US20220316039A1

US20220316039A1 - Steel reinforcement and method for manufacturing the same

Info

Publication number: US20220316039A1
Application number: US17/609,645
Authority: US
Inventors: Jun Ho Chung; Tae Hyung Kim
Original assignee: Hyundai Steel Co
Current assignee: Hyundai Steel Co
Priority date: 2020-06-19
Filing date: 2020-06-19
Publication date: 2022-10-06
Also published as: CN114080466A; JP7262617B2; JP2022542740A; WO2021256590A1

Abstract

Provided is a steel reinforcement including an amount of 0.07 to 0.43 wt % of carbon (C), an amount of 0.5 to 2.0 wt % of manganese (Mn), an amount of 0.05 to 0.5 wt % of silicon (Si), an amount greater than 0 and less than or equal to 0.5 wt % of chromium (Cr), an amount greater than 0 and less than or equal to 4.5 wt % of copper (Cu), an amount greater than 0 and less than or equal to 0.003 wt % of boron (B), an amount greater than 0 and less than or equal to 0.25 wt % of vanadium (V), an amount greater than 0 and less than or equal to 0.012 wt % of nitrogen (N), an amount greater than 0 and less than or equal to 0.03 wt % of phosphorus (P), an amount greater than 0 and less than or equal to 0.03 wt % of sulfur (S), an amount of 0.01 to 0.5 wt % of the sum of one or more of nickel (Ni), niobium (Nb) and titanium (Ti), the balance of iron (Fe), and other inevitable impurities. A final microstructure includes ferrite, bainite, pearlite, retained austenite, and precipitates comprising copper.

Description

TECHNICAL FIELD

Exemplary embodiments of the present invention relate to a steel reinforcement and a method for manufacturing the same, and more particularly, to a high-strength steel reinforcement having excellent fatigue resistance and a method for manufacturing the same.

BACKGROUND

Recently, in order to make better use of a space in installing structures, the installed structures have become larger and longer. It has been analyzed that unexpected natural disasters or climate changes result from continuous global warming due to pollution of global environment. Meanwhile, it has been pointed out that a main factor of global warming is generation of CO₂. Dense arrangement of steel reinforcements may be solved due to a decrease in an amount of steel reinforcement by arranging a high-strength steel reinforcement. Thus, 0.4 tons of CO₂generated when 1 ton of steel reinforcement is produced may be reduced to 0.2 tons of CO₂per household when an ultra-high-strength steel reinforcement is applied. Accordingly, a steel reinforcement having strength higher than the previous steel reinforcement is required. For example, the steel reinforcement that was required to have a yield strength of 500 MPa is recently required to have a yield strength of 600 to 700 MPa, and it is expected that a steel reinforcement having a yield strength of about 1.0 GPa will be needed in the future. However, it is important not only to increase strength of the steel reinforcement, but also to secure safety against loads due to self-weights of buildings that are becoming larger and longer and natural disasters such as earthquake. In a case of simply increasing an amount of added alloying elements, there occurs a problem such as a cost increase, occurrence of a crack defect in the steel reinforcement, and a decrease in toughness and ductility of the steel reinforcement due to the addition of a large amount of alloys. Furthermore, a tempcore process applied in order to harden a surface of a product becomes a burden in terms of productivity.
As the related art, there is Korean Patent Application Publication No. 10-2003-0095071 (entitled “Method for Manufacturing High-Yield Ratio-Type High-Strength Hot-Dip Galvanized Steel Reinforcement).

SUMMARY OF THE INTENTION

Technical Problem

An object of the present invention is to provide a high-strength steel reinforcement capable of reducing a cost in manufacturing a high-strength steel reinforcement and minimizing a tempcore process so that productivity is not reduced without introducing new facilities, and a method for manufacturing the same.

