EP4613902A1

EP4613902A1 - Concrete-reinforcing steel fiber wire rod not undergoing heat treatment, steel fiber, and manufacturing methods therefor

Info

Publication number: EP4613902A1
Application number: EP23907459.4A
Authority: EP
Inventors: Yosep YANG; Jaehwan Kim; Seongryeol JEON
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2022-12-21
Filing date: 2023-11-24
Publication date: 2025-09-10
Also published as: WO2024136173A1; KR20240098409A; CN120418470A; JP2025539890A; EP4613902A4

Abstract

The present invention provides: a concrete-reinforcing steel fiber wire rod, which does not undergo LP heat treatment during drawing, does not break when repeatedly bent at 90-degrees 10 times or more, and has a tensile strength of 1,700 MPa or more; a steel fiber; and manufacturing methods therefor. The concrete-reinforcing steel fiber wire rod according to an embodiment of the present invention comprises, by wt%, 0.005-0.035% of C, 0.07-0.3% of Si, 0.07-0.2% of Mn, 1.0-2.2% of Cr, 0.05% or less of P, 0.05% or less of S and the balance of Fe and other inevitable impurities, and satisfies Expression 1. [Expression 1] [C]+0.17*[Mn]+0.25*[Cr]-0.62≤0 ([C], [Mn] and [Cr] each indicate wt%.).

Description

[Technical Field]

The present invention relates to a wire rod for concrete reinforcing steel fibers that are used as concrete reinforcements in tunnels, floors, and the like, steel fibers, and a method for manufacturing the same.

[Background Art]

Steel fibers vary in diameter and length depending on the use, such as for tunnels and flooring. In the case of tunnels, steel fibers are used as shorts, and have a smaller diameter and shorter length, while in the case of flooring, steel fibers have a relatively larger diameter and longer length. Steel fibers are drawn (dry or wet drawing) using wire rods having a diameter of 5.0 to 7.0 mm, and due to a thin diameter, they require a structure that may withstand high processing amounts. Ultra-low carbon steels having a maximum carbon content of 0.03% or 0.035% are used, as fully ferrites have the best drawability, and although pearlite may be formed at grain boundaries, an increased fraction of pearlite may lead to defects in the hard pearlite, causing wire breakage during processing.
In the past, low-strength steel fibers of 1000 MPa or less were used, but in order to achieve goals such as shortening construction time and reducing manufacturing costs, construction companies now demand high-strength steel fibers (1500 MPa or more). Various approaches haven been attempted to increase the strength of steel fibers, and the direction is toward enhancing the strength by increasing the carbon content. Unlike other elements, carbon is a cost effective element that may effectively increase the strength during drawing. This is because carbon forms hard cementite, which is a component of pearlite, and when carbon increases by 0.1%, the tensile strength may increase by approximately 100 MPa. However, as described above, the formation of pearlite causes wire breakage during processing, and in order to resolve these limitations, isothermal heat treatment (lead patenting) that restores ductility and enables grain refinement during drawing has been introduced. However, since isothermal heat treatment uses lead, which poses environmental contamination, and the additional heat treatment process increases the manufacturing costs.
In addition, due to the strengthening of global environmental regulations, harmful acid pickling has been replaced by mechanical descaling for scale removal. On the other hand, Si forms firelite (Fe₂SiO₄) at the boundary between the scale and the base material, which degrades the scale removability, and thus it is important to minimize SI.

[Disclosure]

[Technical Problem]

To resolve the above-described issues, the present invention is directed to providing a wire rod for concrete reinforcing steel fibers, steel fibers, and a method for manufacturing the same, which have a tensile strength of 1700 MPa or more and do not fracture after at least 10 repeated applications of 90-degree bending, through dry drawing and wet drawing without a lead patenting (LP) heat treatment by controlling Expression1 and microstructure through an alloy composition and a manufacturing process.
The technical objectives of the present invention are not limited to the above, and other objectives that are not described above will be clearly understood by those skilled in the art from the above detailed description.

