Steel fiber wire rod, concrete reinforcement steel fiber, and their manufacturing methods

Abstract

The present invention relates to a steel fiber wire rod and steel fiber having excellent bending strength and used as a concrete reinforcing material for LNG tanks, etc., and a manufacturing method thereof. The present invention provides a high-strength steel fiber for concrete reinforcement that has excellent tensile strength and elongation and also has excellent bending strength when mixed with concrete, and a manufacturing method thereof. SOLUTION: The present invention is directed to a steel sheet containing, by weight, carbon (C): 0.010 to 0.040%, silicon (Si): more than 0% and 0.10% or less, manganese (Mn): 0.10 to 1.50%, nickel (Ni): 0.50 to 1.50%, phosphorus (P): 0.05% or less (including 0%), and sulfur (S): 0.05% or less (including 0%). The remainder relates to wire rods for steel fibers, steel fibers for concrete reinforcement, and methods for producing these, which contain iron (Fe) and inevitable impurities.

Description

[Technical Field]

[0001] The present invention relates to a wire rod for steel fibers, a steel fiber for concrete reinforcement, and a method for manufacturing the same, and more particularly to a wire rod for steel fibers, a high-strength steel fiber for concrete reinforcement, which has excellent tensile strength and elongation and also has excellent bending strength when mixed with concrete, and a method for manufacturing the same. [Background technology]

[0002] Generally, steel fibers are used as concrete reinforcement materials to support earth pressure inside building floors and tunnels, and are often used as a substitute for rebar because there is no reinforcing process like with rebar. Low-strength steel fibers of 1,000 MPa or less are often used, but the strength of steel fibers is gradually increasing due to changes in construction methods such as an increase in concrete compressive strength.

[0003] Steel fibers are divided into different diameters depending on the application, but since they are generally thin, about 0.4 to 1.0 mm in diameter, they are manufactured through the following process: wire → descaling → dry wiredrawing → wet wiredrawing → bundling. During wiredrawing, a true strain (e) of about 4.6 is applied, so the composition must be designed by adding solid solution strengthening elements such as Si and Mn to ultra-low carbon steel with a carbon content of about 0.01 wt%.

[0004] Concrete is also used in cryogenic vessels such as LNG and in seawater, and steel fibers can be used as a reinforcing material in these cases. However, since commonly used steel fibers are vulnerable to low temperatures, specialized steel materials that can withstand cryogenic temperatures are required. For this reason, there is a need to develop steel materials that can significantly improve the bending strength of concrete at low temperatures while suppressing wire breakage during wire drawing. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Korean Patent Publication No. 10-2000-0042052 Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention relates to a steel fiber wire rod and steel fiber having excellent bending strength and used as a concrete reinforcing material for LNG tanks, etc., and a manufacturing method thereof. The present invention provides a high-strength steel fiber for concrete reinforcement that has excellent tensile strength and elongation and also has excellent bending strength when mixed with concrete, and a manufacturing method thereof.

[0007] However, the problems to be solved by this specification are not limited to the above problems, and other problems not mentioned will be clearly understood by those skilled in the art from the following description. [Means for solving the problem]

[0008] The present invention provides a wire rod for steel fibers, which contains, by weight, 0.010 to 0.040% carbon (C), more than 0% and up to 0.10% silicon (Si), 0.10 to 1.50% manganese (Mn), 0.50 to 1.50% nickel (Ni), 0.05% or less (including 0%) phosphorus (P), and 0.05% or less (including 0%) sulfur (S), with the remainder consisting of iron (Fe) and unavoidable impurities. The wire rod for steel fibers of the present invention includes one having a scale with a thickness of 8 to 20 μm formed on the surface, and can have a tensile strength deviation of less than ±40 MPa.

[0009] The wire rod for steel fibers of the present invention may have a tensile strength of 340 MPa or more.

[0010] The wire rod for steel fibers of the present invention may contain polygonal ferrite as a microstructure.