Technical Solution

A steel reinforcement according to an exemplary embodiment of the present invention includes: an amount of 0.07 to 0.43 wt % of carbon (C), an amount of 0.5 to 2.0 wt % of manganese (Mn), an amount of 0.05 to 0.5 wt % of silicon (Si), an amount greater than 0 and less than or equal to 0.5 wt % of chromium (Cr), an amount greater than 0 and less than or equal to 4.5 wt % of copper (Cu), an amount greater than 0 and less than or equal to 0.003 wt % of boron (B), an amount greater than 0 and less than or equal to 0.25 wt % of vanadium (V), an amount greater than 0 and less than or equal to 0.012 wt % of nitrogen (N), an amount greater than 0 and less than or equal to 0.03 wt % of phosphorus (P), an amount greater than 0 and less than or equal to 0.03 wt % of sulfur (S), an amount of 0.01 to 0.5 wt % of the sum of one or more of nickel (Ni), niobium (Nb) and titanium (Ti), the balance of iron (Fe) and other inevitable impurities, wherein a final microstructure includes ferrite, bainite, pearlite, retained austenite, and precipitates including copper.
In the steel reinforcement, the final microstructure may have a bainite fraction of 90% or more and a retained austenite fraction of 5% or less.
The steel reinforcement may have a yield strength (YS) of 750 MPa or greater, a tensile strength (TS) of 1,000 MPa or greater, an elongation of 11% or greater, and a ratio of a tensile strength to a yield strength (TS/YS) of 1.25 or greater.
A method for manufacturing a steel reinforcement according to an embodiment of the present invention includes: (a) reheating a steel at a temperature of 1,050 to 1,230° C., wherein the steel includes an amount of 0.07 to 0.43 wt % of carbon (C), an amount of 0.5 to 2.0 wt % of manganese (Mn), an amount of 0.05 to 0.5 wt % of silicon (Si), an amount greater than 0 and less than or equal to 0.5 wt % of chromium (Cr), an amount greater than 0 and less than or equal to 4.5 wt % of copper (Cu), an amount greater than 0 and less than or equal to 0.003 wt % of boron (B), an amount greater than 0 and less than or equal to 0.25 wt % of vanadium (V), an amount greater than 0 and less than or equal to 0.012 wt % of nitrogen (N), an amount greater than 0 and less than or equal to 0.03 wt % of phosphorus (P), an amount greater than 0 and less than or equal to 0.03 wt % of sulfur (S), an amount of 0.01 to 0.5 wt % of the sum of one or more of nickel (Ni), niobium (Nb) and titanium (Ti), the balance of iron (Fe), and other inevitable impurities; (b) hot rolling the reheated steel material under a condition of a finish rolling temperature of 950 to 1,020° C.; and (c) performing an aging heat treatment on the hot-rolled steel material at a temperature of 400 to 600° C. for 15 to 60 minutes.
In the method for manufacturing a steel reinforcement, the final microstructure after performing step (c) may include ferrite, bainite, pearlite, retained austenite, and precipitates including copper.
In the method for manufacturing a steel reinforcement, the final microstructure may have a bainite fraction of 90% or greater and a retained austenite fraction of 5% or less.
In the method for manufacturing a steel reinforcement, the steel reinforcement after performing the step (c) may have a yield strength (YS) of 750 MPa or greater, a tensile strength (TS) of 1,000 MPa or greater, an elongation of 11% or greater, and a ratio of a tensile strength to a yield strength (TS/YS) of 1.25 or greater.

Advantageous Effects

According to an embodiment of the present invention, it is possible to implement a high-strength steel reinforcement capable of reducing a cost in manufacturing a high-strength and high-toughness steel reinforcement minimizing a tempcore process so that productivity is not reduced without introducing new facilities, and a method for manufacturing the same. The scope of the present invention is not limited by the effects described above.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow chart schematically illustrating a method for manufacturing a steel reinforcement according to an exemplary embodiment in the present invention.

FIG. 2 is a graph illustrating the hardness of each specimen according to Experimental Examples of the present invention.

FIG. 3 is a graph illustrating the hardness of each specimen according to Experimental Examples of the present invention.

FIG. 4 is a photograph obtained by photographing final microstructures of the specimens according to the Experimental Examples of the present invention.