[Technical Solution]

A wire rod for concrete reinforcing steel fibers according to an example of the present invention includes, in percent by weight (wt%), 0.005 to 0.035% of C, 0.07 to 0.3% of Si, 0.07 to 0.2% of Mn, 1.0 to 2.2% of Cr, 0.05% or less of P, 0.05% or less of S, and the balance of Fe and other inevitable impurities, and satisfies Expression 1 below,
$[C] + 0.17 * [Mn] + 0.25 * [Cr] - 0.62 \leq 0$
Here, [C], [Mn], and [Cr] represent the content (wt%) of each element.
In the wire rod for concrete reinforcing steel fibers according to an example of the present invention, a microstructure thereof may include, in area fraction, more than 98% and less than 100% quasi polygonal ferrite and more than 0% and less than 2% cementite.
In the wire rod for concrete reinforcing steel fibers according to an example of the present invention, an average grain size of the quasi polygonal ferrite in a range of 1/4D based on a cross-section may be more than 0 µm and 35 µm or less. D refers to a diameter of the wire rod.
The wire rod for concrete reinforcing steel fibers according to an example of the present invention may have a tensile strength of 850 MPa or more.
A method for manufacturing a wire rod for concrete reinforcing steel fibers according to an example of the present invention includes: maintaining a billet comprising, in percent by weight (wt%), 0.005 to 0.035% of C, 0.07 to 0.3% of Si, 0.07 to 0.2% of Mn, 1.0 to 2.2% of Cr, 0.05% or less of P, 0.05% or less of S, and the balance of Fe and other inevitable impurities at a temperature range of 1,000 to 1,250°C for 90 to 120 minutes, and then rolling the billet to produce a wire rod; coiling the produced wire rod at a temperature range of 880 to 950°C, wherein cooling after the coiling includes a first cooling of cooling the wire rod to a temperature of 800°C at a rate of 1°C/s or less, and a second cooling of cooling the wire rod to a temperature of 300°C at a rate of 20°C/s or more.
In the method for manufacturing a wire rod for concrete reinforcing steel fibers according to an example of the present invention, the wire rod may satisfy Expression 1 below,
$[C] + 0.17 * [Mn] + 0.25 * [Cr] - 0.62 \leq 0,$
Here, [C], [Mn], and [Cr] represent the content (wt%) of each element.
A concrete reinforcing steel fiber according to an example of the present invention includes, in percent by weight (wt%), 0.005 to 0.035% of C, 0.07 to 0.3% of Si, 0.07 to 0.2% of Mn, 1.0 to 2.2% of Cr, 0.05% or less of P, 0.05% or less of S, and the balance of Fe and other inevitable impurities, satisfies Expression 1 below, and has a tensile strength of 1700 MPa or more. $[C] + 0.17 * [Mn] + 0.25 * [Cr] - 0.62 \leq 0,$
Here, [C], [Mn], and [Cr] represent the content (wt%) of each element.
In the concrete reinforcing steel fiber according to an example of the present invention, no fracture occurs after at least 10 repeated applications of 90-degree bending.
A method for manufacturing a concrete reinforcing steel fiber according to an example of the present invention includes: dry drawing a wire rod that comprises, in percent by weight (wt%), 0.005 to 0.035% of C, 0.07 to 0.3% of Si, 0.07 to 0.2% of Mn, 1.0 to 2.2% of Cr, 0.05% or less of P, 0.05% or less of S, and the balance of Fe and other inevitable impurities, and satisfies Expression 1 below; and wet drawing the wire rod, wherein a lead patenting (LP) heat treatment is omitted after the dry drawing and before the wet drawing, and a tensile strength is 1700 MPa or more. $[C] + 0.17 * [Mn] + 0.25 * [Cr] - 0.62 \leq 0 .$
Here, [C], [Mn], and [Cr] represent the content (wt%) of each element.

