CN114080466A - Reinforcing bar and method for manufacturing same - Google Patents

Reinforcing bar and method for manufacturing same Download PDF

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Publication number
CN114080466A
CN114080466A CN202080034356.0A CN202080034356A CN114080466A CN 114080466 A CN114080466 A CN 114080466A CN 202080034356 A CN202080034356 A CN 202080034356A CN 114080466 A CN114080466 A CN 114080466A
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equal
weight
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steel
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郑准镐
金泰亨
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Korea Advanced Institute of Science and Technology KAIST
Korea University Research and Business Foundation
Hyundai Steel Co
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Hyundai Steel Co
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
    • C21D8/065—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires of ferrous alloys
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/009—Pearlite

Abstract

There is provided a reinforcing bar comprising: 0.07 to 0.43 wt% of carbon (C), 0.5 to 2.0 wt% of manganese (Mn), 0.05 to 0.5 wt% of silicon (Si), greater than 0 and less than or equal to 0.5 wt% of chromium (Cr), greater than 0 and less than or equal to 4.5 wt% of copper (Cu), greater than 0 and less than or equal to 0.003 wt% of boron (B), greater than 0 and less than or equal to 0.25 wt% of vanadium (V), greater than 0 and less than or equal to 0.012 wt% of nitrogen (N), greater than 0 and less than or equal to 0.03 wt% of phosphorus (P), greater than 0 and less than or equal to 0.03 wt% of sulfur (S), 0.01 to 0.5 wt% in total of nickel (Ni), niobium (Nb), and titanium (Ti), the balance being iron (Fe) and other unavoidable impurities. The final microstructure includes ferrite, bainite, pearlite, retained austenite and precipitates including copper.

Description

Reinforcing bar and method for manufacturing same
Technical Field
Exemplary embodiments of the present invention relate to a reinforcing bar and a method of manufacturing the same, and more particularly, to a high-strength reinforcing bar having excellent fatigue resistance and a method of manufacturing the same.
Background
Recently, in order to make better use of space when mounting structures, the mounted structures have become larger and longer. According to analysis, the global continuous warming caused by global environmental pollution can cause unexpected natural disasters or climate change. Meanwhile, it has been pointed out that a major factor of global warming is CO2Is generated. By arranging the high-strength reinforcing bars, dense arrangement of the reinforcing bars can be solved because the number of the reinforcing bars is reduced. Thus, when ultra-high strength steel bars are used, 0.4 tons of CO are generated per 1 ton of steel bar produced2Can be reduced to 0.2 ton CO per household2. Therefore, a reinforcing bar having a higher strength than the previous reinforcing bar is required. For example, a steel bar having a yield strength of 500MPa is recently required to have a yield strength of 600MPa to 700MPa, and a steel bar having a yield strength of about 1.0GPa is expected to be required in the future. However, it is important to ensure safety against loads due to natural disasters such as earthquakes and the self weight of buildings that are becoming larger and longer, as well as to increase the strength of the reinforcing bars. When the amount of the alloying element is simply increased, problems such as an increase in cost, a crack defect in the steel bar, and a decrease in toughness and ductility of the steel bar due to the addition of a large amount of the alloying element may occur. In addition, in order to make the surface of the product hardThe hot core tempering process applied by the tempering becomes a burden in productivity.
As a related art, there is korean patent application laid-open No.10-2003-0095071 (entitled "method for manufacturing high-strength hot-dip galvanized steel bars of high yield ratio").
Disclosure of Invention
Technical problem
An object of the present invention is to provide a high-strength reinforcing bar and a method for manufacturing the same, which can reduce the cost of manufacturing the high-strength reinforcing bar and minimize a hot core tempering process, thereby not reducing productivity and not requiring introduction of new equipment.
Technical scheme
A reinforcing bar according to an exemplary embodiment of the present invention includes: 0.07 to 0.43% by weight of carbon (C), 0.5 to 2.0% by weight of manganese (Mn), 0.05 to 0.5% by weight of silicon (Si), greater than 0 and less than or equal to 0.5% by weight of chromium (Cr), greater than 0 and less than or equal to 4.5% by weight of copper (Cu), greater than 0 and less than or equal to 0.003% by weight of boron (B), greater than 0 and less than or equal to 0.25% by weight of vanadium (V), greater than 0 and less than or equal to 0.012% by weight of nitrogen (N), greater than 0 and less than or equal to 0.03% by weight of phosphorus (P), greater than 0 and less than or equal to 0.03% by weight of sulfur (S), one or more of nickel (Ni), niobium (Nb) and titanium (Ti) in a total amount of 0.01 to 0.5% by weight, the balance being iron (Fe) and other unavoidable impurities, wherein the final microstructure comprises ferrite, bainite, pearlite, retained austenite and precipitates comprising copper.