DETAILED DESCRIPTION

A steel reinforcement and a method for 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 functions in the present invention, and definitions of these terms should be defined on the basis of the contents throughout the present specification. Hereinafter, a high-strength steel reinforcement capable of partially reducing expensive alloying elements and minimizing a tempcore process so that productivity is not reduced, and a method for manufacturing the same are provided.
Steel Reinforcement
A steel reinforcements according to an exemplary embodiment of the present invention comprises: an amount of 0.07 to 0.43 wt % of carbon (C), an amount of 0.5 to 2.0 wt % of manganese (Mn), an amount of 0.05 to 0.5 wt % of silicon (Si), an amount greater than 0 and less than or equal to 0.5 wt % of chromium (Cr), an amount greater than 0 and less than or equal to 4.5 wt % of copper (Cu), an amount greater than 0 and less than or equal to 0.003 wt % of boron (B), an amount greater than 0 and less than or equal to 0.25 wt % of vanadium (V), an amount greater than 0 and less than or equal to 0.012 wt % of nitrogen (N), an amount greater than 0 and less than or equal to 0.03 wt % of phosphorus (P), an amount greater than 0 and less than or equal to 0.03 wt % of sulfur (S), an amount of 0.01 to 0.5 wt % of the sum of one or more of nickel (Ni), niobium (Nb) and titanium (Ti), the balance of iron (Fe) and other inevitable impurities.
Hereinafter, the role and content of each component contained in the steel reinforcement according to an embodiment of the present invention will be described.
Carbon (C)
Carbon (C) is the most effective and important element for increasing the strength of steel. In addition, carbon is added and solid-dissolved in austenite to form a martensitic structure during quenching. As the amount of carbon increases, a quenching hardness is improved, but the possibility of deformation during quenching is increased. Furthermore, carbon (C) is combined with elements such as iron, chromium, molybdenum, and vanadium to form carbides, thereby improving strength and hardness. Carbon (C) may be added in an amount of 0.07 to 0.43 wt % of the total weight of the steel reinforcement according to an embodiment of the present invention. If the content of carbon is less than 0.07 wt % of the total weight, the effect described above may not be implemented 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 partly solid-dissolved in steel and partly bonded to sulfur contained in steel to form MnS, a non-metallic inclusion, which is ductile and is elongated in a machining direction during plastic working. However, as MnS is formed to reduce sulfur components in the steel, the grains become weak and the formation of FeS, which is a low-melting compound, is inhibited. Manganese inhibits acid resistance and oxidation resistance of steel, but improves a yield strength by making pearlite fine and solid-solution strengthening ferrite. Manganese may be added in an amount of 0.5 to 2.0 wt % of the total weight of the steel reinforcement according to an embodiment of the present invention. If the content of manganese is less than 0.5 wt %, the above-described effect of adding manganese may not be sufficiently exhibited. In addition, if the content of manganese exceeds 2.0 wt %, quenching crack or deformation is induced, such that weldability may be degraded, and MnS inclusions and center segregation may be generated, which may degrades ductility and corrosion resistance of the steel reinforcement.
Silicon (Si)
Silicon (Si) is well-known as an element that stabilizes ferrite, and is well-known as an element that increases ductility by increasing the ferrite fraction during cooling. Meanwhile, silicon is added together with aluminum as a deoxidizer for removing oxygen in steel in a steelmaking process, and may also exhibit a solid-solution strengthening effect. The silicon may be added in an amount of 0.05 to 0.5 wt % of the total weight of the steel reinforcement according to an embodiment of the present invention. If the content of silicon is less than 0.05 wt % of the total weight, the above-described effect of adding silicon may not be sufficiently exhibited. If a large amount of silicon exceeding 0.5 wt % of the total weight is added, toughness is degraded, plastic workability is degraded, weldability of steel is reduced, softening resistance is increased during tempering, and red scale is generated during reheating and hot rolling, which may cause problems in surface quality.
Chromium (Cr)
Chromium (Cr) is an element that stabilizes ferrite, and when added to C—Mn steel, chromium delays the diffusion of carbon due to a solute interference effect, thereby affecting particle size refinement. The chromium may be added in an amount of greater than 0 and less than or equal to 0.5 wt % of the total weight of the steel reinforcement according to an embodiment of the present invention. If a large amount of chromium content exceeding 0.5 wt % of the total weight is added, toughness may be degraded and workability and machinability may deteriorate.
Copper (Cu)
Copper (Cu) is an element that improves hardenability and low-temperature impact toughness of steel. Copper is solid-dissolved in ferrite at room temperature and exhibits a solid-solution strengthening effect, such that strength and hardness are slightly improved, but elongation is degraded. The copper may be added in an amount greater than 0 and less than or equal to 4.5 wt % of the total weight of the steel reinforcement according to an embodiment of the present invention. If a large amount of copper content exceeding 4.5 wt % of the total weight is added, hot workability may deteriorate, hot brittleness may be caused, and surface quality of the product may be impaired.
Boron (B)
Boron (B) is an important element for securing hardenability. The boron may be added in an amount greater than 0 and less than or equal to 0.003 wt % of the total weight of the steel reinforcement according to an embodiment of the present invention. If a large amount of boron content exceeding 0.003 wt % of the total weight is added, the effect of addition is saturated and an elongation may be reduced, and thus it is preferable to limit the upper limit to 0.003 wt % or less.
Vanadium (V)
