EP4603615A1

EP4603615A1 - Wire rod for steel fiber, steel fiber for concrete reinforcement, and manufacturing methods therefor

Info

Publication number: EP4603615A1
Application number: EP23907460.2A
Authority: EP
Inventors: Yosep YANG; Jaehwan Kim; Seongryeol JEON
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2022-12-20
Filing date: 2023-11-24
Publication date: 2025-08-20
Also published as: WO2024136174A1; JP2026500906A; EP4603615A4; KR20240097400A; CN120077157A

Abstract

The present invention relates to a wire rod for steel fiber, a steel fiber for concrete reinforcement, and manufacturing methods therefor, the wire rod comprising, based on wt%, 0.010-0.040% of carbon (C), greater than 0% and less than or equal to 0.10% of silicon (Si), 0.10-1.50% of manganese (Mn), 0.50-1.50% of nickel (Ni), 0.05% or less (including 0%) of phosphorus (P), 0.05% or less (including 0%) of sulfur (S), and the remainder of iron (Fe) and inevitable impurities.

Description

[Technical Field]

The present disclosure relates to a wire rod for steel fibers, a steel fiber for concrete reinforcement, and manufacturing methods therefor.

[Background Art]

In general, steel fibers, used as a material for concrete reinforcement to support the floor of buildings or to withstand the earth pressure inside tunnels, are widely used as a substitute for rebar because there is no process for placing rebar. Although low-strength steel fibers with a strength of 1,000 MPa or less have been widely used, the strength of steel fibers is gradually increasing by altering methods for improving concrete compression strength.
Although diameters of steel fibers are classified according to their intended use, thin steel fibers having a diameter of about 0.4 to 1.0 mm are generally used, and thus they are manufactured by a process including wire rod → descaling → dry drawing → wet drawing → bundle processes. Because a true strain (e) of about 4.6 is applied to a drawing process, a solid solution strengthening element such as Si and Mn should be added to a ultra-low carbon steel having a carbon content of about 0.01 wt% to design the composition.
Concrete is used in ultra-low temperature vessels such as LNG tanks and in seawater as well, and in this case, steel fibers may also be used as a material for reinforcement. However, because steel fibers commonly used in the art are vulnerable to low temperature, specialized steel materials capable of withstanding extremely low temperature are required. Therefore, there is a need to develop steel materials capable of suppressing fractures during a drawing process and significantly increasing bending strength of concrete at a low temperature.
(Patent Document 1) Korean Patent Application Publication No. 10-2000-0042052

[Disclosure]

[Technical Problem]

Provided is a wire rod for steel fibers having excellent bending strength used as a material for concrete reinforcement such as LNG tanks, a steel fiber, and manufacturing methods therefor. A high-strength steel fiber for concrete reinforcement having excellent tensile strength and elongation and excellent bending strength when mixed with concrete, and manufacturing methods therefor are provided.
However, the technical problems to be solved by the present disclosure are not limited to the aforementioned problems, and any other technical problems not mentioned herein will be clearly understood from the following description by those skilled in the art to which the present disclosure pertains.

[Technical Solution]

