ultra-low-temperature stability.

[0161] Classification DiameterOperating ConditionsRolling Start Temperature (℃)Rolling End Temperature (℃)Cooling Water Amount (m) 3 / hr) Recovery temperature (℃) Comparative material 1D16 1050 900~1000℃ range, each performed above the recrystallization stop temperature (RST) 1005520 D32 1050 1005573 Comparative material 2D25 11001005585 Comparative material 3D32 11001005585 Inventive material 1D16 1050 800~1000℃ range, each performed below the recrystallization stop temperature (RST) 1005525 D32 1050 1005569 Inventive material 2D16 1050 1005523 D32 1050 1005571 Inventive material 3D25 1050 720 565 Inventive material 4D25 1050 720 580 Inventive material 5D25 1050 720 598

[0162] Classification DiameterRoom temperature propertyGrain sizeLow temperature toughnessYS(MPa)EL(%)FerriteAverage grain size(㎛)DBTT(℃)Comparative material 1D1655318.413.3-40D3254017.4Comparative material 2D2557512.512.6-21Comparative material 3D2545713.612.1-28Inventive material 1D1658326.37.2-53D3254724.0Inventive material 2D1656227.26.8-59D3255725.7Inventive material 3D2558826.67.1-59Inventive material 4D2556724.36.8-59Inventive material 5D2556624.05.1-72

[0163] Low temperature Physical properties (-170℃) YS_un (MPa) UE_un (%) TS_n (MPa) NSR Comparative material 1 D168089.28251.02 D327767.17921.02 Comparative material 2 D258224.17560.92 Comparative material 3 D257429.37050.95 Inventive material 1 D1682110.28461.03 D327968.78121.02 Inventive material 2 D1678110.48131.04 D327559.27781.03 Inventive material 3 D257599.67941.05 Inventive material 4 D257579.97941.05 Inventive material 5 D251,01616.710251.01

[0164] With reference to Tables 1 to 5 above, let us examine comparative materials 1 to 3 and inventive materials 1 to 5 in detail.

[0165] Comparative materials 1 to 3 were each subjected to final finishing rolling at a temperature exceeding the recrystallization stop temperature (RST) of each comparative material within the range of 900 to 100°C, which is the finishing temperature of hot deformation.

[0166] And, for invention materials 1 to 5, the final finishing rolling was performed at a temperature lower than the recrystallization stop temperature (RST) of each comparative material within the range of 800 to 1000℃, which is the finishing temperature of hot deformation.

[0167] At this time, the comparative materials were subjected to final finishing rolling at a rolling end temperature higher than the recrystallization stop temperature (RST), thereby forming a coarse average grain size of the ferrite structure.

[0168] However, the inventors formed a very fine average grain size of ferrite structure by performing the final finishing rolling at a rolling end temperature controlled below the recrystallization stop temperature (RST).

[0169] This fact can be confirmed in Figure 2. Looking at Figure 2, it can be easily confirmed with the naked eye that the final structure of the comparative material has a coarse particle size, whereas the final structure of the inventive material has a very fine particle size.

[0170] This difference in the average grain size of the ferrite structure affects the low-temperature toughness of the steel bar. While the ductile-brittle transition temperature (DBTT) of the comparative material was above -40℃, the invented materials had a stable low-temperature impact performance of -50 to -75℃, confirming that the low-temperature impact performance was improved compared to the comparative material.

[0171] Meanwhile, it can be confirmed that the elongation at room temperature (EL) of invention materials 1 to 5 is greater than that of comparative materials 1 to 3.

[0172] In addition, it can be confirmed that the unnotch specimen uniform elongation (UE_un) of invention materials 1 to 5 is also higher than that of comparative materials 1 to 3.

[0173] In addition, it can be confirmed that the notch sensitivity ratio (NSR), which is the value of tensile strength (TS_n) / yield strength (YS_un) at -170℃, is superior to the comparative material.

