Wire rod for steel fiber, steel fiber for concrete reinforcement, and method for producing same

Abstract

The present invention relates to a wire rod for steel fibers, a steel fiber for concrete reinforcement, and a method for manufacturing the same, the wire rod comprising, based on wt%, 0.010 to 0.040% of carbon (C), greater than 0% and less than or equal to 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 a remainder of iron (Fe) and unavoidable impurities.

Description

Technical Field

[0001] The present disclosure relates to a wire rod for steel fibers, a steel fiber for concrete reinforcement, and a method for manufacturing the same. Background Art

[0002] Generally, since there is no process of placing steel bars, steel fibers used as a concrete reinforcement material to support the bottom of a building or withstand the soil pressure inside a tunnel are widely used as a substitute for steel bars. Although low-strength steel fibers with a strength of 1,000 MPa or less are widely used, the strength of steel fibers is gradually increasing by changing the method for improving the compressive strength of concrete.

[0003] Although the diameter of steel fibers is classified according to their intended use, fine steel fibers with a diameter of about 0.4 mm to 1.0 mm are generally used, and thus they are manufactured by a method including a wire rod → descaling → dry drawing → wet drawing → bundling process. Since a true strain (e) of about 4.6 is applied to the drawing process, solid solution strengthening elements such as Si and Mn should be added to an extra-low carbon steel with a carbon content of about 0.01 wt% to design the composition.

[0004] Concrete is used in ultra-low temperature containers such as LNG tanks and is also used in seawater, and in this case, steel fibers can also be used as a reinforcement material. However, since the steel fibers generally used in the art are vulnerable to low temperatures, special steel materials capable of withstanding extremely low temperatures are required. Therefore, it is necessary to develop a steel material capable of suppressing fracture during the drawing process and significantly improving the flexural strength of concrete at low temperatures.

[0005] (Patent Document 1) Korean Patent Application Publication No. 10-2000-0042052 Summary of the Invention

[0006] Technical Problem

[0007] Provided are a wire rod for steel fibers having excellent flexural strength for use as a concrete reinforcement material such as an LNG tank, a steel fiber, and a method for manufacturing the same. Provided are a high-strength steel fiber for concrete reinforcement having excellent tensile strength, elongation, and flexural strength when mixed with concrete and a method for manufacturing the same.

[0008] However, the technical problems to be solved by the present disclosure are not limited to the foregoing problems, and any other technical problems not mentioned herein will be clearly understood by those skilled in the art to which the present disclosure pertains from the following description.

[0009] Technical Solution

[0010] According to one aspect of the present disclosure, the wire rod for steel fibers contains, by weight percentage (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 formed on the surface with a thickness of 8 μm to 20 μm and have a tensile strength variation of less than ±40 MPa.

[0011] In addition, the tensile strength of the wire rod for steel fibers according to an embodiment of the present disclosure may be 340 MPa or greater.

[0012] In addition, the wire rod for steel fibers according to an embodiment of the present disclosure may contain polygonal ferrite as a microstructure.

[0013] According to another aspect of the present disclosure, the steel fibers for concrete reinforcement contain, by weight percentage (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 fibers for concrete reinforcement according to an embodiment of the present disclosure, the number of void defects with a size of 5 μm or greater formed in the grain boundaries is 4×10 7 per mm 2 or less.

[0014] In addition, the tensile strength of the steel fibers for concrete reinforcement according to an embodiment of the present disclosure may be 1,220 MPa or greater. The work hardening rate of the steel fibers for concrete reinforcement may be 405 or greater.

[0015] In addition, the elongation rate of the steel fibers for concrete reinforcement according to an embodiment of the present disclosure may be 5% or greater.

[0016] In addition, the fracture rate of the steel fibers for concrete reinforcement according to an embodiment of the present disclosure may be 1.5 times / ton or less.

[0017] In addition, the tensile strength variation of the steel fibers for concrete reinforcement according to an embodiment of the present disclosure may be less than ±50 MPa.

[0018] According to another aspect of the present disclosure, a method for manufacturing wire rods for steel fibers includes: heating a steel billet, the steel billet containing, by weight percentage (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 steel billet into wire rods; coiling the wire rods in a temperature range of 880 °C to 950 °C; and cooling the coiled wire rods to 300 °C at a rate of 1 °C / second or less.