In accordance with an aspect of the present disclosure, a wire rod for steel fibers includes, in percent by weight (wt%), 0.010 to 0.040% of carbon (C), more than 0% but not more than 0.10% of silicon (Si), 0.10 to 1.50% of manganese (Mn), 0.50 to 1.50% of nickel (Ni), 0.05% or less (including 0%) of phosphorus (P), 0.05% or less (including 0%) of sulfur (S), and the balance of iron (Fe) and inevitable impurities. The wire rod for steel fibers according to an embodiment of the present disclosure may include a scale with a thickness of 8 to 20 µm is formed on the surface and have a tensile strength variation of less than ±40 MPa.
In addition, the wire rod for steel fibers according to an embodiment of the present disclosure may have a tensile strength is 340 MPa or more.
In addition, the wire rod for steel fibers according to an embodiment of the present disclosure may include polygonal ferrite as a microstructure.
In accordance with another aspect of the present disclosure, a steel fiber for concrete reinforcement includes, in percent by weight (wt%) 0.010 to 0.040% of carbon (C), more than 0% but not more than 0.10% of silicon (Si), 0.10 to 1.50% of manganese (Mn), 0.50 to 1.50% of nickel (Ni), 0.05% or less (including 0%) of phosphorus (P), 0.05% or less (including 0%) of sulfur (S), and the balance of iron (Fe) and inevitable impurities. In the steel fiber for concrete reinforcement according to an embodiment of the present disclosure, the number of void defects formed in crystal grains boundaries and having a size of 5 µm or more is 4x10⁷ count/mm² or less.
In addition, the steel fiber for concrete reinforcement according to an embodiment of the present disclosure may have a tensile strength of 1,220 MPa or more. The steel fiber for concrete reinforcement may have a work hardening rate of 405 or more.
In addition, the steel fiber for concrete reinforcement according to an embodiment of the present disclosure may have an elongation of 5% or more.
In addition, the steel fiber for concrete reinforcement according to an embodiment of the present disclosure may have a fracture rate of 1.5 times/ton or less.
In addition, the steel fiber for concrete reinforcement according to an embodiment of the present disclosure may have a tensile strength variation of less than ±50 MPa.
In accordance with another aspect of the present disclosure, a method for manufacturing a wire rod for steel fibers includes: heating a billet comprising, in percent by weight (wt%), 0.010 to 0.040% of carbon (C), more than 0% but not more than 0.10% of silicon (Si), 0.10 to 1.50% of manganese (Mn), 0.50 to 1.50% of nickel (Ni), 0.05% or less (including 0%) of phosphorus (P), 0.05% or less (including 0%) of sulfur (S), and the balance of iron (Fe) and inevitable impurities; rolling the heated billet into a wire rod; coiling the wire rod in a temperature range of 880 to 950 °C; and cooling the coiled wire rod to 300 °C at a rate of 1 °C/s or less .
In addition, the method for manufacturing the wire rod for steel fibers according to an embodiment of the present disclosure may include maintaining the billet in a heating furnace at a temperature of 1,000 to 1,250°C for 90 to 120 minutes in the heating of the billet.
In addition, in the method for manufacturing the wire rod for steel fibers according to an embodiment of the present disclosure, the wire rod for steel fiber may include polygonal ferrite as a microstructure. In addition, a scale with a thickness of 8 to 20 µm may be formed on the surface of the wire rod.
In accordance with another aspect of the present disclosure, a method for manufacturing a steel fiber for concrete reinforcement includes dry drawing and wet drawing the wire rod according to any one of claims 1 to 3 with a true strain of 4.6 or more into a steel fiber.
In addition, in the method for manufacturing the steel fiber for concrete reinforcement according to an embodiment of the present disclosure, the number of void defects formed in crystal grain boundaries and having a size of 5 µm or more may be 4x10⁷ count/mm² or less.
In addition, in the method for manufacturing the steel fiber for concrete reinforcement according to an embodiment of the present disclosure, a tensile strength may be 1,220 MPa or more. In addition, the steel fiber may have a tensile strength variation of less than ±50 MPa. In addition, the steel fiber may have a work hardening rate of 405 or more. In addition, the steel fiber may have an elongation of 5% or more.

[Advantageous Effects]

The use of the high-strength steel fiber for concrete reinforcement according to an embodiment of the present disclosure is advantageous in terms of construction because rebar is not used in concrete and a time for rebar placement is not required. In addition, because the steel fiber according to the present disclosure has improved toughness at a low temperature due to a high content of Ni added thereto, bending strength of concrete may be considerably increased when added to concrete, so that stability and lifespan may be improved.
However, the effects obtainable by the present disclosure are not limited to the aforementioned effects, and any other effects not mentioned herein will be clearly understood from the following description by those skilled in the art to which the present disclosure pertains.

[Description of Drawings]

FIG. 1 is an image of a microstructure observed at the center of a wire rod according to Invention Example 1.
FIG. 2 shows continuous cooling transformation (CCT) of components of Invention Example 1 calculated by using J-mat pro.
FIG. 3 is an image showing defects observed in a cross-section of a steel fiber of Invention Example 1.
FIG. 4 is an image of defects observed in a cross-section of a steel fiber of Comparative Example 6.

[Modes of the Invention]