[Advantageous Effects]

A wire rod for concrete reinforcing steel fibers, steel fibers, and a method for manufacturing the same according to an embodiment of the present invention can increase the initial material strength and work hardening rate by adding a high content of Cr of 1.0% or more, which provides an excellent solid solution strengthening effect, and can enable a reduction in manufacturing cost by minimizing the content of Si and Mn, which have insufficient solid solution strengthening effect or degrade scale removability, and since the processing is performed only through drawing without the need for lead patenting (LP) heat treatment, reduce the manufacturing process cost, and since the use of reinforcing bars in the concrete is omitted, shorten the construction period. Furthermore, since scale is removed through mechanical descaling instead of pickling, the environmentally friendly image of the product can be enhanced and its competitiveness in the global market can be increased.
A wire rod for concrete reinforcing steel fibers, steel fibers, and a method for manufacturing the same according to an embodiment of the present invention can ensure a tensile strength of 1700 MPa or more and not fracture after at least 10 repeated applications of 90-degree bending, through dry drawing and wet drawing without a lead patenting (LP) heat treatment during processing.

[Description of Drawings]

FIG. 1 is a photograph showing the microstructure observed in Inventive Example 4.

[Modes of the Invention]

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The embodiments of the present disclosure may, however, 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.
Also, the terms used herein are merely used to describe particular embodiments. An expression used in the singular encompasses the expression of the plural, unless otherwise indicated. Throughout the specification, the terms such as "including" or "having" are intended to indicate the existence of features, operations, functions, components, or combinations thereof disclosed in the specification, and are not intended to preclude the possibility that one or more other features, operations, 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.
The terms "about", "substantially", etc. used throughout the specification means that when a natural manufacturing and a substance allowable error are suggested, such an allowable error corresponds the value or is similar to the value, and such values are intended for the sake of clear understanding of the present invention or to prevent an unconscious infringer from illegally using the disclosure of the present invention.
A wire rod for concrete reinforcing steel fibers according to an embodiment of the present invention may include, in percent by weight (wt%), 0.005 to 0.035% of C, 0.07 to 0.3% of Si, 0.07 to 0.2% of Mn, 1.0 to 2.2% of Cr, 0.05% or less of P, 0.05% or less of S, and the balance of Fe and other inevitable impurities.
Hereinafter, the reason for numerically limiting the alloy element contents in the embodiments of the present invention will be described.
The content of C may be 0.005 to 0.035 wt%.
C is an element that greatly improves strength during the formation of pearlite or cementite, but an increase in the C content may lead to the formation of pearlite that may cause wire breakage during wet drawing. Therefore, when the C content is less than 0.005%, it is difficult to achieve the target strength, and when the C content exceeds 0.035%, cementite having an area fraction of 2% or more may form along grain boundaries, which may cause wire breakage during drawing. Accordingly, it is desirable to control the C content to less than or equal to 0.035%.
The content of Si may be 0.07 to 0.3 wt%.
Si is a ferrite strengthening element that contributes to improved strength but is unfavorable in terms of scale removability as it forms Fe₂SiO₄, which strongly adheres to the base material. Therefore, when the Si content exceeds 0.3%, the scale removability is inferior, and thus it is desirable to control the Si content to 0.07% or more due to limitations caused by the introduction of exogenous slag.
The content of Mn may be 0.07 to 0.2 wt%.
Mn is an element that contributes to solid solution strengthening and hardenability, and may combine with S present in steel to form MnS, and thus, it may be included in an amount of 0.07% or more. However, since the addition of Mn has a limited effect on strengthening improvement and increase the costs, it is desirable to set the maximum content to 0.2%.
The content of Cr may be 1.0 to 2.2 wt%.
In the present invention, Cr is a primary solid solution strengthening element that is present in ferrite and increases the strength of the material. When Cr is added in an amount of 0.1%, the tensile strength may increase by about 40 MPa. In addition, the addition of Cr may increase the work hardening rate during drawing by 300 or more, enabling strength improvement also in the final product. When the Cr content is less than 1.0%, it is difficult to achieve the target strength, and when the Cr content exceeds 2.2%, cracks may occur due to stress difference between the surface and the center caused by martensite formation in the central region, when a continuously cast bloom is charged into a high-temperature furnace at a low temperature. Therefore, it is desirable to control the Cr content to 2.2% or less.