[0019] In addition, a method for manufacturing wire rods for steel fibers according to an embodiment of the present disclosure may include, in the heating of the steel billet, holding the steel billet in a heating furnace at a temperature of 1,000 °C to 1,250 °C for 90 minutes to 120 minutes.

[0020] In addition, in a method for manufacturing wire rods for steel fibers according to an embodiment of the present disclosure, the wire rods for steel fibers may contain polygonal ferrite as a microstructure. In addition, a scale with a thickness of 8 μm to 20 μm may be formed on the surface of the wire rods.

[0021] According to another aspect of the present disclosure, a method for manufacturing steel fibers for concrete reinforcement includes dry drawing and wet drawing the wire rods manufactured according to the method for manufacturing wire rods for steel fibers at a true strain of 4.6 or greater into steel fibers.

[0022] In addition, in a method for manufacturing steel fibers for concrete reinforcement according to an embodiment of the present disclosure, the number of void defects having a size of 5 μm or greater formed in the grain boundaries may be 4×10 7 per mm 2 or less.

[0023] In addition, in a method for manufacturing steel fibers for concrete reinforcement according to an embodiment of the present disclosure, the tensile strength may be 1,220 MPa or greater. In addition, the variation in the tensile strength of the steel fibers may be less than ±50 MPa. In addition, the work hardening rate of the steel fibers may be 405 or greater. In addition, the elongation of the steel fibers may be 5% or greater.

[0024] Beneficial effects

[0025] It is structurally advantageous to use high-strength steel fibers for concrete reinforcement according to an embodiment of the present disclosure because no steel bars are used in the concrete and no time for steel bar placement is required. In addition, since the steel fibers according to the present disclosure have improved toughness at low temperatures due to the high content of Ni added thereto, when added to concrete, the flexural strength of the concrete can be significantly increased, so that the stability and lifespan can be improved.

[0026] However, the effects obtainable through the present disclosure are not limited to the foregoing effects, and those skilled in the art to which the present disclosure pertains will clearly understand any other effects not mentioned herein from the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 An image of the microstructure observed at the center of the wire according to Invention Example 1.

[0028] Figure 2 Shows the continuous cooling transformation (CCT) of the components of Invention Example 1 calculated using J-mat pro.

[0029] Figure 3 An image showing the defects observed in the cross-section of the steel fiber of Invention Example 1.

[0030] Figure 4 An image of the defects observed in the cross-section of the steel fiber of Comparative Example 6. DETAILED DESCRIPTION

[0031] Hereinafter, embodiments of the present disclosure will be described. However, the present disclosure can be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. On the contrary, these embodiments are provided so that the present disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art.

[0032] The terms used herein are only for describing specific embodiments. Therefore, an expression used in the singular encompasses a plural expression unless it should clearly be singular in context. In addition, it should be understood that terms such as "comprising / including" or "having" are intended to indicate the presence of features, steps, functions, components, or combinations thereof disclosed in the specification, and are not intended to preclude the possibility of the existence or addition of other features, steps, functions, components, or combinations thereof.

[0033] 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 pertains. Accordingly, these terms should not be construed in an idealized or overly formal sense unless explicitly so defined herein. As used herein, unless the context clearly indicates otherwise, the singular forms are also intended to include the plural forms.

[0034] Furthermore, the terms "about", "substantially", etc. used throughout the specification mean that when natural manufacturing and material tolerances are presented, such tolerances correspond to a value or are similar to the value, and such a value is intended for a clear understanding of the present disclosure or to prevent an infringer from using the present disclosure illegally without awareness.

[0035] The present disclosure relates to wire rods for steel fibers, steel fibers for concrete reinforcement, and methods for manufacturing the same.

[0036] The wire rod for steel fibers according to an embodiment of the present disclosure may contain, by weight percentage (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.

[0037] Hereinafter, the reasons for numerically limiting the contents of alloying elements according to an embodiment of the present disclosure will be described. Hereinafter, unless otherwise specified, the unit is wt%.

[0038] The content of carbon (C) may be 0.010 wt% to 0.040 wt%, and preferably 0.010 wt% to 0.035 wt%.

[0039] C is an element that significantly improves the strength of pearlite, but an increase in the C content may lead to the formation of pearlite, thereby causing a problem of processing fracture during the wet drawing process. When the C content is less than 0.010 wt%, it is difficult to achieve the desired strength. On the contrary, when the C content exceeds 0.040 wt%, fracture may occur during the drawing process due to the formation of pearlite grain boundaries, so the C content can be controlled to a level lower than this.