In the steel bar, the final microstructure may have a bainite fraction of 90% or more and a retained austenite fraction of 5% or less.
The steel bar may have a Yield Strength (YS) of 750MPa or more, a Tensile Strength (TS) of 1000MPa or more, an elongation of 11% or more, and a tensile strength to yield strength ratio (TS/YS) of 1.25 or more.
A method of manufacturing a reinforcing bar according to an embodiment of the present invention includes: (a) reheating the steel at a temperature of 1050 ℃ to 1230 ℃, wherein the steel comprises 0.07 wt% to 0.43 wt% of carbon (C), 0.5 wt% to 2.0 wt% of manganese (Mn), 0.05 wt% to 0.5 wt% of silicon (Si), greater than 0 and less than or equal to 0.5 wt% of chromium (Cr), greater than 0 and less than or equal to 4.5 wt% of copper (Cu), greater than 0 and less than or equal to 0.003 wt% of boron (B), greater than 0 and less than or equal to 0.25 wt% of vanadium (V), more than 0 and less than or equal to 0.012 wt% nitrogen (N), more than 0 and less than or equal to 0.03 wt% phosphorus (P), more than 0 and less than or equal to 0.03 wt% sulfur (S), one or more of nickel (Ni), niobium (Nb), and titanium (Ti) in a total amount of 0.01 to 0.5 wt%, the balance being iron (Fe) and other unavoidable impurities; (b) hot rolling the reheated steel at a finish rolling temperature of 950 ℃ to 1020 ℃; and (c) age heat treating the hot rolled steel at a temperature of 400 ℃ to 600 ℃ for 15 minutes to 60 minutes.
In the method of manufacturing the steel bar, the final microstructure after performing the step (c) may include ferrite, bainite, pearlite, residual austenite, and precipitates including copper.
In the method of manufacturing the steel bar, the final microstructure may have a bainite fraction of 90% or more and a retained austenite fraction of 5% or less.
In the method of manufacturing a reinforcing bar, the reinforcing bar after performing step (c) may have a Yield Strength (YS) of 750MPa or more, a Tensile Strength (TS) of 1000MPa or more, an elongation of 11% or more, and a ratio of tensile strength to yield strength (TS/YS) of 1.25 or more.
Advantageous effects
According to the embodiments of the present invention, it is possible to provide a high-strength steel bar and a method for manufacturing the same, which can reduce the cost of manufacturing the high-strength and high-toughness steel bar and minimize a hot core tempering process, thereby not reducing productivity and not requiring the introduction of new equipment. The scope of the present invention is not limited by the above-described effects.
Drawings
Fig. 1 is a flowchart schematically illustrating a method of manufacturing a reinforcing bar according to an exemplary embodiment of the present invention.
Fig. 2 is a graph illustrating the hardness of each sample according to an experimental example of the present invention.
Fig. 3 is a graph illustrating the hardness of each sample according to an experimental example of the present invention.
Fig. 4 is a photograph obtained by photographing the final microstructure of a sample according to an experimental example of the present invention.
Detailed Description
A reinforcing bar and a method of manufacturing the same according to an embodiment of the present invention will be described in detail. The terms described below are appropriately selected in consideration of the functions in the present invention, and the definitions of the terms should be defined based on the contents of the entire specification. Hereinafter, a high-strength steel bar and a method for manufacturing the same are provided, which are capable of partially reducing expensive alloying elements and minimizing a hot core tempering process so as not to lower productivity.
Reinforcing bar
A reinforcing bar according to an exemplary embodiment of the present invention includes: 0.07 to 0.43 wt% of carbon (C), 0.5 to 2.0 wt% of manganese (Mn), 0.05 to 0.5 wt% of silicon (Si), greater than 0 and less than or equal to 0.5 wt% of chromium (Cr), greater than 0 and less than or equal to 4.5 wt% of copper (Cu), greater than 0 and less than or equal to 0.003 wt% of boron (B), greater than 0 and less than or equal to 0.25 wt% of vanadium (V), greater than 0 and less than or equal to 0.012 wt% of nitrogen (N), greater than 0 and less than or equal to 0.03 wt% of phosphorus (P), greater than 0 and less than or equal to 0.03 wt% of sulfur (S), 0.01 to 0.5 wt% in total of nickel (Ni), niobium (Nb), and titanium (Ti), the balance being iron (Fe) and other unavoidable impurities.