Hereinafter, embodiments of the present disclosure will be described. However, the present disclosure may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art.
The terms used herein are merely used to describe particular embodiments. Therefore, an expression used in the singular encompasses the expression of the plural, unless it should be clearly singular in the context. In addition, it is to be understood that the terms such as "including" or "having" are intended to indicate the existence of features, steps, functions, components, or combinations thereof disclosed in the specification, and are not intended to preclude the possibility that other features, steps, functions, components, or combinations thereof may exist or may be added.
Meanwhile, unless otherwise defined, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Thus, these terms should not be interpreted in an idealized or overly formal sense unless expressly so defined herein. As used herein, the singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise.
Also, the terms "about", "substantially", etc. used throughout the specification means that when natural manufacturing and substance allowable errors are suggested, such allowable errors correspond to the value or is similar to the value, and such values are intended for the sake of clear understanding of the present disclosure or to prevent an unconscious infringer from illegally using the present disclosure of the present.
The present disclosure relates to a wire rod for steel fibers, a steel fiber for concrete reinforcement, and manufacturing methods therefor.
A wire rod for steel fibers according to an embodiment of the present disclosure may include, in percent by weight (wt%), 0.010 to 0.040% of carbon (C), more than 0% but not more than 0.10% of silicon (Si), 0.10 to 1.50% of manganese (Mn), 0.50 to 1.50% of nickel (Ni), 0.05% or less (including 0%) of phosphorus (P), 0.05% or less (including 0%) of sulfur (S), and the balance of iron (Fe) and inevitable impurities.
Hereinafter, reasons for numerical limitations on the contents of alloying elements of the aluminum alloy according to an embodiment of the present disclosure will be described. Hereinafter, the unit is wt% unless otherwise stated.
The content of carbon (C) may be from 0.010 to 0.040 wt%, and preferably from 0.010 to 0.035 wt%.
C is an element considerably increasing strength in pearlite, but an increase in the C content may cause a problem of forming pearlite that induces processing fracture during a wet drawing process. With a C content less than 0.010 wt%, a desired strength is difficult to achieve. On the contrary, with a C content exceeding 0.040 wt%, fractures may be caused during a drawing process due to formation of pearlite grain boundaries, and therefore the C content may be controlled to a level therebelow.
The content of silicon (Si) may be more than 0% but not more than 0.10 wt%, and preferably from 0.05 to 0.10 wt%.
Si, as a ferrite hardening element, may improve strength. However, Si may form Fe₂SiO₄ having excellent binding strength with a matrix, thereby disadvantageous in terms of scale peeling property. Therefore, the Si content may be controlled to 0.1% or less to improve scale peeling property.
The content of manganese (Mn) may be from 0.10 to 1.50 wt%, preferably from 0.10 to 1.10 wt%.
Mn is added to increase strength of a wire rod. With a Mn content less than 0.10 wt%, a target strength may be difficult to achieve. On the contrary, with a Mn content exceeding 1.50 wt%, there is a high possibility of occurrence of processing fractures due to segregation, and thus the Mn content may be controlled to a level therebelow.
The content of nickel (Ni) may be from 0.50 to 1.50 wt%, preferably from 0.50 to 1.30 wt%, and more preferably from 0.55 to 1.10 wt%.
Ni, as an austenite-stabilizing element, forms a solid solution in ferrite, so as to improve tensile strength by solid solution strengthening. In addition, Ni significantly improves toughness, which is effective in preventing fractures during a drawing process. Particularly, because Ni improves low-temperature toughness, bending strength of concrete against external force applied thereto may be significantly improved after mixing the steel fiber with concrete and repeating low temperature/room temperature cycles. With a Ni content less than 0.50 wt%, improvement in bending strength may not be observed. On the contrary, with a Ni content exceeding 1.50 wt%, occurrence of processing fractures may increase, and thus the Ni content may be controlled to a level therebelow.
The contents of phosphorus (P) and sulfur (S) may be each independently 0.05 wt% or less (including 0%), and preferably 0.040% or less. P and S are harmful elements regarded as impurities. With the contents exceeding 0.05%, segregation at the center may cause fractures during a drawing process, and therefore the contents may be controlled to levels therebelow.
The remaining component of the composition of the present disclosure is iron (Fe). However, the composition may include unintended impurities inevitably incorporated from raw materials or surrounding environments, and thus addition of other alloy components is not excluded. The impurities are not specifically mentioned in the present disclosure, as they are known to any person skilled in the art of manufacturing.
The wire rod for steel fibers according to an embodiment may include a scale with a thickness of 8 to 20 µm formed on the surface. During the manufacture of steel fibers, a scale is removed from drawn wires by mechanical peeling by using a bending rolling without a separate chemical scale peeling process. In this case, a too thin scale may make a scale peeling process of the wire rod difficult. Therefore, in order to use the wire rod as a product suitable for mechanical peeling, the oxide scale may preferably be formed to have a thickness of 8 µm or more. On the contrary, in the case where the thickness of the oxide scale exceeds 20 µm, a scale scattering phenomenon, where the oxide scale breaks and flies off, occurs during processing due to the too large thickness of the oxide scale, impairing the shape of coils, and accordingly making the manufacture of steel fibers difficult. Therefore, the thickness of the oxide scale may preferably be controlled to a level therebelow. Specifically, because the wire rod scale is a hard defect, a cemented carbide die inside a drawing die may break while processing the wire rod into the steel fiber in the case where the scale of the wire rod is too thick. Accordingly, deep die grooves are formed on the surface of a material, resulting in fractures during the drawing process and an increase in the number of defects (voids) formed by a large deformation applied to a local area of the surface, compared to a normal area, thereby increasing a processing fracture rate.