[0040] The content of silicon (Si) may be more than 0% but not more than 0.10 wt%, and preferably 0.05 wt% to 0.10 wt%.

[0041] Si, as a ferrite hardening element, can improve the strength. However, Si may form Fe2 SiO 4 , which is disadvantageous in terms of scale peeling characteristics. Therefore, the Si content can be controlled to 0.1% or less to improve the scale peeling characteristics.

[0042] The content of manganese (Mn) can be from 0.10% by weight to 1.50% by weight, preferably from 0.10% by weight to 1.10% by weight.

[0043] Mn is added to increase the strength of the wire rod. When the Mn content is less than 0.10% by weight, it may be difficult to achieve the target strength. On the contrary, when the Mn content exceeds 1.50% by weight, there is a high possibility of processing fracture due to segregation, so the Mn content can be controlled to a level lower than this.

[0044] The content of nickel (Ni) can be from 0.50% by weight to 1.50% by weight, preferably from 0.50% by weight to 1.30% by weight, and more preferably from 0.55% by weight to 1.10% by weight.

[0045] Ni, as an austenite stabilizing element, forms a solid solution in ferrite to improve the tensile strength by solid solution strengthening. In addition, Ni significantly improves toughness, which is effective in preventing fracture during the drawing process. In particular, since Ni improves low-temperature toughness, after mixing steel fibers with concrete and repeating low-temperature / room-temperature cycles, the flexural strength of the concrete against the external force applied to it can be significantly improved. When the Ni content is less than 0.50% by weight, improvement in flexural strength may not be observed. On the contrary, when the Ni content exceeds 1.50% by weight, the occurrence of processing fracture may increase, so the Ni content can be controlled to a level lower than this.

[0046] The contents of phosphorus (P) and sulfur (S) can each independently be 0.05% by weight or less (including 0%), and are preferably 0.040% or less. P and S are harmful elements regarded as impurities. When the content exceeds 0.05%, segregation at the center may cause fracture during the drawing process, so the content can be controlled to a level lower than this.

[0047] The remaining component of the composition of the present disclosure is iron (Fe). However, the composition may contain unexpected impurities inevitably incorporated from raw materials or the surrounding environment, so the addition of other alloy components is not excluded. Since impurities are known to any person skilled in the art of manufacturing, they are not specifically mentioned in the present disclosure.

[0048] The wire rod for steel fibers according to an embodiment may include a scale formed on the surface with a thickness of 8 μm to 20 μm. During the manufacture of the steel fibers, the scale is removed from the drawn wire rod without a separate chemical scale removal process by mechanical peeling using bending rolling. In this case, too thin a scale may make the scale removal process of the wire rod difficult. Therefore, in order to use the wire rod as a product suitable for mechanical peeling, the scale may preferably be formed to have a thickness of 8 μm or more. On the contrary, when the thickness of the scale exceeds 20 μm, due to the too large thickness of the scale, a scale scattering phenomenon occurs in which the scale breaks and scatters during processing, thereby damaging the shape of the coil and thus making it difficult to manufacture the steel fibers. Therefore, it is preferable to control the thickness of the scale to a level lower than this. Specifically, since the wire rod scale is a hard defect, when the scale of the wire rod is too thick, the cemented carbide die inside the drawing die head may break when the wire rod is processed into steel fibers. Therefore, compared with the normal area, deep die grooves are formed on the surface of the material, resulting in fracture during the drawing process and an increase in the number of defects (voids) formed by large deformation applied to the local area of the surface, thereby increasing the processing fracture rate.

[0049] The tensile strength of the wire rod for steel fibers according to an embodiment may be 340 MPa or more. A low tensile strength of the wire rod may cause a decrease in the tensile strength of the steel fibers. Therefore, the tensile strength of the wire rod may satisfy at least 340 MPa to manufacture steel fibers having a certain level of strength or higher strength. As the strength of the steel fibers decreases, the amount of steel fibers used in the concrete increases, resulting in an increase in the manufacturing cost. In addition, the use of a large amount of steel fibers causes a problem of an increase in the time required to mix with the concrete and cure the concrete. That is, when the tensile strength of the wire rod is less than 340 MPa, it is difficult to obtain the effect of reducing the amount of steel fibers mixed with the concrete.