The contents of P and S may be 0.05 wt% or less.
P and S are harmful elements, and when the contents exceed 0.05%, wire breakage may occur during drawing due to segregation at the center. Therefore, it is desirable to control P and S to 0.05 wt% or less.
The remaining component(s) of the disclosed invention is iron (Fe). However, unintended impurities may inevitably be introduced from raw materials or the surrounding environment in a typical manufacturing process, and thus cannot be excluded. Since such impurities may be well known to those skilled in the art of conventional manufacturing processes, details thereof are not described in this specification.
Hereinafter, a wire rod for concrete reinforcing steel fibers according to an embodiment of the present invention having the above-described alloy composition will be described.
The wire rod for concrete reinforcing steel fibers according to an embodiment of the present invention may satisfy Expression 1. $[C] + 0.17 * [Mn] + 0.25 * [Cr] - 0.62 \leq 0$
Here, [C], [Mn], and [Cr] represent the content (wt%) of each element.
With respect to Expression 1 above, C, Mn, and Cr are elements related to the strength of the material. In particular, the addition of Cr at a high content of 1.0% or more, which has an excellent solid solution strengthening effect, may increase the initial material strength and work hardening rate. However, excessive addition of Cr content may cause internal cracks when the billet is cooled and then charged into the heating furnace, which may lead to breakage inside the heating furnace or cause cobbling during rolling. Therefore, Expression 1 represents a relationship for appropriate contents of C, Mn, and Cr, and when the value of Expression 1 is more than 0, billet cracks may occur, resulting in cobbling formation during wire rod rolling. Therefore, it is desirable to control the value to be less than or equal to 0.
In addition, by controlling Expression 1 to maintain an appropriate Cr content, internal cracks and cobble formation may be prevented, thereby increasing the productivity and achieving a reduction in the manufacturing cost of the wire rod.
The microstructure of the wire rod for concrete reinforcing steel fibers according to an embodiment of the present invention may include, in area fraction, more than 98% and less than 100% of quasi polygonal ferrite and more than 0% and less than 2% of cementite.
Since the primary structure of the steel is composed of quasi polygonal ferrite, which is a low-temperature transformation structure, through a large amount of Cr content, wire breakage under load may be prevented even when a lead patenting (LP) heat treatment is omitted, and compared to when the primary structure is composed of polygonal ferrite, the strength reduction may be suppressed without inhibition of elongation, and the like, thereby enabling a higher tensile strength. In addition, the formation of carbides such as CrC due to the addition of Cr reduces the cementite formation compared to when Cr is not added, which may lower the probability of wire breakage during drawing.
In addition, when the microstructure of the wire rod includes, in area fraction, 2% or more of cementite, the formation of cementite along grain boundaries may cause a wire breakage during drawing. Therefore, it is desirable to control the area fraction of cementite to be more than 0 and less than 2%.
In addition, in the wire for concrete reinforcing steel fibers according to an embodiment of the present invention, an average grain size of the quasi polygonal ferrite in a range of 1/4D based on a cross-section may be more than 0 µm and 35 µm or less. Here, D refers to a diameter of the wire rod.
When the average grain size of the quasi-polygonal ferrite is more than 0 µm and 35 µm or less, a wire breakage may be prevented during drawing, and the 90-degree bending performance of the final steel fiber may be greatly improved to withstand at least 10 repetition.
In addition, the wire rod for concrete reinforcing steel fiber according to an embodiment of the present invention may have a tensile strength of 850 MPa or more.
Hereinafter, a method for manufacturing a wire rod for concrete reinforcing steel fibers according to an embodiment of the present invention having the above described alloy composition will be described.
The wire rod for concrete reinforcing steel fibers according to an embodiment of the present invention may be manufactured by preparing a billet having the alloy composition described above, and then subjecting the billet to reheating, wire rod rolling, coiling, and cooling processes.
The method for manufacturing a wire rod for concrete reinforcing steel fibers may include maintaining a billet comprising, in percent by weight (wt%), 0.005 to 0.035% of C, 0.07 to 0.3% of Si, 0.07 to 0.2% of Mn, 1.0 to 2.2% of Cr, 0.05% or less of P, 0.05% or less of S, and the balance of Fe and other inevitable impurities at a temperature range of 1,000 to 1,250°C for 90 to 120 minutes, and then rolling the billet to produce a wire rod; coiling the produced wire rod at a temperature range of 880 to 950°C, wherein cooling after the coiling includes a first cooling of cooling the wire rod to a temperature of 800°C at a rate of 1°C/s or less, and a second cooling of finally cooling the wire rod to a temperature of 300°C at a rate of 20°C/s or more.