[0011] The present invention provides a steel fiber for concrete reinforcement, which contains, by weight, 0.010 to 0.040% carbon (C), more than 0% and up to 0.10% silicon (Si), 0.10 to 1.50% manganese (Mn), 0.50 to 1.50% nickel (Ni), 0.05% or less (including 0%) phosphorus (P), and 0.05% or less (including 0%) sulfur (S), with the remainder consisting of iron (Fe) and unavoidable impurities. The steel fiber for concrete reinforcement of the present invention has a grain boundary void defect count of 4×10 7 pieces / mm 2 It can be the following:

[0012] The steel fiber for concrete reinforcement of the present invention may have a tensile strength of 1,220 MPa or more, and a work hardening ratio of 405 or more.

[0013] Furthermore, the concrete reinforcing steel fiber of the present invention can have an elongation ratio of 5% or more.

[0014] Furthermore, the steel fiber for concrete reinforcement of the present invention can have a fiber breakage rate of 1.5 times / ton or less.

[0015] Furthermore, the steel fiber for concrete reinforcement of the present invention can have a tensile strength deviation of less than ±50 MPa.

[0016] The present invention provides a method for producing a wire rod for steel fibers, comprising the steps of heating a billet containing, by weight, 0.010 to 0.040% carbon (C), more than 0% and up to 0.10% silicon (Si), 0.10 to 1.50% manganese (Mn), 0.50 to 1.50% nickel (Ni), 0.05% or less (including 0%) phosphorus (P), 0.05% or less (including 0%) sulfur (S), and the remainder being iron (Fe) and unavoidable impurities; rolling the heated billet to obtain a wire rod; coiling the wire rod at a temperature in the range of 880 to 950°C; and cooling the coiled wire rod to 300°C at a rate of 1°C / s or less.

[0017] The method for producing a wire rod for steel fiber of the present invention may also include maintaining the billet at a furnace temperature of 1,000 to 1,250° C. for 90 to 120 minutes during heating.

[0018] In the method for producing a wire rod for steel fibers of the present invention, the wire rod for steel fibers may contain polygonal ferrite as a microstructure. Also, the wire rod for steel fibers may have a scale having a thickness of 8 to 20 μm formed on its surface.

[0019] The present invention provides a method for manufacturing steel fibers for concrete reinforcement, which includes the step of dry drawing and wet drawing the wire rod manufactured by the method for manufacturing a wire rod for steel fiber to a true strain of 4.6 or more to manufacture steel fibers.

[0020] In the method for producing a steel fiber for concrete reinforcement of the present invention, the steel fiber has a grain boundary void defect count of 4×10 7 pieces / mm 2 It can be the following:

[0021] In the method for producing steel fibers for concrete reinforcement according to the present invention, the steel fibers may have a tensile strength of 1,220 MPa or more, a tensile strength deviation of less than ±50 MPa, a work hardening ratio of 405 or more, and an elongation ratio of 5% or more. [Effects of the Invention]

[0022] When the high-strength steel fiber for concrete reinforcement of the present invention is used, the use of rebar in concrete is omitted, eliminating the need for rebar placement time, which is advantageous from the perspective of construction. Furthermore, the steel fiber of the present invention has improved toughness at low temperatures due to the addition of high Ni, so when added to concrete, it can significantly increase the flexural strength of concrete, thereby improving stability and extending the service life of the concrete.

[0023] The effects that can be obtained in this specification are not limited to the effects described above, and other effects not mentioned here will be clearly understood by those having ordinary skill in the art to which the present invention pertains from the following description. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a photograph of a microstructure observed at the center of a wire rod of Example 1. [Figure 2] This is the CCT for the component system of Example 1 calculated using J-mat pro. [Figure 3] 1 is a photograph showing defects observed in the cross section of steel fiber of Example 1. [Figure 4] 1 is a photograph showing defects observed in the cross section of the steel fiber of Comparative Example 6. DETAILED DESCRIPTION OF THE INVENTION

[0025] The following describes preferred embodiments of the present invention. However, the embodiments of the present invention can be modified into various other forms, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the art.

[0026] The terms used in this application are merely used to describe specific examples. Thus, for example, singular expressions include plural expressions unless the context clearly dictates otherwise. Note that the terms "comprise" or "include" used in this application are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not intended to preclude the presence of other features, steps, functions, components, or combinations thereof.

[0027] On the other hand, unless otherwise defined, all terms used herein should be considered to have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention belongs. Therefore, unless clearly defined herein, specific terms should not be interpreted in an overly ideal or formal sense. For example, singular expressions in this specification include plural expressions unless there is a clear exception in the context.