[0050] In addition, in the wire rod for steel fibers according to an embodiment, the change in tensile strength between the overlapping portion and the non-overlapping portion of the coil may be less than ±40 MPa, preferably ±30 MPa or less, and more preferably ±25 MPa or less. During the process of manufacturing the wire rod, the change in tensile strength between the overlapping portion and the non-overlapping portion of the coil may occur during cooling at the Stelmor cooling tower. By controlling the change in tensile strength of the wire rod as low as possible, the change in tensile strength of the steel fibers can also be controlled to decrease.

[0051] In addition, since both the thickness of the scale of 8 μm to 20 μm and the change in the tensile strength of less than ±40 MPa are satisfied, the wire rod for steel fibers according to an embodiment can be processed into steel fibers for concrete reinforcement, and the void defects and fracture rate can be significantly reduced during the manufacture of the steel fibers. In addition, in the case of manufacturing steel fibers using a wire rod satisfying such physical properties, the compressive strength can be normally applied to the concrete mixed with the steel fibers to provide concrete that stably resists external stress.

[0052] 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 with an area fraction of 98% or more, and preferably 99% or more. In the case where pearlite or cementite is included in the grain boundary with an area fraction of 2% or more, cracking may occur during wet wire drawing, resulting in fracture.

[0053] The steel fibers 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.

[0054] The reason for numerically limiting the content of alloying elements is as described above.

[0055] The steel fibers according to an embodiment may include polygonal ferrite as a microstructure, that is, polygonal ferrite in the form of fibers stretched through a wire drawing process. For example, the steel fibers may include polygonal ferrite with an area fraction of 98% or more.

[0056] In the steel fibers for concrete reinforcement according to an embodiment, the number of void defects having a size of about 5 μm or more formed in the grain boundary may be 4×10 7 per mm 2 or less, preferably 3×10 7 per mm 2 or less, and more preferably 2×10 7 per mm 2 or less. As the number of void defects decreases, the elongation is improved. In the present disclosure, the elongation refers to the sum of the uniform elongation and the fracture elongation. If there are fewer cracks in the plastic deformation region, the material can be further stretched, so the uniform elongation can also increase.

[0057] The tensile strength of the steel fiber for concrete reinforcement according to an embodiment may be 1,220 MPa or greater, and preferably 1,230 MPa or greater. The higher tensile strength of the steel fiber can provide an excellent concrete reinforcement effect.

[0058] In addition, in the steel fiber for concrete reinforcement according to an embodiment, the change in tensile strength between the overlapping portion and the non-overlapping portion of the coil may be less than ±50 MPa, preferably ±40 MPa or less, and more preferably ±30 MPa or less. A large change in the tensile strength of the steel fiber may cause defects in the product. In addition, a small change in the tensile strength in the steel fiber can solve the problem of detachment from the concrete. Further, as the change in the tensile strength of the steel fiber decreases, the compressive strength can be more normally applied to the concrete, so that the concrete mixed with the steel fiber according to the present disclosure can be stable under external stresses such as impact.

[0059] When a total true strain of 4.6 or greater, and preferably 4.6 to 5.0, is applied during the drawing process, the work hardening rate of the steel fiber for concrete reinforcement according to an embodiment may be 405 or greater. For example, when a wire with a thickness of 6.5 mm is drawn into a steel fiber with a thickness of 0.55 mm, the total true strain may be 4.93. When a wire with a thickness of 6.0 mm is drawn into a steel fiber with a thickness of 0.55 mm, the total true strain may be 4.78. When a wire with a thickness of 5.5 mm is drawn into a steel fiber with a thickness of 0.55 mm, the total true strain may be 4.61. In addition, when the work hardening rate satisfies 405 or greater, the desired steel fiber strength can be obtained. In this regard, the work hardening rate can be calculated by using the following equation (1).

[0060] Equation (1): TS = A * Exp(e / 4) + B

[0061] In Equation (1), TS is the tensile strength, A is the work hardening rate, B is the extrapolated value when Exp(e / 4) of the initial tensile strength is 0, and e is the true strain.

[0062] The elongation of the steel fiber for concrete reinforcement according to an embodiment may be 5% or greater, and preferably 5.8% or greater.