Hereinafter, the effects and contents of the components included in the reinforcing steel bar according to an embodiment of the present invention will be described.
Carbon (C)
Carbon (C) is the most effective and important element for improving the strength of steel. In addition, carbon is added and dissolved in austenite during quenching to form a martensitic structure. As the amount of carbon increases, the quenching hardness increases, but the possibility of deformation during quenching increases. In addition, carbon (C) combines with elements such as iron, chromium, molybdenum, and vanadium to form carbides, thereby improving strength and hardness. According to an embodiment of the present invention, the carbon (C) may be added in an amount of 0.07 to 0.43 wt% based on the total weight of the reinforcing bar. If the content of carbon is less than 0.07 wt% of the total weight, the above effects may not be achieved and it may be difficult to secure sufficient strength. If the content of carbon exceeds 0.43 wt% of the total weight, excessive strength and poor weldability may occur.
Manganese (Mn)
Manganese (Mn) is partially dissolved in steel and partially combined with sulfur contained in steel to form MnS, a kind of nonmetallic inclusion, which has ductility and is elongated in a working direction during plastic working. However, since the formation of MnS reduces the sulfur component in the steel, the crystal grains are weakened and the formation of FeS, which is a low melting point compound, is suppressed. Manganese inhibits acid resistance and oxidation resistance of steel, but increases yield strength by refining pearlite and solid-solution strengthening ferrite. According to an embodiment of the present invention, manganese may be added in an amount of 0.5 to 2.0 wt% based on the total weight of the reinforcing bar. If the content of manganese is less than 0.5% by weight, the above-mentioned effect of adding manganese cannot be sufficiently exhibited. Further, if the content of manganese exceeds 2.0 wt%, quench cracking or deformation is induced, so that weldability may be deteriorated, and MnS inclusions and center segregation may be generated, which may reduce ductility and corrosion resistance of the steel bar.
Silicon (Si)
Silicon (Si) is an element known to stabilize ferrite and is also an element known to increase ductility by increasing the fraction of ferrite during cooling. Meanwhile, in the steel-making process, silicon and aluminum are added as a deoxidizer together so as to remove oxygen in steel, and the silicon can also play a solid solution strengthening role. According to an embodiment of the present invention, the silicon may be added in an amount of 0.05 to 0.5 wt% based on the total weight of the reinforcing bar. If the content of silicon is less than 0.05 wt% based on the total weight, the above-mentioned effect of adding silicon cannot be sufficiently exerted. If a large amount of silicon exceeding 0.5 wt% of the total weight is added, toughness is reduced, plastic workability is reduced, weldability of steel is reduced, softening resistance during tempering is increased, and red scale is generated during reheating and hot rolling, which may cause problems in surface quality.
Chromium (Cr)
Chromium (Cr) is an element stabilizing ferrite, and when added to C — Mn steel, it retards the diffusion of carbon due to solute interference effect, thereby affecting grain size refinement. According to an embodiment of the present invention, the chromium may be added in an amount of more than 0 and less than or equal to 0.5 wt% based on the total weight of the steel bar. If a large amount of chromium content exceeding 0.5 wt% of the total weight is added, toughness may be reduced and workability and machinability may be deteriorated.
Copper (Cu)
Copper (Cu) is an element that improves hardenability and low-temperature impact toughness of steel. Copper is solid-dissolved in ferrite at normal temperature and exhibits a solid-solution strengthening effect, so that strength and hardness are slightly improved, but elongation is reduced. According to an embodiment of the present invention, the copper may be added in an amount of more than 0 and less than or equal to 4.5 wt% based on the total weight of the reinforcing steel. If a large amount of copper content exceeding 4.5 wt% of the total weight is added, hot workability may be deteriorated, hot brittleness may be caused, and the surface quality of the product may be deteriorated.
Boron (B)
Boron (B) is an important element for ensuring hardenability. According to an embodiment of the present invention, the amount of boron added may be greater than 0 and less than or equal to 0.003 wt% of the total weight of the steel reinforcement. If a large amount of boron content exceeding 0.003 wt% of the total weight is added, the addition effect is saturated and the elongation may be reduced, so that the upper limit is preferably limited to 0.003 wt% or less.