After preparing the billet having the above-described alloy composition, in order to achieve normalizing and austenite formation, the billet is maintained at a heating temperature of 1,000 to 1,250°C for 90 to 120 minutes and then rolled. When the temperature is maintained below 1,000°C, the charging time becomes long, and when the temperature is maintained above 1,250°C, it imposes an increased thermal load. Therefore, it is desirable to control the temperature to 1,000 to 1,250°C. In addition, when the temperature is maintained below 90 minutes, it may be difficult to form central austenite, and when the temperature is maintained above 120 minutes, coarse grain growth may occur. Therefore, it is desirable to maintain the temperature for 90 to 120 minutes.
The heated billet is rolled under conventional rolling conditions. That is, the heated billet is subjected to hot rolling sequentially including rough rolling, intermediate rough rolling/finish rolling, and final rolling to produce a wire rod.
For mechanical descaling, it is required to form scale at an appropriate thickness (8~20um). To this end, the coiling temperature is controlled to 880°C~950°C through water cooling. When the temperature is below 880°C, the minimum thickness of 8um is not achieved, and when the temperature is above 950°C, the thickness of 20um is achieved, but a coiling shape defect (requiring equipment investment) occurs. Therefore, it is desirable to control the coiling temperature within the range.
In a first cooling following the coiling, the wire rod is cooled to 800°C at a rate of 1°C/s or less such that the scale thickens since the scale is removed through mechanical descaling. When the cooling rate exceeds 1°C/s in the first cooling, a desired scale thickness for mechanical descaling may not be achieved.
In a second cooling following the first cooling, since the scale transformation from FeO to Fe₂O₄ needs to be suppressed to prevent scattering, the wire rod is cooled to 300°C at a rate of 20°C/s or more using a reforming tube in a Stelmore cooling zone, thereby suppressing the transformation and increasing the tensile strength. Preferably, the cooling rate in the second cooling may be 30°C/s or less. When the cooling rate exceeds 30°C/s, the tensile strength targeted by the present invention may not be achieved.
In addition, the wire rod may satisfy Expression 1. $[C] + 0.17 * [Mn] + 0.25 * [Cr] - 0.62 \leq 0$
Here, [C], [Mn], and [Cr] represent the content (wt%) of each element.
Hereinafter, a concrete reinforcing steel fiber according to an embodiment of the present invention having the above-described alloy composition will be described.
The concrete reinforcing steel fiber according to the present invention may include, in percent by weight (wt%), 0.005 to 0.035% of C, 0.07 to 0.3% of Si, 0.07 to 0.2% of Mn, 1.0 to 2.2% of Cr, 0.05% or less of P, 0.05% or less of S, and the balance of Fe and other inevitable impurities, and satisfy Expression 1 below, and have a tensile strength of 1700 MPa or more, $[C] + 0.17 * [Mn] + 0.25 * [Cr] - 0.62 \leq 0$
Here, [C], [Mn], and [Cr] represent the content (wt%) of each element.
In addition, the concrete reinforcing steel fiber according to the present invention may have no fracture after at least 10 repeated applications of 90-degree bending.
In addition, the concrete reinforcing steel fiber according to the present invention may be manufactured by drawing the wire rod for the concrete reinforcing steel fiber manufactured as the above.
Hereinafter, a method for manufacturing a concrete reinforcing steel fiber according to an embodiment of the present invention having the above-described alloy composition will be described.
The method for manufacturing the concrete reinforcing steel fiber according to the present invention may include: dry drawing a wire rod that includes, in percent by weight (wt%), 0.005 to 0.035% of C, 0.07 to 0.3% of Si, 0.07 to 0.2% of Mn, 1.0 to 2.2% of Cr, 0.05% or less of P, 0.05% or less of S, and the balance of Fe and other inevitable impurities, and satisfies Expression 1 below; and wet drawing the wire rod, wherein a lead patenting (LP) heat treatment may be omitted after the dry drawing and before the wet drawing, and a tensile strength of 1700 MPa or more may be ensured. $[C] + 0.17 * [Mn] + 0.25 * [Cr] - 0.62 \leq 0$
Here, [C], [Mn], and [Cr] represent the content (wt%) of each element.
The wire rod may be mechanically descaled, then subjected to dry-drawing to reduce its size, and may be finished by wet-drawing after the dry-drawing without an intermediate LP heat treatment.
Hereinafter, the present invention will be described in more detail through embodiments. However, the descriptions of the embodiments are only for illustrating the implementation of the present invention, and the present invention is not limited by the descriptions of the embodiments. This is because the scope of the rights of the present invention is determined by matters described in the scope of claims and matters reasonably inferred therefrom.