Vanadium (V) is a useful component for solid-solution strengthening and precipitation strengthening, and has a stronger carbide-forming ability than chromium and refines grains, and thus, exhibits an effect of suppressing the amount of carbon added, and is an element contributing to strength improvement by acting as pinning at grain boundaries. 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 reinforcement according to an embodiment of the present invention. If a large amount of vanadium content exceeding 0.25 wt % of the total weight is added, there is a problem in that the manufacturing cost of the steel is excessively increased in comparison with the strength improvement effect.
Nitrogen (N)
Nitrogen (N) is an element that increases strength by precipitating vanadium and nitride or carbonitride. 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 steel reinforcement according to an embodiment of the present invention. If a large amount of nitrogen content exceeding 0.012 wt % of the total weight is added, the nitrogen may act as an element impairing toughness.
Phosphorus (P)
Phosphorus (P) may increase the strength of steel by solid-solution strengthening, and may perform a function of suppressing the formation of carbides. 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 steel reinforcement according to an embodiment of the present invention. If the content of phosphorus exceeds 0.03 wt %, impact resistance is degraded, temper brittleness is promoted, and a low-temperature impact value is reduced by precipitation behavior.
Sulfur (S)
Sulfur (S) may be combined with manganese, titanium, etc. to improve the machinability of steel, and may form the precipitates of fine MnS to improve processability. The sulfur may be added in an amount greater than 0 and less than or equal to 0.03 wt % of the total weight of the steel reinforcement according to an embodiment of the present invention. If the content of sulfur exceeds 0.03 wt %, toughness and weldability may be impaired, and a low-temperature impact value may be reduced. If the amount of manganese in the steel reinforcement is not sufficient, sulfur combines with iron to form FeS. This FeS is very brittle and has a low melting point, which causes cracks during hot and cold working. Therefore, in order to avoid the formation of such FeS inclusions, a ratio of manganese and sulfur may be adjusted to 5:1.
Nickel (Ni), Niobium (Nb), Titanium (Ti)
Nickel (Ni) is an element that increases hardenability and improves toughness, and niobium (Nb) is an element that precipitates in the form of NbC or Nb(C,N) to improve the strength of a base material and a weld zone, and titanium (Ti)) is an element that suppresses the formation of AlN due to the formation of TiN at a high temperature and exhibits the effect of refining the grain size due to the formation of Ti(C,N), etc. The steel reinforcement according to an embodiment of the present invention may contain one or more of nickel (Ni), niobium (Nb), and titanium (Ti), but the sum of the content thereof may be added in an amount of 0.01 to 0.5 wt % of the total weight of the steel reinforcement. If the sum of the content of at least one of nickel (Ni), niobium (Nb) and titanium (Ti) contained in the steel reinforcement according to an embodiment of the present invention is less than 0.01 wt %, the effect described above may not be expected. If the sum thereof exceeds 0.5 wt %, problems may arise in that the manufacturing cost of the parts is increased, brittle cracks are generated, and the content of carbon in the matrix is reduced, such that the characteristics of the steel are degraded.
The final microstructure of the steel reinforcement according to an embodiment of the present invention having the above-described alloying element composition includes ferrite, bainite, pearlite, retained austenite, and precipitates including copper. Furthermore, in the final microstructure, a bainite fraction may be 90% or greater, and a retained austenite fraction may be 5% or less.
In addition, the steel reinforcement according to an embodiment of the present invention having the above-described alloy element composition has a yield strength (YS) of 750 MPa or greater, a tensile strength (TS) of 1,000 MPa or greater, an elongation of 11% or greater, and a ratio of a tensile strength to a yield strength (TS/YS) of 1.25 or greater.
For example, the steel reinforcement may have a yield strength (YS) of 750 to 1,000 MPa, a tensile strength (TS) of 1,000 to 1,300 MPa, and an elongation of 11 to 25%, and a ratio of a tensile strength to a yield strength (TS/YS) of 1.25 to 1.40.
Hereinafter, a method for manufacturing the steel reinforcement according to an embodiment of the present invention having the above-described alloying element composition will be described.
Manufacturing Method of Steel Reinforcement
FIG. 1 is a flow chart schematically illustrating a method for manufacturing a steel reinforcement according to an embodiment in the present invention.
Referring to FIG. 1, the method for manufacturing a steel reinforcement according to an embodiment of the present invention includes: (a) reheating a steel material at a temperature of 1,050 to 1,230° C. (S100), wherein the steel material includes an amount of 0.07 to 0.43 wt % of carbon (C), an amount of 0.5 to 2.0 wt % of manganese (Mn), an amount of 0.05 to 0.5 wt % of silicon (Si), an amount greater than 0 and less than or equal to 0.5 wt % of chromium (Cr), an amount greater than 0 and less than or equal to 4.5 wt % of copper (Cu), an amount greater than 0 and less than or equal to 0.003 wt % of boron (B), an amount greater than 0 and less than or equal to 0.25 wt % of vanadium (V), an amount greater than 0 and less than or equal to 0.012 wt % of nitrogen (N), an amount greater than 0 and less than or equal to 0.03 wt % of phosphorus (P), an amount greater than 0 and less than or equal to 0.03 wt % of sulfur (S), an amount of 0.01 to 0.5 wt % of the sum of one or more of nickel (Ni), niobium (Nb) and titanium (Ti), the balance of iron (Fe) and other inevitable impurities; (b) hot rolling the reheated steel material under a condition of a finish rolling temperature of 950 to 1,020° C. (S200); and (c) performing an aging heat treatment on the hot-rolled steel material at 400 to 600° C. for 15 to 60 minutes (S300).