Vanadium (V)
Vanadium (V) is a component useful for solid solution strengthening and precipitation strengthening, has a carbide forming ability stronger than that of chromium, and has a function of refining crystal grains, and therefore has an effect of suppressing the amount of added carbon, and is an element contributing to improvement of strength by pinning at grain boundaries. According to an embodiment of the present invention, the vanadium may be added in an amount greater than 0 and less than or equal to 0.25 wt% of the total weight of the steel bar. If a large amount of vanadium is added in excess of 0.25 wt% of the total weight, there is a problem in that the manufacturing cost of the steel is excessively increased as compared with the strength-improving effect.
Nitrogen (N)
Nitrogen (N) is an element that increases strength by precipitating vanadium and nitrides or carbonitrides. According to an embodiment of the present invention, the nitrogen may be added in an amount greater than 0 and less than or equal to 0.012 wt% of the total weight of the rebar. If a large amount of nitrogen content exceeding 0.012 wt% of the total weight is added, nitrogen may act as an element that impairs toughness.
Phosphorus (P)
Phosphorus (P) may increase the strength of steel through solid solution strengthening, and may play a role in inhibiting carbide formation. According to an embodiment of the present invention, the phosphorus may be added in an amount greater than 0 and less than or equal to 0.03 wt% of the total weight of the reinforcing bar. If the phosphorus content exceeds 0.03% by weight, the impact resistance is lowered, the temper brittleness is improved, and the low-temperature impact value is lowered by precipitation behavior.
Sulfur (S)
Sulfur (S) may be combined with manganese, titanium, etc. to improve machinability of steel, and precipitates of fine MnS may be formed to improve workability. According to an embodiment of the present invention, the sulfur may be added in an amount of more than 0 and less than or equal to 0.03 wt% based on the total weight of the steel bar. If the content of sulfur exceeds 0.03 wt%, toughness and weldability may be impaired, and the low-temperature impact value may be reduced. If the steel bar has insufficient manganese content, the sulfur combines with the iron to form FeS. This FeS is very brittle and has a low melting point, which can lead to cracks during hot and cold working. Therefore, in order to avoid the formation of such FeS inclusions, the manganese-sulfur ratio can be adjusted to 5: 1.
Nickel (Ni), niobium (Nb), titanium (Ti)
Nickel (Ni) is an element that increases hardenability and improves toughness, niobium (Nb) is an element that precipitates in the form of NbC or Nb (C, N) to improve the strength of the substrate and the weld region, titanium (Ti) is an element that suppresses AlN formation by forming TiN at high temperature, and plays a role in refining the grain size due to the formation of Ti (C, N), and the like. The reinforcing steel bar according to an embodiment of the present invention may include one or more of nickel (Ni), niobium (Nb), and titanium (Ti), but the total content thereof may be 0.01 wt% to 0.5 wt% of the total weight of the reinforcing steel bar. If the total content of at least one of nickel (Ni), niobium (Nb), and titanium (Ti) contained in the steel bar according to the embodiment of the present invention is less than 0.01 wt%, it is expected that the above-described effects cannot be achieved. If the total content thereof exceeds 0.5 wt%, there may occur problems that the manufacturing cost of the parts is increased, brittle cracks are generated, the carbon content in the matrix is reduced, and the steel characteristics are deteriorated.
The final microstructure of the steel bar having the above-described composition of alloying elements according to an embodiment of the present invention includes ferrite, bainite, pearlite, retained austenite, and precipitates containing copper. Further, in the final microstructure, the bainite fraction may be 90% or more, and the retained austenite fraction may be 5% or less.
Further, the steel bar having the above-described alloying element composition according to an embodiment of the present invention has a Yield Strength (YS) of 750MPa or more, a Tensile Strength (TS) of 1000MPa or more, an elongation of 11% or more, and a ratio of tensile strength to yield strength (TS/YS) of 1.25 or more.
For example, the rebar can have a Yield Strength (YS) of 750MPa to 1000MPa, a Tensile Strength (TS) of 1000MPa to 1300MPa, an elongation of 11% to 25%, and a ratio of tensile strength to yield strength (TS/YS) of 1.25 to 1.40.
Hereinafter, a method of manufacturing a reinforcing bar having the above-described composition of alloying elements according to an embodiment of the present invention will be described.
Method for manufacturing steel bar
Fig. 1 is a flowchart schematically illustrating a method of manufacturing a reinforcing bar according to an embodiment of the present invention.