The wire rod for steel fibers according to an embodiment may have a tensile strength of 340 MPa or more. A low tensile strength of the wire rod may cause a decrease in tensile strength of the steel fiber, and accordingly, the tensile strength of the wire rod may satisfy at least 340 MPa to manufacture a steel fiber having a certain level of strength or higher. As the strength of the steel fiber decreases, the amount of the steel fiber used in concrete increases so that manufacturing costs increase. In addition, the use of a large amount of the steel fiber causes problems of increasing time required for mixing with concrete and curing the concrete. That is, with a tensile strength of the wire rod less than 340 MPa, it is difficult to obtain the effect in reducing the amount of the steel fiber mixed with concrete.
In addition, in the wire rod for steel fibers according to an embodiment, a tensile strength variation between coil overlap and non-overlap sections may be less than ±40 MPa, preferably ±30 MPa or less, and more preferably ±25 MPa or less. During a process of manufacturing a wire rod, a tensile strength variation between coil overlap and non-overlap sections may occur during cooling at a Stelmor cooling tower. By controlling the tensile strength variation of the wire rod as low as possible, the tensile strength variation of the steel fiber may also be controlled to be lowered.
In addition, because both the thickness of the oxide scale of 8 to 20 µm and the tensile strength variation less than ±40 MPa are satisfied, the wire rod for steel fibers according to an embodiment may be processed to a steel fiber for concrete reinforcement, and void defects and fracture rates may be considerably lowered during the manufacturing of the steel fiber. Furthermore, in the case of manufacturing a steel fiber using the wire rod satisfying such physical properties, compression strength may be normally applied to concrete mixed with the steel fiber, so as to provide concrete stable against external stress.
The wire rod for steel fibers according to an embodiment may include polygonal ferrite as a microstructure. For example, the wire rod for steel fibers may include polygonal ferrite in an area fraction of 98% or more, and preferably 99% or more. In the case where pearlite or cementite is included in grain boundaries in an area fraction of 2% or more, breakage may occur during wet drawing to cause fractures.
The steel fiber for concrete reinforcement according to an embodiment of the present disclosure may include 0.010 to 0.040% of carbon (C), more than 0% but not more than 0.10% of silicon (Si), 0.10 to 1.50% of manganese (Mn), 0.50 to 1.50% of nickel (Ni), 0.05% or less (including 0%) of phosphorus (P), 0.05% or less (including 0%) of sulfur (S), and the balance of iron (Fe) and inevitable impurities.
Reasons for numerical limitations on the contents of alloying elements are as described above.
The steel fiber according to an embodiment may include polygonal ferrite as a microstructure, i.e., polygonal ferrite in the form of fibers elongated by a drawing process. For example, the steel fiber may include polygonal ferrite in an area fraction of 98% or more.
In the steel fiber for concrete reinforcement according to an embodiment, the number of void defects formed in crystal grain boundaries and having a size of about 5 µm or more may be 4x10⁷ count/mm² or less, preferably 3x10⁷ count/mm² or less, and more preferably 2x10⁷ count/mm². As the void defect number decreases, elongation is improved. In the present disclosure, the elongation refers to a sum of uniform elongation and fracture elongation. If there are fewer cracks in a plastic deformation region, a material may be elongated further, and thus the uniform elongation may also increase.
The steel fiber for concrete reinforcement according to an embodiment may have a tensile strength of 1,220 MPa or more, and preferably 1,230 MPa or more. A higher tensile strength of the steel fiber may provide a superior concrete reinforcement effect.
In addition, in the steel fiber for concrete reinforcement according to an embodiment, a tensile strength variation between coil overlap and non-overlap sections may be less than ±50 MPa, preferably ±40 MPa or less, and more preferably ±30 MPa or less. A large tensile strength variation of a steel fiber may cause defects in products. Furthermore, a small tensile strength variation in the steel fiber may solve a problem of being detached from concrete. In addition, as the tensile strength variation of the steel fiber decreases, compression strength may be more normally applied to concrete, and thus concrete mixed with the steel fiber according to the present disclosure may be stable under external stress such as impact.
The steel fiber for concrete reinforcement according to an embodiment may have a work hardening rate of 405 or more when a total true strain of 4.6 or more, and preferably 4.6 to 5.0, is applied during a drawing process. For example, when a 6.5 mm-thick wire rod is drawn into a 0.55 mm-thick steel fiber, a total true strain may be 4.93. When a 6.0 mm-thick wire rod is drawn into a 0.55 mm-thick steel fiber, a total true strain may be 4.78. When a 5.5 mm-thick wire rod is drawn into a 0.55 mm-thick steel fiber, a total true strain may be 4.61. In addition, with a work hardening rate satisfying 405 or more, a desired steel fiber strength may be obtained. In this regard, the work hardening rate may be calculated by using Equation (1) below. $TS = A * Exp (e / 4) + B$
In Equation (1), TS is tensile strength, A is work hardening rate, B is extrapolated value when initial tensile strength Exp(e/4) is 0, and e is true strain.
The steel fiber for concrete reinforcement according to an embodiment may have an elongation of 5% or more, and preferably 5.8% or more.
The steel fiber for concrete reinforcement according to an embodiment may have a fracture rate of 5.0 times/ton or less, and preferably 1.5 times/ton or less. If the fracture rate per ton is too high, the steel fiber may not be suitable for processing steel fibers for concrete.
A bending strength of the steel fiber according to an embodiment when mixed with concrete may be 45 MPa or more, and preferably 50 MPa or more. The higher the bending strength, the better the low-temperature toughness. The steel fiber according to the present disclosure may have improved bending strength by designing the composition of alloying elements to contain a high Ni content. In this regard, the bending strength may be measured by repeating 100 cycles, each cycle consisting of cooling a specimen to a low temperature of -20°C, maintaining the specimen at the temperature for 1 hours, and maintaining the specimen at room temperature of 25°C (-20°Cxlhr → 25°Cxlhr → -20°Cx1hr), and measuring a bending strength under a load of 130 kg/mm³ applied to the center.