[0028] Furthermore, in this specification, the terms "about," "substantially," and the like are used to mean from or near a numerical value when manufacturing and material tolerances inherent in the stated meaning are given, and are used to prevent unscrupulous infringers from unfairly taking advantage of disclosures in which precise or absolute numerical values ​​are stated to aid in the understanding of the present invention.

[0029] The present invention relates to a wire rod for steel fibers, a steel fiber for concrete reinforcement, and a method for producing the same.

[0030] The wire rod for steel fibers of the present invention may contain, by weight, carbon (C): 0.010 to 0.040%, silicon (Si): more than 0% and 0.10% or less, manganese (Mn): 0.10 to 1.50%, nickel (Ni): 0.50 to 1.50%, phosphorus (P): 0.05% or less (including 0%), sulfur (S): 0.05% or less (including 0%), and the remainder may be iron (Fe) and unavoidable impurities.

[0031] The reasons for limiting the range of each alloy element will be explained below. Unless otherwise specified, the units are % by weight.

[0032] The C (carbon) content may be 0.010 to 0.040% by weight, and preferably 0.010 to 0.035% by weight.

[0033] Carbon (C) is an element that significantly improves strength when formed as pearlite, but increasing the C content can cause the formation of pearlite, which can lead to wire breakage during wet drawing. If the C content is less than 0.010% by weight, it is difficult to achieve the strength target. On the other hand, if the C content exceeds 0.040% by weight, pearlite grain boundary formation can cause wire breakage during drawing, so it is desirable to keep it below that level.

[0034] The content of Si (silicon) may be more than 0% by weight and 0.10% by weight or less, and preferably 0.05 to 0.10% by weight.

[0035] Silicon is a ferrite hardening element and has the advantage of improving strength, but it forms Fe2SiO4, which has excellent bonding strength with the base material, which can be disadvantageous in terms of scale removal. Therefore, in order to improve scale removal, it is desirable to control the Si content to 0.1% or less.

[0036] The Mn (manganese) content may be 0.10 to 1.50% by weight, and preferably 0.10 to 1.10% by weight.

[0037] Mn is added to increase the strength of the wire. If the Mn content is less than 0.10 wt%, it may be difficult to achieve the target strength. On the other hand, if the Mn content exceeds 1.50 wt%, there is a high possibility of wire breakage due to segregation, so it is desirable to control it below that level.

[0038] The Ni (nickel) content may be 0.50 to 1.50% by weight, preferably 0.50 to 1.30% by weight, and more preferably 0.55 to 1.10% by weight.

[0039] Ni is an austenite-stabilizing element and dissolves in ferrite, increasing tensile strength through solid solution strengthening. At the same time, Ni significantly improves toughness, which is effective in preventing wire breakage during wire drawing. In particular, Ni improves low-temperature toughness, so after mixing steel fibers into concrete, it can significantly improve the flexural strength of concrete when subjected to external force after repeatedly maintaining low and room temperatures. If the Ni content is less than 0.50 wt%, no improvement in flexural strength may be observed. On the other hand, if the Ni content exceeds 1.50 wt%, the occurrence of wire breakage during drawing may increase, so it is recommended to keep it below that level.

[0040] The contents of P (phosphorus) and S (sulfur) are each independently 0.05% by weight or less (including 0%), preferably 0.040% or less. P and S are harmful elements that fall under the category of impurities, and if they exceed 0.05%, they may cause breakage during wire drawing due to segregation in the center, so it is desirable to control them below that level.

[0041] The remaining component of steel fiber wire rods is iron (Fe). However, in normal manufacturing processes, unintentional impurities may inevitably be mixed in from the raw materials or the surrounding environment, and this cannot be excluded. Since these impurities are known to anyone skilled in normal manufacturing processes, not all of their contents will be specifically mentioned in this specification.