Referring to fig. 1, a method of manufacturing a reinforcing bar according to an embodiment of the present invention includes: (a) reheating the steel at a temperature of 1050 to 1230 ℃ (S100), wherein the steel comprises 0.07 to 0.43 wt% of carbon (C), 0.5 to 2.0 wt% of manganese (Mn), 0.05 to 0.5 wt% of silicon (Si), greater than 0 and less than or equal to 0.5 wt% of chromium (Cr), greater than 0 and less than or equal to 4.5 wt% of copper (Cu), greater than 0 and less than or equal to 0.003 wt% of boron (B), greater than 0 and less than or equal to 0.25 wt% of vanadium (V), greater than 0 and less than or equal to 0.012 wt% of nitrogen (N), greater than 0 and less than or equal to 0.03 wt% of phosphorus (P), greater than 0 and less than or equal to 0.03 wt% of sulfur (S), one or more of nickel (Ni), niobium (Nb), and titanium (Ti) in a total amount of 0.01 to 0.5 wt%, the balance being iron (Fe), and other unavoidable impurities; (b) hot rolling the reheated steel at a finish rolling temperature of 950 ℃ to 1020 ℃ (S200); and (c) age heat treating the hot rolled steel at 400 to 600 ℃ for 15 to 60 minutes (S300).
The reinforcing bar according to the embodiment of the present invention is manufactured through a reheating process, a hot deformation process, and a cooling process. In the reheating process, the semi-finished blank is reheated to a temperature of 1050 ℃ to 1230 ℃. Then, the hot rolling process is characterized in that the steel is subjected to final finish rolling at a temperature of 950 ℃ to 1020 ℃ while passing through respective rolls (RM, IM and FM) to complete rolling, the rolled steel is air-cooled to a temperature of 400 ℃ to 600 ℃, and then aging heat treatment is performed according to desired physical properties while maintaining the air-cooled steel for a certain time before the cooling stage and at a temperature of 400 ℃ to 600 ℃ for 15 minutes to 60 minutes in the cooling stage.
The steel bar manufacturing/continuous casting process generally consists of an electric furnace, a Ladle Furnace (LF), and continuous casting. In order to improve fatigue resistance, after LF (secondary refining process), a Vacuum Degassing (VD) process may be performed to reduce the oxygen content to a predetermined level or less, and then the steel may be solidified into a semi-finished product in a continuous casting process.
The steel material may include: 0.07 to 0.43 wt% of carbon (C), 0.5 to 2.0 wt% of manganese (Mn), 0.05 to 0.5 wt% of silicon (Si), greater than 0 and less than or equal to 0.5 wt% of chromium (Cr), greater than 0 and less than or equal to 4.5 wt% of copper (Cu), greater than 0 and less than or equal to 0.003 wt% of boron (B), greater than 0 and less than or equal to 0.25 wt% of vanadium (V), greater than 0 and less than or equal to 0.012 wt% of nitrogen (N), greater than 0 and less than or equal to 0.03 wt% of phosphorus (P), greater than 0 and less than or equal to 0.03 wt% of sulfur (S), 0.01 to 0.5 wt% in total of nickel (Ni), niobium (Nb), and titanium (Ti), the balance being iron (Fe) and other unavoidable impurities.
In one embodiment, the steel may be reheated at a temperature of 1050 ℃ to 1230 ℃. When the steel is reheated at the above temperature, the segregation components during the continuous casting process may be solid-dissolved again. The present invention aims to improve strength by precipitation strengthening and solid solution strengthening. Therefore, these elements need to be sufficiently solid-solubilized to austenite before hot rolling, and thus the blank needs to be heated to a temperature of 1050 ℃ or more. If the reheating temperature is lower than 1050 ℃, the solid solution of various carbides may be insufficient, and the segregation components may not be uniformly distributed during the continuous casting process. However, if the reheating temperature exceeds 1230 ℃, there are adverse effects such as coarsening and decarburization of austenite, and the desired strength may not be obtained. That is, if the reheating temperature exceeds 1230 ℃, very coarse austenite grains are formed, and it is difficult to secure the strength. In addition, if the reheating temperature exceeds 1230 ℃, the heating cost may increase and the process time may increase, resulting in an increase in manufacturing cost and a decrease in productivity.
In the hot rolling step (S200), the reheated steel is hot rolled. The hot deformation finishing temperature may range from 950 to 1020 ℃. The hot deformation finishing temperature is a very important factor affecting the final material, and the temperature of 950 ℃ to 1020 ℃ at which rolling is performed is a temperature at which austenite can be refined. However, when rolling is performed at a hot rolling temperature lower than 950 ℃, the rolling load may increase and a double grain size structure may be generated at the edge portion. Further, when rolling is performed at a high temperature range exceeding 1020 ℃, the target mechanical properties may not be obtained due to coarsening of crystal grains.