The steel reinforcement according to an 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 billet is reheated to a temperature of 1,050 to 1,230° C. Next, the hot rolling process is characterized in that final finishing rolling is performed on the steel material at a temperature of 950 to 1,020° C. while the steel material passes through the respective rolling rolls (RM, IM, and FM) to complete the rolling, the rolled steel material is air-cooled to a temperature of 400 to 600° C., and the aging heat treatment is then performed while maintaining the air-cooled steel material before a cooling phase for a specific time and at a temperature of 400 to 600° C. for 15 minutes to 60 minutes in the cooling phase according to required physical properties.
The steel reinforcement manufacturing/continuous casting process generally consists of an electric furnace, a ladle furnace (LF), and continuous casting. In order to improve fatigue resistance properties, after the LF, which is a secondary refining process, a vacuum degassing (VD) process may be performed to lower the content of oxygen to a predetermined level or less, and then the steel material may be solidified into a semi-material in the continuous casting process.
The steel material may include: an amount of 0.07 to 0.43 wt % of carbon (C), an amount of 0.5 to 2.0 wt % of manganese (Mn), an amount of 0.05 to 0.5 wt % of silicon (Si), an amount greater than 0 and less than or equal to 0.5 wt % of chromium (Cr), an amount greater than 0 and less than or equal to 4.5 wt % of copper (Cu), an amount greater than 0 and less than or equal to 0.003 wt % of boron (B), an amount greater than 0 and less than or equal to 0.25 wt % of vanadium (V), an amount greater than 0 and less than or equal to 0.012 wt % of nitrogen (N), an amount greater than 0 and less than or equal to 0.03 wt % of phosphorus (P), an amount greater than 0 and less than or equal to 0.03 wt % of sulfur (S), an amount of 0.01 to 0.5 wt % of the sum of one or more of nickel (Ni), niobium (Nb) and titanium (Ti), the balance of iron (Fe) and other inevitable impurities.
In an embodiment, the steel material may be reheated at a temperature of 1,050 to 1,230° C. When the steel material is reheated at the temperature described above, the segregated component during the continuous casting process may be solid-dissolved again. The present invention aims to improve strength through precipitation and solid-solution strengthening. Therefore, it is necessary to sufficiently solid-dissolve these elements in austenite before hot rolling, so that it is necessary to heat the billet to a temperature of 1,050° C. or greater. If the reheating temperature is less than 1,050° C., there may be a problem that the solid-dissolution of various carbides are not sufficient, and the segregated components are not sufficiently evenly distributed during the continuous casting process. However, if the reheating temperature exceeds 1,230° C., there are adverse effects such as the coarsening of austenite and decarburization, and the desired strength may not be obtained. That is, if the reheating temperature exceeds 1,230° C., very coarse austenite grains are formed, making it difficult to secure strength. In addition, if the reheating temperature exceeds 1,230° C., the heating cost may be increased and process time may be added, resulting in an increase in manufacturing cost and a decrease in productivity.
In the hot rolling step (S200), the reheated steel material is hot-rolled. The range of a hot deformation finishing temperature may be 950 to 1,020° C. The hot deformation finishing temperature is a very important factor affecting the final material, and the temperature of 950 to 1,020° C. at which rolling is performed is the temperature at which austenite may be refined. However, when the rolling is performed at a hot rolling temperature of less than 950° C., the rolling load may be increased and a duplex grain size structure at the edge portion may be generated. In addition, when the rolling is performed at a high temperature region exceeding 1,020° C., the target mechanical properties may not be obtained due to the coarsening of grains.
Immediately after hot rolling and after air cooling, the steel material is directly put into a thermal insulation tank or a thermal insulation bath capable of maintaining the temperature of 400 to 600° C. A temperature of a rod wire in the thermal insulation bath is about 400 to 600° C. Compared to a steel reinforcement having a yield strength of 600 MPa to which tempcore is applied, martensite is not generated on the surface, which is also advantageous for ductility and toughness. In addition, since the temperature is maintained at the product's own temperature, an additional heat treatment is not required, and thus, the production cost is reduced. It was confirmed that it is advantageous to increase strength, a TS/YS ratio, and ductility when maintained at a temperature of 400 to 600° C. for 15 to 60 minutes during warming. At a temperature lower than 400° C., the tempering effect is insufficient, and at a temperature higher than 600° C., the effect of increasing the strength is insufficient, and thus, a temperature range of 400 to 600° C. was determined.
In the case of the steel reinforcement not subjected to an aging heat treatment, the final microstructure thereof has a ferrite and pearlite structure. However, in the case of the steel reinforcement of the present invention subjected to an aging heat treatment, the temperature was maintained to form bainite, and the retained austenite was also formed to be 5% or less, and the steel reinforcement of the present invention may secure mechanical properties in which a yield strength (YS) is 750 MPa or greater, a tensile strength (TS) is 1,000 MPa or greater, an elongation is 11% or greater, and a ratio of a tensile strength to a yield strength (TS/YS) is 1.25 or greater. That is, through the method for manufacturing a steel reinforcement according to an embodiment of the present invention, an ultra-simple process and cost-reducing manufacturing method are disclosed, which are capable of securing high functionality due to the formation of nano-precipitates through the addition of copper and maximizing a sufficient precipitation-strengthening effect through thermal insulation at a specific temperature without performing a tempcore process even in an existing vanadium system. Therefore, even though a process is simplified, the method may implement high strength, high seismic resistance, and a high ratio of a tensile strength to a yield strength that exceed conventional mechanical properties, and is expected to further improve stability of the building structure.