Hereinafter, a method for manufacturing a wire rod for high-strength steel fibers according to an embodiment of the present disclosure including the above composition of alloying elements will be described.
The wire rod for high strength steel fibers of the present disclosure may be manufactured by manufacturing a billet having the above-described composition of alloying elements, followed by reheating-wire rod rolling-coiling-cooling processes.
The method for manufacturing the wire rod for steel fibers according to an embodiment of the present disclosure includes: heating a billet including, by wt%, 0.010 to 0.040% of carbon (C), more than 0% but not more than 0.10% of silicon (Si), 0.10 to 1.50% of manganese (Mn), 0.50 to 1.50% of nickel (Ni), 0.05% or less (including 0%) of phosphorus (P), 0.05% or less (including 0%) of sulfur (S), and the balance of iron (Fe) and inevitable impurities; rolling the heated billet into a wire rod; coiling the wire rod in a temperature range of 880 to 950°C; and cooling the coiled wire rod to 300°C at a rate of 1°C/s or less.
Reasons for limitations on the composition of alloying elements are as described above, and hereinafter, processes of the manufacturing method thereof will be described in more detail.
After manufacturing the billet having the above-described composition of alloying elements, the billet is maintained in a heating furnace at a temperature of 1,000 to 1,250°C for 90 to 120 minute for normalizing and formation of austenite, and rolled. If maintained at a temperature below 1,000°C, a loading time may increase. If maintained at a temperature above 1,250°C, the heating furnace is under load. Therefore, the temperature may be preferably controlled in the range of 1,000 to 1,250°C. In addition, if the maintenance time is less than 90 minutes, austenite may be difficult to form at the center. If the maintenance time exceeds 120, coarse crystal grains may grow. Therefore, the maintenance time may preferably be from 90 to 120 minutes.
The heated billet is rolled under common rolling conditions. That is, the heated billet is subjected to a hot rolling process sequentially including rough rolling, intermediate rough rolling/finish rolling, and final rolling to manufacture a wire rod.
In order to form the oxide scale on the surface of the wire rod to an appropriate thickness, a coiling temperature is controlled to a range of 880 to 950°C by water quenching. For use as products for mechanical scale peeling, a scale with a thickness of 8 to 20 µm needs to be formed. At a coiling temperature below 880°C, the scale is formed to a thickness of less than 8 µm, failing to satisfy an appropriate thickness requirement. On the contrary, at a coiling temperature above 950°C, the appropriate scale thickness of 20 µm or less is satisfied, but defects may be caused in a coiling shape. Therefore, the scale thickness needs to be controlled to a level therebelow. Additional investment in equipment may be required to resolve the defects in the coiling shape.
Subsequently, the coiled wire rod is cooled in a Stelmor cooling tower from the coiling temperature to 300°C at a rate of 1°C/s or less. Because there is overlap sections in a ring in the Stelmor cooling tower, there is a tensile strength variation with non-overlap sections. When the cooling rate exceeds 1°C/s, the tensile strength variation between the wire rod and the steel fiber may be ±40 MPa or more. Because there is no homogenization treatment between the manufacturing of the wire rod and the drawing process, the steel fiber may have excellent properties by reducing the tensile strength variation in the wire rod. To suppress the tensile strength variation in the Stelmor cooling tower, it is preferably to cover the material and controlling the cooling rate to 1°C/s or less by minimizing the airflow.
The wire rod for steel fibers manufactured by a method according to an embodiment may include polygonal ferrite as a microstructure and may have a tensile strength of 340 MPa or more.
The wire rod for steel fibers manufactured by a method according to an embodiment may have a tensile strength variation between coil overlap and non-overlap sections of less than ±40 MPa, preferably ±30 MPa or less, and more preferably ±25 MPa or less.
The wire rod for steel fibers manufactured by a method according to an embodiment may be a wire rod having a scale with a thickness of 8 to 20 µm formed on the surface.
Hereinafter, a method for manufacturing the steel fiber for concrete reinforcement having the above-described composition of alloying elements according to an embodiment of the present disclosure will be described.
The steel fiber for concrete reinforcement according to the present disclosure may be manufactured by dry drawing and wet drawing the above-described wire rod for steel fibers.
The method for manufacturing the steel fiber for concrete reinforcement according to an embodiment of the present disclosure may include manufacturing a steel fiber by dry drawing and wet drawing the wire rod manufactured by the method for manufacturing a wire rod for steel fibers with a total true strain 4.6 or more, and preferably 4.6 to 5.0. By drawing the wire rod with a high true strain, the tensile strength variation in products may be reduced.
In the steel fiber manufactured by using the method according to an embodiment, the number of void defects formed in crystal grain boundaries and having a size of about 5 µm or more may be 4x10⁷ count/mm² or less.
The steel fiber manufactured by using the method according to an embodiment may have a tensile strength of 1,220 MPa or more, a work hardening rate of 405 or more, and an elongation of 5% or more.
The steel fiber manufactured by using the method according to an embodiment may have a fracture rate of 1.5 times/ton or less and a bending strength of 45 MPa or more when mixed with concrete.
The steel fiber manufactured by using the method according to an embodiment may have a tensile strength variation between coil overlap and non-overlap sections of less than ±50 MPa, preferably ±40 MPa or less, and more preferably ±30 MPa or less.
Hereinafter, the present disclosure will be described in more detail with reference to the following examples. However, the following examples are merely presented to exemplify the present disclosure, and the scope of the present disclosure is not limited thereto.