{Examples}

A steel having the alloy composition shown in Table 1 below was refined in a converter and cast under conventional conditions to produce a 160 x 160mm² continuous casting billet. The billet was maintained at a temperature of 1,080°C for 98 minutes, followed by rolling under conventional conditions, and after finish rolling, the coiling temperature was controlled to 905°Cthrough cooling in a water cooling zone. The first cooling was performed up to a temperature of 800°C at a rate of 0.08°C/s in the Stelmore cooling zone, and the second cooling was performed at a rate of 22°C/s to the reforming tube to complete the cooling and produce a wire rod.
Tables 1 and 2 show the test compositions, the microstructures of the wire rod observed under an optical microscope in a 1/4D region of the wire rod cross-section, and the mechanical properties of the wire rod, in which D represents the diameter of the wire rod. In addition, the average grain size of quasi-polygonal ferrite was defined by measuring the area of all grains measured at 100 x magnification, converting the area into a grain size while assuming it as a circular shape, and then dividing the grain size by the number of grains.

FIG. 1 shows that the wire rod of Inventive Example 4, which satisfies the alloy composition according to the present invention,, has a microstructure composed of quasi-polygonal ferrite and cementite, as observed through an optical microscope.

[Table 1]

	C	Si	Mn	P	S	Cr	Expression [1]	Wire rod rollability
Inventive Example 1	0.005	0.30	0.20	0.04	0.05	2.20	-0.031	Good
Inventive Example 2	0.015	0.30	0.20	0.04	0.05	2.20	-0.021	Good
Inventive Example 3	0.022	0.30	0.20	0.04	0.05	2.20	-0.014	Good
Inventive Example 4	0.030	0.30	0.20	0.04	0.05	2.20	-0.006	Good
Inventive Example 5	0.030	0.07	0.20	0.04	0.04	2.20	-0.006	Good
Inventive Example 6	0.030	0.30	0.20	0.04	0.04	2.20	-0.006	Good
Inventive Example 7	0.030	0.30	0.07	0.04	0.02	2.20	-0.028	Good
Inventive Example 8	0.030	0.30	0.20	0.03	0.04	1.00	-0.306	Good
Inventive Example 9	0.030	0.30	0.20	0.04	0.03	1.60	-0.156	Good
Comparative Example 1	0.040	0.30	0.20	0.43	0.04	2.20	0.004	Cobble
Comparative Example 2	0.030	0.50	0.20	0.04	0.03	2.20	-0.006	Good
Comparative Example 3	0.030	0.30	0.50	0.04	0.04	2.20	0.045	Cobble
Comparative Example 4	0.030	0.30	0.20	0.04	0.03	0.50	-0.431	Good
Comparative Example 5	0.030	0.30	0.20	0.03	0.04	2.50	0.069	Cobble

[Table 2]