[0063] The fracture rate of the steel fiber for concrete reinforcement according to an embodiment may be 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 fiber for concrete.

[0064] When mixed with concrete, the flexural strength of steel fibers according to one embodiment can be 45 MPa or greater, and preferably 50 MPa or greater. The higher the flexural strength, the better the low-temperature toughness. The steel fibers according to the present disclosure can have improved flexural strength by designing the alloy element composition to include a high Ni content. In this regard, the flexural strength can be measured by repeating 100 cycles, each cycle including cooling the specimen to a low temperature of -20 °C, holding the specimen at this temperature for 1 hour, and holding the specimen at room temperature of 25 °C for 1 hour (-20 °C × 1 hour → 25 °C × 1 hour → -20 °C × 1 hour), and measuring the flexural strength under a load of 130 kg / mm 3 applied to the center.

[0065] Hereinafter, a method for manufacturing a wire rod for high-strength steel fibers according to one embodiment of the present disclosure will be described.

[0066] The wire rod for high-strength steel fibers of the present disclosure can be manufactured by manufacturing a steel billet having the above alloy element composition, followed by reheating - wire rod rolling - coiling - cooling process.

[0067] A method for manufacturing a wire rod for steel fibers according to one embodiment of the present disclosure includes: heating a steel billet containing, by weight%, 0.010% to 0.040% of carbon (C), greater than 0% but not greater 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 steel billet into a wire rod; coiling the wire rod in a temperature range of 880 °C to 950 °C; and cooling the coiled wire rod to 300 °C at a rate of 1 °C / second or less.

[0068] The reasons for restricting the alloy element composition are as described above, and hereinafter, the processes of its manufacturing method will be described in more detail.

[0069] After manufacturing a steel billet having the above alloy element composition, the steel billet is held in a heating furnace at a temperature of 1,000 °C to 1,250 °C for 90 minutes to 120 minutes for normalizing and forming austenite, and then rolling is performed. If held at a temperature below 1,000 °C, the holding time may increase. If held at a temperature above 1,250 °C, the heating furnace is under load. Therefore, the temperature can be preferably controlled in the range of 1,000 °C to 1,250 °C. In addition, if the holding time is shorter than 90 minutes, it may be difficult to form austenite at the center. If the holding time exceeds 120 minutes, coarse grains may grow. Therefore, the holding time can be preferably 90 minutes to 120 minutes.

[0070] The heated steel billet is rolled under general rolling conditions. That is, the heated steel billet is subjected to a hot rolling process that sequentially includes rough rolling, intermediate rough rolling / finish rolling, and finish rolling to manufacture wire rods.

[0071] In order to form a scale of an appropriate thickness on the surface of the wire rod, the coiling temperature is controlled in the range of 880 °C to 950 °C by water quenching. In order to be used as a product for mechanical scale peeling, a scale with a thickness of 8 μm 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, and the appropriate thickness requirement cannot be met. On the contrary, at a coiling temperature above 950 °C, the appropriate scale thickness of 20 μm or less is satisfied, but defects may occur in terms of the coiling shape. Therefore, it is necessary to control the scale thickness to a level lower than this. Additional investment in equipment may be required to solve the defects in terms of the coiling shape.

[0072] Subsequently, the coiled wire rod is cooled from the coiling temperature to 300 °C at a rate of 1 °C / second or less in a Stelmor cooling tower. Since there are overlapping parts in the loop of the Stelmor cooling tower, there is a change in the tensile strength between the overlapping parts and the non-overlapping parts. When the cooling rate exceeds 1 °C / second, the change in the tensile strength between the wire rod and the steel fiber can be ±40 MPa or more. Since there is no homogenization treatment between the manufacturing and drawing processes of the wire rod, by reducing the change in the tensile strength in the wire rod, the steel fiber can have excellent properties. In order to suppress the change in the tensile strength in the Stelmor cooling tower, it is preferable to cover the material and control the cooling rate to 1 °C / second or less by minimizing the air flow.

[0073] The wire rod for steel fiber manufactured by the method according to an embodiment can contain polygonal ferrite as a microstructure and can have a tensile strength of 340 MPa or more.

[0074] The change in tensile strength between the overlapping and non-overlapping portions of the coil of wire for steel fibers manufactured by the method according to one embodiment can be less than ±40 MPa, preferably ±30 MPa or less, and more preferably ±25 MPa or less.