[Examples]

In this embodiment, steels having the compositions of alloying elements shown in Table 1 below were made in a converter and cast under common conditions to produce cast billets having a size of 160x160 mm². After maintaining the billet in a heating furnace at a temperature of 1,050°C for 90 minutes, the billet was rolled under common conditions, controlled to the coiling temperature shown in Table 2 below by finish rolling and cooling in a cooling bed, and cooled in a Stelmor cooling tower to 300°C at a cooling rate shown Table 2 below, thereby producing a wire rod.
In addition, tensile strength (TS), tensile strength variation, oxide scale thickness of the surface, and microstructure fraction of the produced wire rod were measured, and the results are shown in Table 2 below.
The tensile test was performed according to the ISO6892-1 standard, and a tensile speed (cross head speed) was 20 m/min. The collected wire rod was cut into 400 mm pieces, continuously (12 pieces), and then tensile strength thereof was measured and average and deviation were identified.
To measure the thickness of surface scale, a specimen of the tensile test was cut by 1 cm in the lengthwise direction using a micro cutter, followed by mirror polishing by section polishing. Thicknesses of the scale were measured at different positions of the section by using an optical microscope, and an average thickness was identified.

In addition, images were obtained at 200x by using an optical microscope to figure out microstructure fractions. After obtaining area fractions of a total of 10 sheets, an average was obtained.

Table 1

Steel type	Composition of alloying elements of wire rod (wt%)
Steel type	C	Si	Mn	Ni	P	S
Steel 1	0.021	0.08	0.20	0.99	0.035	0.032
Steel 2	0.011	0.09	0.19	1.02	0.035	0.035
Steel 3	0.021	0.08	1.02	1.00	0.035	0.032
Steel 4	0.020	0.10	0.21	0.51	0.034	0.033
Steel 5	0.020	0.10	0.21	1.01	0.034	0.033
Steel 6	0.045	0.09	0.23	1.02	0.033	0.030
Steel 7	0.022	0.32	0.20	1.00	0.035	0.030
Steel 8	0.023	0.11	1.59	0.98	0.035	0.031
Steel 9	0.020	0.09	0.22	1.61	0.031	0.035

Table 2

Category	Steel type	Wire rod
Category	Steel type	Coiling tempera ture (°C)	Coolin g rate (°C/s)	Tensile strength (MPa)	Strength variatio n (MPa)	Scale thicknes s (µm)	Polygonal ferrite area fraction (%)	Pearlite or cementite area fraction (%)
Invention Example 1	Steel 1	908	0.8	360	±20	13.2	99.2	0.8
Invention Example 2	Steel 2	907	0.7	348	±25	13.8	99.9	0.1
Invention Example 3	Steel 3	910	0.9	375	±22	12.8	99.4	0.6
Invention Example 4	Steel 4	911	1.0	341	±24	13.7	99.5	0.5
Comparative Example 1	Steel 5	832	0.7	362	±25	3.5	99.8	0.2
Comparative Example 2	Steel 1	908	7.5	360	±48	13.2	99.7	0.3
Comparative Example 3	Steel 6	912	0.9	412	±24	12.1	95.8	4.2
Comparative Example 4	Steel 7	911	0.8	378	±24	13.8	99.7	0.3
Comparative Example 5	Steel 8	915	0.8	390	±21	14.0	99.2	0.8
Comparative Example 6	Steel 9	912	0.7	400	±24	12.5	99.7	0.3

Tables 1 and 2 show compositions of alloying elements of the wire rods and mechanical properties thereof. Representative alloying elements of Invention Example 1 include 0.021C-0.08Si-0.2Mn-0.99Ni (wt%). The coiling temperature was 908°C, and the cooling rate was 0.8°C/s in this case. FIG. 1 is an image of a microstructure observed at the center of the wire rod according to Invention Example 1. Based thereon, it may be confirmed that the microstructure of the wire rod of Invention Example 1 is polygonal ferrite. Although the cooling rate was high, conditions allowing formation of only ferrite were confirmed based on the J-mat pro of FIG. 2. In this case, the wire rod of Invention Example 1 had a tensile strength of 360 MPa and a scale thickness of 13.2 µm. In Invention Examples 2 to 4, the contents of carbon, manganese, and nickel were controlled, respectively, within the ranges of the present disclosure, based on Invention Example 1 and satisfied the conditions of the tensile strength of 340 MPa or more and the scale thickness of 8 to 20 µm.
Meanwhile, although Comparative Example 1 satisfied the composition of alloying elements according to the present disclosure, the scale thickness was 3.5 µm due to a low coiling temperature of 830°C, which is considerably reduced in comparison with Invention Example 1.
Although the composition of alloying elements of Comparative Example 2 was the same as that of Invention Example 1, the increased cooling rate of 7.5°C/s considerably increased the tensile strength variation to ±48 MPa.
Comparative Examples 3 to 6 did not satisfy the range of the composition of alloying elements according to the present disclosure.