Example

Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred examples of the present invention. However, the following Examples are intended to assist in the 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 in order to assist in the understanding of the present invention. However, the following Experimental Examples are intended to assist in the understanding of the present invention, and the present invention is not limited by the following Experimental Examples.
1. Preparation of Specimens
In the present Experimental Examples, the specimens implemented with the alloy element composition (unit: wt %) and process conditions of Tables 1 and 2 are provided.

TABLE 1

	Composition

N

Division

C

Si

Mn

P

S

Cr

Cu

Ni

Nb

Ti

V

B

(ppm)

Comparative	0.34	0.2	1.35	0.03	0.03	0.2	0.25	0.01	—	—	0.05	—	100
Example 1
Example 1	0.36	0.42	1.68	0.03	0.03	0.25	2.17	0.3	0.01	0.008	0.002	0.0011	80
Comparative
Example 2
Example 2	0.34	0.4	1.66	0.03	0.03	0.24	3.01	0.32	0.01	0.008	0.002	0.001	80
Comparative
Example 3
Example 3	0.36	0.41	1.77	0.03	0.03	0.22	4.22	0.31	0.009	0.009	0.002	0.0009	80
Comparative
Example 4
Example 4	0.35	0.35	1.62	0.03	0.03	0.23	0.24	0.3	0.012	0.008	0.1	0.001	80
Example 5	0.33	0.34	1.65	0.03	0.03	0.23	0.25	0.32	0.008	0.007	0.15	0.0012	80

TABLE 2

	Process conditions

		Finishing rolling	Recuperation	Aging
	Reheating	temperature	temperature	temperature	Aging time
Division	temperature	(° C.)	(° C.)	(° C.)	(min.)

Comparative	1,080	1,020	630	—	—
Example 1
Example 1		980	—	600/500	30
					60
Comparative					120
Example 2
Example 2		991	—	600/500	30
					60
Comparative					120
Example 3
Example 3		988	—	600/500	30
					60
Comparative					120
Example 4
Example 4		998	—	400	60
Example 5		1,012	—	400	60