[0075] The wire for steel fibers manufactured by the method according to one embodiment can be a wire having a scale with a thickness of 8 μm to 20 μm formed on the surface.

[0076] Hereinafter, a method for manufacturing steel fibers for concrete reinforcement having the above alloy element composition according to one embodiment of the present disclosure will be described.

[0077] The steel fibers for concrete reinforcement according to the present disclosure can be manufactured by dry drawing and wet drawing of the above wire for steel fibers.

[0078] The method for manufacturing steel fibers for concrete reinforcement according to one embodiment of the present disclosure can include manufacturing steel fibers by dry drawing and wet drawing of a wire manufactured by the method for manufacturing a wire for steel fibers with a total true strain of 4.6 or more, and preferably 4.6 to 5.0. By drawing the wire with a high true strain, the change in tensile strength in the product can be reduced.

[0079] In the steel fibers manufactured by using the method according to one embodiment, the number of void defects having a size of about 5 μm or more formed in the grain boundaries can be 4×10 7 per mm 2 or less.

[0080] The steel fibers manufactured by using the method according to one embodiment can 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.

[0081] When mixed with concrete, the steel fibers manufactured by using the method according to one embodiment can have a fracture rate of 1.5 times / ton or less and a flexural strength of 45 MPa or more.

[0082] The change in tensile strength between the overlapping and non-overlapping portions of the coil of steel fibers manufactured by using the method according to one embodiment can be less than ±50 MPa, preferably ±40 MPa or less, and more preferably ±30 MPa or less.

[0083] Hereinafter, the present disclosure will be described in more detail with reference to the following examples. However, the following examples are presented only for illustrating the present disclosure, and the scope of the present disclosure is not limited thereto.

[0084] [Examples]

[0085] <Evaluation of Alloy Element Composition, Manufacture, and Physical Properties of Wire Rods>

[0086] In this embodiment, steel having the alloy element composition shown in Table 1 below is manufactured in a converter and cast under general conditions to produce a slab having dimensions of 160×160 mm 2 . After holding the slab in a heating furnace at a temperature of 1,050 °C for 90 minutes, the slab is rolled under general conditions, controlled to a coiling temperature shown in Table 2 below by finish rolling and cooling in a cooling bed, and cooled to 300 °C at a cooling rate shown in Table 2 below in a Stelmor cooling tower, thereby producing wire rods.

[0087] In addition, the tensile strength (TS), change in tensile strength, scale thickness on the surface, and microstructure fraction of the produced wire rods are measured, and the results are shown in Table 2 below.

[0088] Tensile tests are conducted in accordance with the ISO6892-1 standard, and the tensile speed (crosshead speed) is 20 m / min. The collected wire rods are continuously cut into 400-mm pieces (12 pieces), and then their tensile strengths are measured and the average value and deviation are determined.

[0089] To measure the scale thickness on the surface, the specimens for the tensile tests are cut 1 cm in the longitudinal direction using a micro-cutting machine, and then mirror-polished by cross-section polishing. The scale thickness is measured at different positions of the cross-section using an optical microscope, and the average thickness is determined.

[0090] In addition, images are obtained at 200 times magnification using an optical microscope to calculate the microstructure fraction. After obtaining the area fractions of a total of 10 sheets, the average value is obtained.

[0091] Table 1

[0092]

[0093] Table 2

[0094]

[0095] Tables 1 and 2 show the alloy element composition of the wire rods and their mechanical properties. Representative alloy elements of Invention Example 1 include 0.021C - 0.08Si - 0.2Mn - 0.99Ni (wt%). In this case, the coiling temperature is 908 °C, and the cooling rate is 0.8 °C / sec. Figure 1 An image of the microstructure observed at the center of the wire rod according to Invention Example 1. Based on this, it can be determined that the microstructure of the wire rod of Invention Example 1 is polygonal ferrite. Although the cooling rate is high, based on Figure 2The J-mat pro determines the conditions that allow for the formation of only ferrite. In this case, the wire rod of Invention Example 1 has a tensile strength of 360 MPa and a scale thickness of 13.2 μm. In Invention Examples 2 to 4, based on Invention Example 1, the contents of carbon, manganese, and nickel are respectively controlled within the scope of the present disclosure, and the conditions of a tensile strength of 340 MPa or more and a scale thickness of 8 μm to 20 μm are satisfied.