The manufactured wire rods were dry and wet drawn to produce steel fibers, and tensile strength (TS), tensile strength variation of overlap sections, work hardening rate (A), elongation, the number of void defects having a size of 5 µm or more, and processing fracture rate per ton were measured and shown in Table 3 below.
The tensile test was performed according to the ISO6892-1 standard, and a tensile speed (cross head speed) was 50 m/min. The specimen was cut into 300 mm pieces, continuously (20 pieces), and then tensile strengths thereof were measured and average and deviation were identified.
The number of void defects was identified by using a scanning electron microscope, and the image was obtained at x1000.
The processing fracture rate per ton refers to interruption of processing by fractures during a drawing process. After counting the number of fractures from a total supply of 100 tone of the wire rod, an average was calculated.
In addition, 20 kg of each of the steel fibers manufactured according to Table 1 and 2 was mixed with 135 kg of concrete to construct specimens (cuboids) each having a length of 3,400 mm, a width of 1,200 mm, and a thickness of 200 mm.

In addition, after repeating cycles (-20°Cxl hour → 25°Cx1 hour → -20°Cx1 hour →...; 100 times), bending strength of the specimen were measured by applying a load of 130 kg/mm³ using a node at the center, and the results are shown in Table 3 below.

Table 3

Category	Steel fiber						Concrete
Category	Tensile strength (MPa)	Strength variation (MPa)	Work hardenin g rate (A)	Elongatio n (%)	Defect number (count/m m²)	Fractur e rate (times/t on)	Bending strength (MPa)
Invention Example 1	1,250	±25	410	8.5	2x10⁷	0.5	55
Invention Example 2	1,230	±29	408	8.7	2x10⁷	0.6	57
Invention Example 3	1,320	±25	414	7.1	3x10⁷	0.9	52
Invention Example 4	1,227	±27	405	5.8	4x10⁷	1.2	45
Comparative Example 1	1,240	±28	411	8.9	5x10⁷	8.2	54
Comparative Example 2	1,250	±65	409	8.7	9x10⁷	0.9	52
Comparative Example 3	impossible to process	-	-	-	3x10⁷	20.0	-
Comparative Example 4	1,280	±28	413	7.4	5x10⁷	12.0	55
Comparative Example 5	impossible to process	-	-	-	9x10⁷	18.0	-
Comparative Example 6	impossible to process	-	-	-	6x10⁷	7.0	-

Referring to Table 3, the steel fiber manufactured using the wire rod of Invention Example 1 had a tensile strength of 1,250 MPa and an elongation of 8.5%, and a work hardening rate (A) calculated by Equation (1) below was 410. $TS = A * Exp (e / 4) + B$
In Equation (1), TS is tensile strength, A is work hardening rate, B is extrapolated value when initial tensile strength Exp(e/4) is 0, and e is true strain.
In addition, FIG. 3 is an SEM image of a cross-section of a steel fiber of Invention Example 1. Referring thereto, in the steel fiber of Invention Example 1, the number of defects, such as voids, having a size of 5 µm or more was 2*10⁷ count/mm². In this case, a fair processing fracture rate per ton of 0.5 was obtained.
Meanwhile, it was confirmed that concrete manufactured by mixing with the steel fiber of Invention Example 1 had a bending strength of 55 MPa.
Although the steel fiber of Invention Example 2 having the C content of 0.011 % had similar properties such as the fracture rate to those of the steel fiber of Invention Example 1, the strength decreased and the elongation slightly increased.
In the steel fiber of Invention Example 3 having the Mn content of 1.02%, fractures did not seriously occur during working. Although the steel fiber had a higher strength and a slightly lower elongation than those of Invention Example 1, fair bending strength of the concrete was obtained.
Although the steel fiber of Invention Example 4 had slightly inferior physical properties such as tensile strength and elongation due to the lower Ni content of 0.51% than that of the steel fiber of Invention Example 1, it was confirmed that physical properties desired by the present disclosure were achieved.
In addition, in the case of using conventional steel fibers, the bending strength of 45 MPa or more was satisfied by adding about 25 kg of the steel fiber per 135 g of concrete. On the contrary, about 20 kg of each of the steel fibers of Examples 1 to 4 per 135 g of concrete was sufficient to achieve the bending strength of 45 MPa or more. Thus, it was confirmed that excellent effect may be obtained while using a reduced amount of the steel fiber.
Meanwhile, Comparative Examples 1 to 6 exhibited differences from Invention Example 1 in the case of varying the coiling temperature, the Stelmor cooling rate, the C content, the Si content, the Mn content, or the Ni content, respectively, from those of Invention Example 1.
In Comparative Example 1, a low coiling temperature of 830 °C was used while manufacturing the wire rod, and the number of fractures per ton was 8.2, which was significantly inferior to that of Invention Examples 1 to 4. Specifically, a drawing process was able to be performed in the case of Comparative Example 1, but fractures easily occurred in the wire rod due to a too thin scale of the wire rod with a thickness of 3.5 µm.
Comparative Example 2 is related to Stelmor cooling rate and provides conditions in which the cooling rate is significantly increased to 7.5 °C /s while manufacturing the wire rod compared to Invention Example 1. Although the tensile strength was similar to that of Invention Example 1, it was confirmed that the tensile strength variation of the product after the drawing process significantly increased to ±65 MPa and the number of defects, such as voids, having a size of 5 µm or more significantly increased.
In Comparative Example 3, the C content was increased to 0.045%. Pearlite or cementite was formed in large quantities in grain boundaries, failing to perform a drawing process and the fracture rate significantly increased.
In Comparative Example 4, the Si content was increased to 0.32%. Due to the high Si content, a scale having a composition of Fe₂SiO₄ is formed on the surface of the wire rod, so that the fracture rate per ton deteriorated.
In Comparative Example 5, the Mn content was increased to 1.59%. It was confirmed that segregation occurring at the center due to the high Mn content makes the drawing process impossible.
In Comparative Example 6, the Ni content was increased to 1.61%. Ni, which forms a relatively hard low-temperature structure by increasing ductility, induces Chevron uniformity during cold drawing, failing to perform a drawing process. In addition, the fracture rate per ton of 7 was not suitable for processing. In addition, FIG. 4 is an SEM image of a cross-section of a steel fiber of Comparative Example 6. Based thereon, it was confirmed that a lot of void defects (6x10⁷ count/mm²) occurred.
While the present disclosure has been particularly described with reference to exemplary embodiments, it should be understood by those of skilled in the art that the scope of the present disclosure is not limited thereby and various changes in form and details may be made without departing from the spirit and scope of the present disclosure.