Immediately after hot rolling and air cooling, the steel is directly placed in a holding tank or bath capable of maintaining a temperature of 400 ℃ to 600 ℃. The temperature of the rod and wire in the heat-preservation bath is about 400 ℃ to 600 ℃. Compared with the steel bar with the yield strength of 600MPa which is tempered by applying a hot core, no martensite is generated on the surface, which is also beneficial to the ductility and the toughness. In addition, since the temperature is maintained at the temperature of the product itself, an additional heat treatment is not required, thereby reducing the production cost. It was confirmed that the holding at a temperature of 400 to 600 ℃ for 15 to 60 minutes during the heating process is advantageous in increasing the strength, the TS/YS ratio and the ductility. The tempering effect at a temperature lower than 400 c is insufficient, and the effect of improving strength at a temperature higher than 600 c is insufficient, and thus a temperature range of 400 c to 600 c is determined.
In the case of a steel bar which has not been subjected to aging heat treatment, its final microstructure has a ferrite structure and a pearlite structure. However, in the case of the aging heat-treated reinforcing steel bar of the present invention, the temperature is maintained to form bainite, and 5% or less of retained austenite is also formed, and the reinforcing steel bar of the present invention can secure the following mechanical properties: a Yield Strength (YS) of 750MPa or more, a Tensile Strength (TS) of 1000MPa or more, an elongation of 11% or more, and a ratio of the tensile strength to the yield strength (TS/YS) of 1.25 or more. That is, by the method of manufacturing a reinforcing bar according to the embodiment of the present invention, an ultra-simple process and a cost-reduced manufacturing method are disclosed, which can ensure high functionality due to formation of nano precipitates by adding copper, and can maximize a sufficient precipitation strengthening effect by adiabatic heat at a specific temperature without performing a hot core tempering process, even in the existing vanadium system. Therefore, even though the process is simplified, the method can achieve high strength, high shock resistance and a high ratio of tensile strength to yield strength, which exceed conventional mechanical properties, and is expected to further improve the stability of the building structure.
Modes for carrying out the invention
Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred embodiments of the present invention. However, the following examples are intended to aid understanding of the present invention, and the scope of the present invention is not limited to the following examples.
Experimental examples
Hereinafter, preferred experimental examples will be presented to aid understanding of the present invention. However, the following experimental examples are intended to aid in understanding the present invention, and the present invention is not limited to the following experimental examples.
1. Preparation of samples
In the present experimental example, samples achieved using the alloy element compositions (unit: weight%) and process conditions of tables 1 and 2 are provided.
[ Table 1]
Figure BDA0003342543780000101
[ Table 2]
Figure BDA0003342543780000102
The compositions disclosed in table 1 satisfy all compositional ranges including: 0.07 to 0.43 wt% of carbon (C), 0.5 to 2.0 wt% of manganese (Mn), 0.05 to 0.5 wt% of silicon (Si), greater than 0 and less than or equal to 0.5 wt% of chromium (Cr), greater than 0 and less than or equal to 4.5 wt% of copper (Cu), greater than 0 and less than or equal to 0.003 wt% of boron (B), greater than 0 and less than or equal to 0.25 wt% of vanadium (V), greater than 0 and less than or equal to 0.012 wt% of nitrogen (N), greater than 0 and less than or equal to 0.03 wt% of phosphorus (P), greater than 0 and less than or equal to 0.03 wt% of sulfur (S), one or more of nickel (Ni), niobium (Nb), and titanium (Ti) in a total amount of 0.01 to 0.5 wt%, the balance being iron (Fe).
Meanwhile, the examples in table 2 satisfy the following process conditions: reheating a steel material having the above composition at a temperature of 1050 ℃ to 1230 ℃; hot rolling the reheated steel at a finish rolling temperature of 950 ℃ to 1020 ℃; the hot rolled steel is subjected to aging heat treatment at a temperature of 400 to 600 ℃ for 15 to 60 minutes. On the other hand, in comparative example 1 of table 2, the hot rolled steel was subjected to the hot core tempering process, but not to the aging heat treatment process. In addition, in comparative example 2, comparative example 3 and comparative example 4 of table 2, the aging heat treatment was performed at a temperature of 500 ℃ to 600 ℃ for 120 minutes, more than 15 minutes to 60 minutes.
2. Evaluation of physical Properties and microstructures
Fig. 2 and 3 are graphs illustrating the hardness of each sample according to an experimental example of the present invention.