The compositions disclosed in Table 1 satisfy all composition ranges including: an amount of 0.07 to 0.43 wt % of carbon (C), an amount of 0.5 to 2.0 wt % of manganese (Mn), an amount of 0.05 to 0.5 wt % of silicon (Si), an amount greater than 0 and less than or equal to 0.5 wt % of chromium (Cr), an amount greater than 0 and less than or equal to 4.5 wt % of copper (Cu), an amount greater than 0 and less than or equal to 0.003 wt % of boron (B), an amount greater than 0 and less than or equal to 0.25 wt % of vanadium (V), an amount greater than 0 and less than or equal to 0.012 wt % of nitrogen (N), an amount greater than 0 and less than or equal to 0.03 wt % of phosphorus (P), an amount greater than 0 and less than or equal to 0.03 wt % of sulfur (S), an amount of 0.01 to 0.5 wt % of the sum of one or more of nickel (Ni), niobium (Nb) and titanium (Ti), and the balance of iron (Fe).
Meanwhile, the Examples in Table 2 satisfy the process conditions for reheating a steel material of the composition described above at a temperature of 1,050 to 1,230° C.; hot rolling the reheated steel material under a condition of a finish rolling temperature of 950 to 1,020° C.; and performing an aging heat treatment on the hot-rolled steel material at a temperature of 400 to 600° C. for 15 to 60 minutes. On the other hand, in Comparative Example 1 in Table 2, the tempcore process was performed on the hot-rolled steel, but the aging heat treatment process was not applied. In addition, in Comparative Example 2, Comparative Example 3, and Comparative Example 4 in Table 2, the aging heat treatment was performed at a temperature of 500 to 600° C. for 120 minutes in excess of 15 to 60 minutes.
2. Evaluation of Physical Properties and Microstructure
FIGS. 2 and 3 are graphs illustrating the hardness of each specimen according to the Experimental Examples of the present invention.
Item ▪ (Example 1) of FIG. 2 corresponds to the specimen to which the composition condition of Example 1 in Table 1 and the reheating temperature and finish rolling temperature process conditions of Example 1 in Table 2 were applied, and, the aging heat treatment at a temperature of 600° C. was performed on the specimen of Example 1 for 0 minute, 30 minutes, 60 minutes, 90 minutes, and 120 minutes. Item (Example 2) of FIG. 2 corresponds to the specimen to which the composition condition 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 at a temperature of 600° C. was performed on the specimen of Example 2 for 0 minute, 30 minutes, 60 minutes, 90 minutes, and 120 minutes. Item ▴ (Example 3) of FIG. 2 corresponds to the specimen to which the composition condition of Example 3 in Table 1 and the reheating temperature and finish rolling temperature process conditions of Example 3 in Table 2 were applied, and the aging heat treatment at a temperature of 600° C. was performed on the specimen of Example 3 for 0 minute, 30 minutes, 60 minutes, 90 minutes, and 120 minutes. Item ▾ (Comparative Example 1) of FIG. 2 corresponds to the specimen to which the composition condition of Comparative Example 1 in Table 1 and the reheating temperature and finish rolling temperature process conditions of Comparative Example 1 in Table 2 were applied, and the tempcore process having a heat recuperation condition at a temperature of 630° C. was performed on the specimen of Comparative Example 1.
Item ▪ (Example 1) of FIG. 3 corresponds to the specimen to which the composition condition of Example 1 in Table 1 and the reheating temperature and finish rolling temperature process conditions of Example 1 in Table 2 were applied, and the aging heat treatment at a temperature of 500° C. was performed on the specimen of Example 1 for 0 minute, 30 minutes, 60 minutes, 90 minutes, and 120 minutes. Item (Example 2) of FIG. 3 corresponds to the specimen 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 at a temperature of 500° C. was performed on the specimen of Example 2 for 0 minute, 30 minutes, 60 minutes, 90 minutes, and 120 minutes. Item ▴ (Example 3) of FIG. 3 corresponds to the specimen to which the composition conditions of Example 3 in Table 1 and the reheating temperature and finish rolling temperature process conditions of Example 3 in Table 2 were applied, and the aging heat treatment at a temperature of 500° C. was performed on the specimen of Example 3 for 0 minute, 30 minutes, 60 minutes, 90 minutes, and 120 minutes. Item ▾ (Comparative Example 1) of FIG. 3 corresponds to the specimen to which the composition condition of Comparative Example 1 in Table 1 and the reheating temperature and finish rolling temperature process conditions of Comparative Example 1 in Table 2 were applied, and the tempcore process having a heat recuperation condition at a temperature of 630° C. was performed on the specimen of Comparative Example 1.

TABLE 3

	Hardness	Y.S	T.S	T.S/	EL
	(Hv)	(MPa)	(MPa)	Y.S	(%)

Comparative	260	658	836	1.27	11.3
Example 1
Example 1	349	783	1,018	1.30	14.7
Example 2	387	935	1,178	1.26	12.6
Example 3	408	991	1,239	1.25	11.3

Table 3 shows strength, a yield strength (YS), a tensile strength (TS), an elongation, and a ratio of a tensile strength to a yield strength (TS/YS) for each steel reinforcement subjected to the aging heat treatment for 30 minutes at a temperature of 500° C. for each specimen according to the Experimental Example of the present invention. Example 1 in table 3 corresponds to the specimen 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 mechanical properties of the specimen of Example 1 were evaluated after the specimen of Example 1 was subjected to the aging heat treatment at 500° C. for 30 minutes. Example 2 in Table 3 corresponds to the specimen 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 mechanical properties of the specimen of Example 2 were evaluated after the specimen of Example 2 was subjected to the aging heat treatment at 500° C. for 30 minutes. Example 3 in Table 3 corresponds to the specimen to which the composition conditions of Example 3 in Table 1 and the reheating temperature and finish rolling temperature process conditions of Example 3 in Table 2 are applied, and the mechanical properties of the specimen of Example 3 were evaluated after the specimen of Example 3 was subjected to the aging heat treatment at 500° C. for 30 minutes. Comparative Example 1 in Table 3 corresponds to the specimen to which the composition conditions of Comparative Example 1 in Table 1 and the reheating temperature and finish rolling temperature process conditions of Comparative Example 1 in Table 2 are applied, and the mechanical properties of the specimen of Comparative Example 1 were evaluated after the specimen of Comparative Example 1 was subjected the tempcore process without the aging heat treatment.