Claims

A wire rod for steel fibers comprising, in percent by weight (wt%), 0.010 to 0.040% of carbon (C), more than 0% but not more than 0.10% of silicon (Si), 0.10 to 1.50% of manganese (Mn), 0.50 to 1.50% of nickel (Ni), 0.05% or less (including 0%) of phosphorus (P), 0.05% or less (including 0%) of sulfur (S), and the balance of iron (Fe) and inevitable impurities,
wherein a scale with a thickness of 8 to 20 µm is formed on the surface, and

a tensile strength variation is less than ±40 MPa.
The wire rod according to claim 1, wherein a tensile strength is 340 MPa or more.
The wire rod according to claim 1, comprising polygonal ferrite as a microstructure.
A steel fiber for concrete reinforcement comprising, in percent by weight (wt%) 0.010 to 0.040% of carbon (C), more than 0% but not more than 0.10% of silicon (Si), 0.10 to 1.50% of manganese (Mn), 0.50 to 1.50% of nickel (Ni), 0.05% or less (including 0%) of phosphorus (P), 0.05% or less (including 0%) of sulfur (S), and the balance of iron (Fe) and inevitable impurities,
wherein the number of void defects formed in crystal grains boundaries and having a size of 5 µm or more is 4x10⁷ count/mm² or less.
The steel fiber according to claim 4, wherein a tensile strength is 1,220 MPa or more, and
a work hardening rate is 405 or more.
The steel fiber according to claim 4, wherein an elongation is 5% or more.
The steel fiber according to claim 4, wherein a fracture rate is 1.5 times/ton or less.
The steel fiber according to claim 4, wherein a tensile strength variation is less than ±50 MPa.
A method for manufacturing a wire rod for steel fibers, the method comprising:
heating a billet comprising, in percent by weight (wt%), 0.010 to 0.040% of carbon (C), more than 0% but not more than 0.10% of silicon (Si), 0.10 to 1.50% of manganese (Mn), 0.50 to 1.50% of nickel (Ni), 0.05% or less (including 0%) of phosphorus (P), 0.05% or less (including 0%) of sulfur (S), and the balance of iron (Fe) and inevitable impurities;

rolling the heated billet into a wire rod;

coiling the wire rod in a temperature range of 880 to 950°C; and

cooling the coiled wire rod to 300°C at a rate of 1°C/s or less.
The method according to claim 9, wherein the billet is maintained in a heating furnace at a temperature of 1,000 to 1,250°C for 90 to 120 minutes in the heating of the billet.
The method according to claim 9, wherein the wire rod for steel fibers comprises polygonal ferrite as a microstructure, and a scale with a thickness of 8 to 20 µm is formed on the surface of the wire rod.
A method for manufacturing a steel fiber for concrete reinforcement, the method comprising dry drawing and wet drawing the wire rod according to any one of claims 1 to 3 with a true strain of 4.6 or more into a steel fiber,
wherein a fracture rate during drawing is 1.5 times/ton or less.
The method according to claim 12, wherein the number of void defects formed in crystal grain boundaries and having a size of 5 µm or more is 4x10⁷ count/mm² or less.
The method according to claim 12, wherein the steel fiber has
a tensile strength of 1,220 MPa or more,

a tensile strength variation of less than ±50 MPa,

a work hardening rate of 405 or more, and

an elongation of 5% or more.