Item ■ (example 1) of fig. 2 corresponds to a sample to which the composition conditions of example 1 in table 1 and the reheating temperature and finish rolling temperature process conditions of example 1 in table 2 are applied, and the aging heat treatment is performed on the sample of example 1 at a temperature of 600 ℃ for 0 minute, 30 minutes, 60 minutes, 90 minutes, and 120 minutes. Item ● (example 2) of fig. 2 corresponds to a sample to which the composition conditions of example 2 in table 1 and the reheating temperature and finish rolling temperature process conditions of example 2 in table 2 were applied, and the aging heat treatment was performed on the sample of example 2 at a temperature of 600 ℃ for 0 minute, 30 minutes, 60 minutes, 90 minutes, and 120 minutes. Item a-up (example 3) of figure 2 corresponds to a sample applying the compositional conditions of example 3 in table 1 and the reheating temperature and finishing temperature process conditions of example 3 in table 2 and the ageing heat treatment is carried out on the sample of example 3 at a temperature of 600 ℃ for 0 min, 30 min, 60 min, 90 min and 120 min. Item t' of fig. 2 (comparative example 1) corresponds to a sample to which the composition conditions of comparative example 1 in table 1 and the reheating temperature and finishing temperature process conditions of comparative example 1 in table 2 were applied, and a hot core tempering process under the heat recovery condition of 630 ℃ was performed on the sample of comparative example 1.
Item ■ (example 1) of fig. 3 corresponds to a sample to which the composition conditions of example 1 in table 1 and the reheating temperature and finish rolling temperature process conditions of example 1 in table 2 are applied, and the aging heat treatment is performed on the sample of example 1 at a temperature of 500 ℃ for 0 minute, 30 minutes, 60 minutes, 90 minutes, and 120 minutes. Item ● (example 2) of fig. 3 corresponds to a sample to which the composition conditions of example 2 in table 1 and the reheating temperature and finish rolling temperature process conditions of example 2 in table 2 are applied, and the aging heat treatment is performed on the sample of example 2 at a temperature of 500 ℃ for 0 minute, 30 minutes, 60 minutes, 90 minutes, and 120 minutes. Item a-up (example 3) of figure 3 corresponds to a sample applying the compositional conditions of example 3 in table 1 and the reheating temperature and finishing temperature process conditions of example 3 in table 2 and the ageing heat treatment is carried out on the sample of example 3 at a temperature of 500 ℃ for 0 min, 30 min, 60 min, 90 min and 120 min. Item t' of fig. 3 (comparative example 1) corresponds to a sample to which the composition conditions of comparative example 1 in table 1 and the reheating temperature and finishing temperature process conditions of comparative example 1 in table 2 were applied, and a hot core tempering process under the heat recovery condition of 630 ℃ was performed on the sample of comparative example 1.
[ Table 3]
Figure BDA0003342543780000121
Table 3 shows the strength, Yield Strength (YS), Tensile Strength (TS), elongation and tensile strength to yield strength ratio (TS/YS) of each steel bar subjected to the aging heat treatment for 30 minutes at a temperature of 500 c for each sample according to the experimental example of the present invention. Example 1 in table 3 corresponds to a sample to which the composition condition of example 1 in table 1 and the reheating temperature and finishing temperature process condition of example 1 in table 2 are applied, and the mechanical properties of the sample of example 1 are evaluated after the sample of example 1 is subjected to the aging heat treatment at a temperature of 500 ℃ for 30 minutes. Example 2 in table 3 corresponds to a sample to which the composition condition of example 2 in table 1 and the reheating temperature and finishing temperature process condition of example 2 in table 2 are applied, and the mechanical properties of the sample of example 2 are evaluated after the sample of example 2 is subjected to the aging heat treatment at a temperature of 500 ℃ for 30 minutes. Example 3 in table 3 corresponds to a sample to which the composition condition of example 3 in table 1 and the reheating temperature and finishing temperature process condition of example 3 in table 2 are applied, and the mechanical properties of the sample of example 3 are evaluated after the sample of example 3 is subjected to the aging heat treatment at a temperature of 500 ℃ for 30 minutes. Comparative example 1 in table 3 corresponds to a sample to which the composition conditions of comparative example 1 in table 1 and the reheating temperature and finishing temperature process conditions of comparative example 1 in table 2 are applied, and the mechanical properties of the sample of comparative example 1 are evaluated after the sample of comparative example 1 is subjected to the hot core tempering process without being subjected to the aging heat treatment.