	Tensile strength of wire rod (MPa)	Microstructure of wire rod	Area fraction of cementite (%)	Average grain size of quasi-polygona l ferrite (µm)	Drawability of the wire rod
Inventive Example 1	960	Quasi-polygonal ferrite + cementite	1.5	35.0	Good
Inventive Example 2	969	Quasi-polygonal ferrite + cementite	1.4	34.8	Good
Inventive Example 3	961	Quasi-polygonal ferrite + cementite	1.7	34.7	Good
Inventive Example 4	985	Quasi-polygonal ferrite + cementite	1.4	35.0	Good
Inventive Example 5	962	Quasi-polygonal ferrite + cementite	1.6	34.8	Good
Inventive Example 6	977	Quasi-polygonal ferrite + cementite	1.8	35.0	Good
Inventive Example 7	972	Quasi-polygonal ferrite + cementite	1.9	34.8	Good
Inventive Example 8	850	Quasi-polygonal ferrite + cementite	1.8	34.7	Good
Inventive Example 9	920	Quasi-polygonal ferrite + cementite	1.7	35.0	Good
Comparative Example 1	No material	No material	-	-	No material
Comparative Example 1	No material
Comparative Example 2	990	Quasi-polygonal ferrite + cementite	1.9	34.9	Wire breakage
Comparative Example 3	No material	No material	-	-	No material
Comparative Example 4	520	Quasi-polygonal ferrite + cementite	1.8	-	Good
Comparative Example 5	No material	No material	-	-	No material

Here, no material indicates that there is no material to be tested due to cobble. Table 3 below shows the properties of steel fibers manufactured by removing scale on the surface of the manufactured wire rod using a mechanical descaling method, and performing dry drawing and then wet drawing without performing a LP heat treatment. The tensile test was conducted in accordance with ISO 6892-1 standard, and the tensile speed (cross head speed) was 50 m/min. The test specimen had a length of 300 mm, and was continuously cut into 20 pieces, whose tensile strengths were measured, and the average and deviation were checked. In addition, the 90-degree bending test was performed using steel fibers having a length of 300 mm as test specimens. A pin having a size of 2.5R (R is the diameter of the steel fiber: 0.55 mm) was fixed at the midpoint along the length, and 90-degree bending was repeated in one direction. The number of times until fracture occurred during the repeated 90-degree bending is shown in Table 3 below.

[Table 3]

	Tensile Strength (MPa)	Repetitions of 90° Bending (Count)
Inventive Example 1	1,881	13
Inventive Example 2	1,893	12
Inventive Example 3	1,886	14
Inventive Example 4	1,913	12
Inventive Example 5	1,875	11
Inventive Example 6	1,889	13
Inventive Example 7	1,882	11
Inventive Example 8	1,790	16
Inventive Example 9	1,848	14
Comparative Example 1	No material	No material
Comparative Example 2	Wire breakage	Wire breakage
Comparative Example 3	No material	No material
Comparative Example 4	1,430	19
Comparative Example 5	No material	No material

Through the above Tables 1 and 2, it can be seen that the Inventive Examples 1 to 9 satisfying the alloy composition, Expression 1, and microstructure according to the present invention ensured a tensile strength of 850 MPa or more for the wire rod for steel fibers. In addition, through Table 3, it can be seen that the steel fibers in Inventive Examples 1 to 9 had a tensile strength of 1700 MPa or more, and that no fracture occurred after more than 10 repetitions of 90-degree bending. In the case of Comparative Example 1, the C content was 0.04 wt%, which was excessive, and Expression 1 was not satisfied, resulting in the occurrence of cobbling during the rolling of the wire rod.
In the case of Comparative Example 2, the Si content was 0.5 wt%, which was excessive, leading to the formation of Fe₂SiO₄, which degraded the scale removability, and resulted in wire breakage during processing.
In the case of Comparative Example 3, the Mn content was 0.5 wt%, which was excessive, failing to satisfy Expression 1. As a result, cracks occurred due to the internal and external stress differences during charging into the wire rod reheating furnace after cooling at the room temperature following continuous casting, thereby leading to cobbling during wire rod rolling.
In the case of Comparative Example 4, the Cr content was 0.5 wt%, and although there was no issue throughout the drawing process, the tensile strength of the steel fiber was 1430 MPa due to the insufficient Cr content, which was inferior to the Inventive Examples of the present invention.
In the case of Comparative Example 5, the Cr content was 2.5 wt%, which was excessive, resulting in a failure to satisfy Expression 1, and thus cobbling occurred during wire rod rolling.