[0174] 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) 0.08 to 0.26 wt%, silicon (Si) 0.50 wt% or less (excluding 0), manganese (Mn) 0.6 to 3.0 wt%, phosphorus (P) 0.025 wt% or less (excluding 0), sulfur (S) 0.025 wt% or less (excluding 0), chromium (Cr) 0.002 to 0.50 wt%, copper (Cu) 0.50 wt% or less (excluding 0), nickel (Ni) 0.01 to 3.40 wt%, molybdenum (Mo) 0.002 to 0.10 wt%, aluminum (Al) 0.005 to 0.040 wt%, vanadium (V) 0.10 wt% or less (excluding 0), niobium (Nb) 0.1 wt% or less (excluding 0), titanium (Ti) 0.001 to Contains 0.050 wt% of iron (Fe), 0.015 wt% or less of nitrogen (N) (excluding 0), and the remainder of iron (Fe) and unavoidable impurities. The yield strength at room temperature (YS) is 500 MPa or more. High performance steel.

2. In paragraph 1, Containing at least one of tin (Sn) of 0.15 wt% or less, magnesium (Mg) of 0.10 wt% or less, and calcium (Ca) of 0.05 wt% or less. High performance steel.

3. In paragraph 1, The average grain size of the ferrite structure is 5.0 to 13.2 ㎛. High performance steel.

4. In paragraph 1, The ductile brittle transition temperature (DBTT) is -41℃ or lower. High performance steel.

5. In paragraph 1, The final microstructure includes polygonal ferrite, acicular ferrite, pearlite, and bainite. High performance steel.

6. In paragraph 1, The elongation at room temperature is 14% or more, High performance steel. 7.(a) Carbon (C) 0.08 to 0.26 wt%, Silicon (Si) 0.50 wt% or less (excluding 0), Manganese (Mn) 0.6 to 3.0 wt%, Phosphorus (P) 0.025 wt% or less (excluding 0), Sulfur (S) 0.025 wt% or less (excluding 0), Chromium (Cr) 0.002 to 0.50 wt%, Copper (Cu) 0.50 wt% or less (excluding 0), Nickel (Ni) 0.01 to 3.40 wt%, Molybdenum (Mo) 0.002 to 0.10 wt%, Aluminum (Al) 0.005 to 0.040 wt%, Vanadium (V) 0.10 wt% or less (excluding 0), Niobium (Nb) 0.1 wt% or less (excluding 0), Titanium (Ti) 0.001 to A step of reheating steel containing 0.050 wt% of steel, 0.015 wt% or less of nitrogen (N) (excluding 0), and the remainder of iron (Fe) and unavoidable impurities; (b) a step of hot rolling the steel by controlling the rolling start temperature to 950 to 1100℃ and the rolling end temperature to 800 to 1000℃; and (c) comprising a step of cooling the steel material; Method for manufacturing high-performance steel bars.

8. In paragraph 7, The above steel further contains at least one of tin (Sn) of 0.15 wt% or less, magnesium (Mg) of 0.10 wt% or less, and calcium (Ca) of 0.05 wt% or less. Method for manufacturing high-performance steel bars.

9. In paragraph 7, Step (b) above, The rolling end temperature is controlled below the recrystallization stop temperature of the steel. Method for manufacturing high-performance steel bars.

10. In paragraph 7, Step (c) above, It is performed by accelerated controlled cooling, Cooled at a cooling rate of 30 to 400℃ / sec and cooled to a final cooling temperature of 500 to 700℃. Method for manufacturing high-performance steel bars.

11. In paragraph 7, The steel bar that performed the above step (c) is The average grain size of the ferrite structure is 5.0 to 13.2 ㎛. Method for manufacturing high-performance steel bars.

12. In paragraph 7, The steel bar that performed the above step (c) is The ductile brittle transition temperature (DBTT) is -41℃ or lower. Method for manufacturing high-performance steel bars.

13. In paragraph 7, The steel bar that performed the above step (c) is The final microstructure comprises polygonal ferrite, acicular ferrite, pearlite and bainite. Method for manufacturing high-performance steel bars.

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