[ Table 4]
Figure BDA0003342543780000131
It was confirmed that holding the sample at a temperature of 400 ℃ to 600 ℃ for 15 minutes to 60 minutes is advantageous in increasing strength, TS/YS ratio and ductility. It was confirmed that in the case of the steel bar which was not subjected to the aging heat treatment, the final microstructure had a ferrite structure and a pearlite structure, whereas in the case of the steel bar which was subjected to the aging heat treatment of the present invention, the temperature was maintained to form bainite, and 5% or less of retained austenite was also formed, and the steel bar of the present invention can secure the following mechanical properties: a Yield Strength (YS) of 750MPa or more, a Tensile Strength (TS) of 1000MPa or more, an elongation of 11% or more, and a ratio of the tensile strength to the yield strength of 1.25 or more.
By the method of manufacturing a reinforcing bar according to an exemplary embodiment of the present invention, an ultra-simple process and a cost-reduced manufacturing method are disclosed, which can ensure high functionality due to formation of nano precipitates by adding copper, and can maximize a sufficient precipitation strengthening effect by adiabatic heat at a specific temperature without performing a hot core tempering process, even in the existing vanadium system. Therefore, even though the process is simplified, the method can achieve high strength, high shock resistance and a high ratio of tensile strength to yield strength, which exceed conventional mechanical properties, and is expected to further improve the stability of the building structure.
While the above description has focused on exemplary embodiments of the invention, various changes or modifications may be made at the level of those skilled in the art. Such changes and modifications may be considered to fall within the scope of the present invention, unless they depart therefrom. Accordingly, the scope of the invention should be determined from the following claims.

Claims (7)

1. A rebar, comprising:
0.07 to 0.43% by weight of carbon (C), 0.5 to 2.0% by weight of manganese (Mn), 0.05 to 0.5% by weight of silicon (Si), greater than 0 and less than or equal to 0.5% by weight of chromium (Cr), greater than 0 and less than or equal to 4.5% by weight of copper (Cu), greater than 0 and less than or equal to 0.003% by weight of boron (B), greater than 0 and less than or equal to 0.25% by weight of vanadium (V), greater than 0 and less than or equal to 0.012% by weight of nitrogen (N), greater than 0 and less than or equal to 0.03% by weight of phosphorus (P), greater than 0 and less than or equal to 0.03% by weight of sulfur (S), one or more of nickel (Ni), niobium (Nb) and titanium (Ti) in a total amount of 0.01 to 0.5% by weight, the balance being iron (Fe) and other unavoidable impurities,
wherein the final microstructure comprises ferrite, bainite, pearlite, retained austenite and precipitates comprising copper.
2. The reinforcing bar of claim 1, wherein the final microstructure has a bainite fraction of 90% or greater and a retained austenite fraction of 5% or less.
3. The reinforcing bar of claim 1, wherein the Yield Strength (YS) is 750MPa or more, the Tensile Strength (TS) is 1000MPa or more, the elongation is 11% or more, and the ratio of the tensile strength to the yield strength (TS/YS) is 1.25 or more.
4. A method of manufacturing a rebar, the method comprising:
(a) reheating the steel at a temperature of 1050 ℃ to 1230 ℃, wherein the steel comprises 0.07 wt% to 0.43 wt% of carbon (C), 0.5 wt% to 2.0 wt% of manganese (Mn), 0.05 wt% to 0.5 wt% of silicon (Si), greater than 0 and less than or equal to 0.5 wt% of chromium (Cr), greater than 0 and less than or equal to 4.5 wt% of copper (Cu), greater than 0 and less than or equal to 0.003 wt% of boron (B), greater than 0 and less than or equal to 0.25 wt% of vanadium (V), more than 0 and less than or equal to 0.012 wt% nitrogen (N), more than 0 and less than or equal to 0.03 wt% phosphorus (P), more than 0 and less than or equal to 0.03 wt% sulfur (S), one or more of nickel (Ni), niobium (Nb), and titanium (Ti) in a total amount of 0.01 to 0.5 wt%, the balance being iron (Fe) and other unavoidable impurities;
(b) hot rolling the reheated steel at a finish rolling temperature of 950 ℃ to 1020 ℃; and
(c) the hot rolled steel is subjected to aging heat treatment at a temperature of 400 to 600 ℃ for 15 to 60 minutes.
5. The method of claim 4, wherein the final microstructure after performing step (c) comprises ferrite, bainite, pearlite, retained austenite, and precipitates comprising copper.
6. The method of claim 5, wherein the final microstructure has a bainite fraction of 90% or greater and a retained austenite fraction of 5% or less.
7. The method of claim 4, wherein the rebar after performing step (c) has a Yield Strength (YS) of 750MPa or greater, a Tensile Strength (TS) of 1000MPa or greater, an elongation of 11% or greater, and a tensile strength to yield strength ratio (TS/YS) of 1.25 or greater.
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