TABLE 4

	Hardness	Y.S	T.S	T.S/	EL
	(Hv)	(MPa)	(MPa)	Y.S	(%)

Comparative	260	658	836	1.27	11.3
Example 1
Example 4	285	778	1,026	1.32	14.2
Example 5	297	828	1,095	1.32	11.9

It was confirmed that it is advantageous to increase strength, a TS/YS ratio, and ductility when the specimens was maintained at a temperature of 400 to 600° C. for 15 to 60 minutes. It was confirmed that in the case of the steel reinforcement not subjected to the aging heat treatment, the final microstructure had a ferrite and pearlite structure, whereas in the case of the steel reinforcement of the present invention subjected to the aging heat treatment, the temperature was maintained to form bainite, and the retained austenite was also formed to be 5% or less, and the steel reinforcement of the present invention may secure mechanical properties in which a yield strength (YS) is 750 MPa or greater, a tensile strength (TS) is 1,000 MPa or greater, an elongation is 11% or greater, and a ratio of a tensile strength to a yield strength (TS/YS) is 1.25 or greater.
Through the method for manufacturing a steel reinforcement according to an exemplary embodiment of the present invention, an ultra-simple process and cost-reducing manufacturing method are disclosed which are capable of securing high functionality due to the formation of nano-precipitates through the addition of copper and maximizing a sufficient precipitation-strengthening effect through thermal insulation at a specific temperature without performing a tempcore process even in an existing vanadium system. Therefore, even though a process is simplified, the method may implement high strength, high seismic resistance, and a high ratio of a tensile strength to a yield strength that exceed conventional mechanical properties, and is expected to further improve stability of the building structure.
Although the above description has been focused on the exemplary embodiments of the present invention, various changes or modifications may be made at the level of those skilled in the art. Such changes and modifications can be said to belong to the present invention unless they deviate from the scope of the present invention. Accordingly, the scope of the present invention should be judged by the claims described below.

Claims

1. A steel reinforcement, comprising:

an amount of 0.07 to 0.43 wt % of carbon (C), an amount of 0.5 to 2.0 wt % of manganese (Mn), an amount of 0.05 to 0.5 wt % of silicon (Si), an amount greater than 0 and less than or equal to 0.5 wt % of chromium (Cr), an amount greater than 0 and less than or equal to 4.5 wt % of copper (Cu), an amount greater than 0 and less than or equal to 0.003 wt % of boron (B), an amount greater than 0 and less than or equal to 0.25 wt % of vanadium (V), an amount greater than 0 and less than or equal to 0.012 wt % of nitrogen (N), an amount greater than 0 and less than or equal to 0.03 wt % of phosphorus (P), an amount greater than 0 and less than or equal to 0.03 wt % of sulfur (S), an amount of 0.01 to 0.5 wt % of the sum of one or more of nickel (Ni), niobium (Nb) and titanium (Ti), the balance of iron (Fe) and other inevitable impurities,

wherein a final microstructure comprises ferrite, bainite, pearlite, retained austenite, and precipitates comprising copper.

2. The steel reinforcement 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 steel reinforcement of claim 1, wherein a yield strength (YS) is 750 MPa or greater, a tensile strength (TS) is 1,000 MPa or greater, an elongation is 11% or greater, and a ratio of a tensile strength to a yield strength (TS/YS) is 1.25 or greater.

4. A method for manufacturing a steel reinforcement, the method comprising:

(a) reheating a steel at a temperature of 1,050 to 1,230° C., wherein the steel comprises an amount of 0.07 to 0.43 wt % of carbon (C), an amount of 0.5 to 2.0 wt % of manganese (Mn), an amount of 0.05 to 0.5 wt % of silicon (Si), an amount greater than 0 and less than or equal to 0.5 wt % of chromium (Cr), an amount greater than 0 and less than or equal to 4.5 wt % of copper (Cu), an amount greater than 0 and less than or equal to 0.003 wt % of boron (B), an amount greater than 0 and less than or equal to 0.25 wt % of vanadium (V), an amount greater than 0 and less than or equal to 0.012 wt % of nitrogen (N), an amount greater than 0 and less than or equal to 0.03 wt % of phosphorus (P), an amount greater than 0 and less than or equal to 0.03 wt % of sulfur (S), an amount of 0.01 to 0.5 wt % of the sum of one or more of nickel (Ni), niobium (Nb) and titanium (Ti), the balance of iron (Fe), and other inevitable impurities;

(b) hot rolling the reheated steel under a condition of a finish rolling temperature of 950 to 1,020° C.; and

(c) performing an aging heat treatment on the hot-rolled steel at a temperature of 400 to 600° C. for 15 to 60 minutes.

5. The method of claim 4, wherein the final microstructure after performing step (c) includes 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 more and a retained austenite fraction of 5% or less.

7. The method of claim 4, wherein the steel reinforcement after performing the step (c) has a yield strength (YS) of 750 MPa or greater, a tensile strength (TS) of 1,000 MPa or greater, an elongation of 11% or greater, and a ratio of a tensile strength to a yield strength (TS/YS) of 1.25 or greater.