[0096] Meanwhile, although Comparative Example 1 satisfies the alloy element composition according to the present disclosure, due to the low coiling temperature of 830°C, the scale thickness is 3.5 μm, which is significantly reduced compared to Invention Example 1.

[0097] Although the alloy element composition of Comparative Example 2 is the same as that of Invention Example 1, the increased cooling rate of 7.5°C / second causes a significant change in the tensile strength to increase to ±48 MPa.

[0098] Comparative Examples 3 to 6 do not satisfy the range of the alloy element composition according to the present disclosure.

[0099] <Manufacture and Evaluation of Steel Fibers>

[0100] The manufactured wire rods are dry drawn and wet drawn to produce steel fibers, and the tensile strength (TS), change in tensile strength of the overlapping part, work hardening rate (A), elongation, number of void defects with a size of 5 μm or more, and processing fracture rate per ton are measured and shown in Table 3 below.

[0101] The tensile test is carried out according to the ISO6892-1 standard, and the tensile speed (crosshead speed) is 50 m / min. The specimens are continuously cut into pieces of 300 mm (20 pieces), and then their tensile strengths are measured and the average value and deviation are determined.

[0102] The number of void defects is determined by using a scanning electron microscope, and images are obtained at 1000 times magnification.

[0103] The processing fracture rate per ton refers to the processing interruption caused by fracture during the drawing process. After counting the number of fractures from a total supply of 100 tons of wire rods, the average value is calculated.

[0104] In addition, 20 kg of each of the steel fibers manufactured according to Tables 1 and 2 is 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.

[0105] In addition, after repeating the cycle (-20°C × 1 hour → 25°C × 1 hour → -20°C × 1 hour →...; 100 times), by applying 130 kg / mm at the node at the center3 The load was used to measure the flexural strength of the specimen, and the results are shown in Table 3 below.

[0106] Table 3

[0107]

[0108] Referring to Table 3, the steel fibers manufactured using the wire material of Invention Example 1 had a tensile strength of 1,250 MPa, an elongation of 8.5%, and a work hardening rate (A) calculated by the following equation (1) of 410. Equation (1): TS = A * Exp(e / 4) + B

[0109] In Equation (1), TS is the tensile strength, A is the work hardening rate, B is the extrapolated value when the initial tensile strength Exp(e / 4) is 0, and e is the true strain.

[0110] In addition, Figure 3 is an SEM image of the cross-section of the steel fibers of Invention Example 1. Referring to it, in the steel fibers of Invention Example 1, the number of defects such as voids with a size of 5 μm or more was 2 * 10 7 per mm 2 . In this case, a good processing fracture rate of 0.5 per ton was obtained.

[0111] At the same time, the flexural strength of the concrete manufactured by mixing with the steel fibers of Invention Example 1 was determined to be 55 MPa.

[0112] Although the steel fibers of Invention Example 2 with a C content of 0.011% had characteristics such as fracture rate similar to those of the steel fibers of Invention Example 1, the strength decreased and the elongation increased slightly.

[0113] In the steel fibers of Invention Example 3 with a Mn content of 1.02%, fracture did not occur severely during processing. Although the steel fibers had a higher strength and slightly lower elongation compared to Invention Example 1, a fairly good flexural strength of the concrete was obtained.

[0114] Although the steel fibers of Invention Example 4 had slightly inferior physical properties such as tensile strength and elongation due to a lower Ni content of 0.51% compared to the steel fibers of Invention Example 1, it was determined that the physical properties desired by the present disclosure were achieved.

[0115] In addition, in the case of using conventional steel fibers, a flexural strength of 45 MPa or more was satisfied by adding about 25 kg of steel fibers per 135 kg of concrete. In contrast, each of the steel fibers of Examples 1 to 4 at about 20 kg per 135 kg of concrete was sufficient to achieve a flexural strength of 45 MPa or more. Therefore, it was determined that excellent effects could be obtained while using a reduced amount of steel fibers.

[0116] Meanwhile, when the coiling temperature, Stelmor cooling rate, C content, Si content, Mn content, or Ni content was changed separately from Invention Example 1, Comparative Examples 1 to 6 showed differences from Invention Example 1.