Claims

A wire rod for concrete reinforcing steel fibers, comprising in percent by weight (wt%), 0.005 to 0.035% of C, 0.07 to 0.3% of Si, 0.07 to 0.2% of Mn, 1.0 to 2.2% of Cr, 0.05% or less of P, 0.05% or less of S, and the balance of Fe and other inevitable impurities, and satisfying Expression 1 below, $[C] + 0.17 * [Mn] + 0.25 * [Cr] - 0.62 \leq 0,$ wherein [C], [Mn], and [Cr] represent the content (wt%) of each element.
The wire rod for concrete reinforcing steel fibers of claim 1, wherein a microstructure thereof includes, in area fraction, more than 98% and less than 100% quasi polygonal ferrite and more than 0% and less than 2% cementite.
The wire rod for concrete reinforcing steel fibers of claim 2, wherein an average grain size of the quasi polygonal ferrite in a range of 1/4D based on a cross-section is,more than 0 µm and 35 µm or less,
wherein D refers to a diameter of the wire rod.
The wire rod for concrete reinforcing steel fibers of claim 1, having a tensile strength of 850 MPa or more.
A method for manufacturing a wire rod for concrete reinforcing steel fibers, the method comprising: maintaining a billet comprising, in percent by weight (wt%), 0.005 to 0.035% of C, 0.07 to 0.3% of Si, 0.07 to 0.2% of Mn, 1.0 to 2.2% of Cr, 0.05% or less of P, 0.05% or less of S, and the balance of Fe and other inevitable impurities at a temperature range of 1,000 to 1,250°C for 90 to 120 minutes, and then rolling the billet to produce a wire rod;
coiling the produced wire rod at a temperature range of 880 to 950°C,

wherein cooling after the coiling includes a first cooling of cooling the wire rod to a temperature of 800°C at a rate of 1°C/s or less, and a second cooling of cooling the wire rod to a temperature of 300°C at a rate of 20°C/s or more.
The method for manufacturing a wire rod for concrete reinforcing steel fibers of claim 5, wherein the wire rod satisfies Expression 1 below, $[C] + 0.17 * [Mn] + 0.25 * [Cr] - 0.62 \leq 0,$ wherein [C], [Mn], and [Cr] represent the content (wt%) of each element.
A concrete reinforcing steel fiber, comprising in percent by weight (wt%), 0.005 to 0.035% of C, 0.07 to 0.3% of Si, 0.07 to 0.2% of Mn, 1.0 to 2.2% of Cr, 0.05% or less of P, 0.05% or less of S, and the balance of Fe and other inevitable impurities, satisfying Expression 1 below, and having a tensile strength of 1700 MPa or more, $[C] + 0.17 * [Mn] + 0.25 * [Cr] - 0.62 \leq 0,$ wherein [C], [Mn], and [Cr] represent the content (wt%) of each element.
The concrete reinforcing steel fiber of claim 7, wherein no fracture occurs after at least 10 repeated applications of 90-degree bending.
A method for manufacturing a concrete reinforcing steel fiber, the method comprising:
dry drawing a wire rod that comprises, in percent by weight (wt%), 0.005 to 0.035% of C, 0.07 to 0.3% of Si, 0.07 to 0.2% of Mn, 1.0 to 2.2% of Cr, 0.05% or less of P, 0.05% or less of S, and the balance of Fe and other inevitable impurities, and satisfies Expression 1 below; and

wet drawing the wire rod,

wherein a lead patenting (LP) heat treatment is omitted after the dry drawing and before the wet drawing, and a tensile strength is 1700 MPa or more, $[C] + 0.17 * [Mn] + 0.25 * [Cr] - 0.62 \leq 0,$

wherein [C], [Mn], and [Cr] represent the content (wt%) of each element.