[0117] In Comparative Example 1, a low coiling temperature of 830 °C was used during wire rod manufacturing, and the number of breaks per ton was 8.2, which was significantly inferior to that of Invention Examples 1 to 4. Specifically, in the case of Comparative Example 1, the drawing process could be carried out, but since the scale of the wire rod with a thickness of 3.5 μm was too thin, breaks were likely to occur in the wire rod.

[0118] Comparative Example 2 related to the Stelmor cooling rate, and compared with Invention Example 1, conditions were provided in which the cooling rate was significantly increased to 7.5 °C / second during wire rod manufacturing. Although the tensile strength was similar to that of Invention Example 1, it was determined that the change in the tensile strength of the product after the drawing process increased significantly to ±65 MPa, and the number of defects such as voids with a size of 5 μm or larger increased significantly.

[0119] In Comparative Example 3, the C content was increased to 0.045%. A large amount of pearlite or cementite was formed at the grain boundaries, making the drawing process impossible and the fracture rate increasing significantly.

[0120] In Comparative Example 4, the Si content was increased to 0.32%. Due to the high Si content, scale with a composition of Fe 2 SiO 4 was formed on the surface of the wire rod, deteriorating the fracture rate per ton.

[0121] In Comparative Example 5, the Mn content was increased to 1.59%. It was determined that segregation occurring at the center due to the high Mn content made the drawing process impossible.

[0122] In Comparative Example 6, the Ni content was increased to 1.61%. Ni, which forms a relatively hard low-temperature structure by increasing ductility, caused chevron uniformity during cold drawing, making the drawing process impossible. In addition, the fracture rate per ton of 7 was not suitable for processing. In addition, Figure 4 is a SEM image of the cross-section of the steel fiber of Comparative Example 6. Based on this, it was determined that a large number of void defects (6×10 7 per mm 2 ) occurred.

[0123] Although the present disclosure has been specifically described with reference to exemplary embodiments, those skilled in the art should understand that the scope of the present disclosure is not limited thereby, and various changes in form and detail can be made without departing from the spirit and scope of the present disclosure.

Claims

1. A wire rod for steel fibers, comprising, by weight percentage (% by weight): 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 μm to 20 μm is formed on the surface, and the change in tensile strength is less than ±40 MPa.

2. The wire rod according to claim 1, wherein the tensile strength is 340 MPa or more.

3. The wire rod according to claim 1, wherein the wire rod contains polygonal ferrite as the microstructure.

4. A steel fiber for concrete reinforcement, comprising, by weight percentage (% by weight): 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 number of void defects with a size of 5 μm or more formed in the grain boundaries is 4×10 7 per mm 2 or less.

5. The steel fiber according to claim 4, wherein the tensile strength is 1,220 MPa or more, and the work hardening rate is 405 or more.

6. The steel fiber according to claim 4, wherein the elongation is 5% or more.

7. The steel fiber according to claim 4, wherein the fracture rate is 1.5 times / ton or less.

8. The steel fiber according to claim 4, wherein the change in tensile strength is less than ±50 MPa.

9. A method for manufacturing a wire rod for steel fibers, the method comprising: heating a steel billet, the steel billet comprising, by weight percentage (% by weight): 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 steel billet into a wire rod; coiling the wire rod in a temperature range of 880°C to 950°C; and cooling the coiled wire rod to 300°C at a rate of 1°C / second or less.

10. The method according to claim 9, wherein in the heating of the steel billet, the steel billet is held in a heating furnace at a temperature of 1,000°C to 1,250°C for 90 minutes to 120 minutes.

11. The method according to claim 9, wherein the wire rod for steel fibers contains polygonal ferrite as the microstructure, and a scale with a thickness of 8 μm to 20 μm is formed on the surface of the wire rod.

12. A method for manufacturing steel fibers for concrete reinforcement, the method comprising dry-drawing and wet-drawing the wire rod according to any one of claims 1 to 3 into steel fibers at a true strain of 4.6 or greater, wherein the fracture rate during drawing is 1.5 times / ton or less.

13. The method according to claim 12, wherein the number of void defects having a size of 5 μm or greater formed in the grain boundaries is 4×10 7 per mm 2 or less.

14. The method according to claim 12, wherein the steel fibers have: a tensile strength of 1,220 MPa or greater, a tensile strength variation of less than ±50 MPa, a work hardening rate of 405 or greater, and an elongation of 5% or greater.

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