[0042] Wire rods for steel fibers may include those with a surface scale of 8 to 20 μm thick. During steel fiber production, scale is removed from the drawn wire by mechanical peeling using a bending roller without a separate chemical descaling process. However, if the scale thickness is too thin, the wire rod may not be able to be descaled properly. Therefore, to use wire rods as products for mechanical peeling, it is desirable for the oxide scale to be formed to a thickness of 8 μm or more. On the other hand, if the oxide scale thickness exceeds 20 μm, scale scattering occurs, a phenomenon in which the oxide scale cracks and flies off during the processing process, resulting in poor winding shape and making it difficult to manufacture into steel fiber. Therefore, it is desirable to keep the oxide scale thickness below 20 μm. Specifically, because wire rod scale is a hard defect, if the wire rod scale is too thick, the carbide dies inside the wiredrawing die may crack when the wire rod is processed into steel fiber. This results in deep die grooves being formed on the surface of the material, which can induce wire breakage during the wire drawing process, and large deformation is applied to localized areas on the surface, which increases the number of voids compared to normal areas and increases the wire breakage rate during processing.

[0043] Steel fiber wire rods can have a tensile strength of 340 MPa or more. If the tensile strength of the wire rod is low, the tensile strength of the steel fiber will also be low, so in order to produce steel fiber with a certain strength or more, it is desirable for the wire rod tensile strength to be at least 340 MPa or more. The lower the strength of the steel fiber, the more steel fiber is used in concrete, which increases costs. In addition, using a large amount of steel fiber creates problems such as the time it takes to mix it with concrete and cure it. In other words, if the tensile strength of the wire rod is less than 340 MPa, the effect of reducing the amount of steel fiber mixed into concrete cannot be achieved.

[0044] Furthermore, the wire rod for steel fibers may have a tensile strength deviation between the coil overlapping portion and the non-overlapping portion of less than ±40 MPa, preferably ±30 MPa or less, and more preferably ±25 MPa or less. In the wire rod manufacturing process, a tensile strength deviation occurs between the coil overlapping portion and the non-overlapping portion during cooling on the Stelmor cooling table, but the more the tensile strength deviation of the wire rod is controlled low, the more the tensile strength deviation of the steel fiber can be controlled low.

[0045] In addition, the wire rod for steel fibers simultaneously satisfies both an 8-20 μm thick oxide scale and a tensile strength deviation of less than ±40 MPa, making it possible to process it into steel fibers for concrete reinforcement, and significantly reducing void defects and the rate of wire breakage during steel fiber production. Furthermore, when steel fibers are mixed with concrete during production using wire rods that satisfy these physical properties, the compressive strength within the concrete can be properly applied, providing concrete that is stable against external stress.

[0046] The wire rod for steel fibers may contain polygonal ferrite as a microstructure. For example, the wire rod for steel fibers preferably contains polygonal ferrite at an area fraction of 98% or more, preferably 99% or more. If pearlite or cementite is contained at grain boundaries at an area fraction of 2% or more, fracture may occur during wet wiredrawing, resulting in wire breakage.

[0047] The steel fiber for concrete reinforcement of the present invention may contain carbon (C): 0.010 to 0.040%, silicon (Si): more than 0% and not more than 0.10%, manganese (Mn): 0.10 to 1.50%, nickel (Ni): 0.50 to 1.50%, phosphorus (P): 0.05% or less (including 0%), sulfur (S): 0.05% or less (including 0%), and the remainder may consist of iron (Fe) and unavoidable impurities.

[0048] The reasons for limiting the ranges of the respective alloy compositions are as described above.

[0049] The steel fiber may include polygonal ferrite as a microstructure, and may include polygonal ferrite in the form of a fiber that has been drawn long by wire drawing. For example, the steel fiber preferably contains polygonal ferrite at an area fraction of 98% or more.

[0050] Steel fibers for concrete reinforcement have a void defect count of 4x10 with a size of approximately 5 μm or more generated at the grain boundaries. 7 pieces / mm 2 less than or equal to 3x10 7 pieces / mm 2 or less, and more preferably 2x10 7 pieces / mm 2 The fewer the number of void defects, the better the elongation ratio. In the present invention, the elongation ratio may refer to the sum of the uniform elongation ratio and the elongation ratio at break. If there are fewer cracks in the plastic deformation region, the film can be elongated for a longer period of time, and therefore the uniform elongation ratio can also be increased.

[0051] The steel fiber for concrete reinforcement may have a tensile strength of 1,220 MPa or more, preferably 1,230 MPa or more. The higher the tensile strength of the steel fiber, the better the concrete reinforcing effect.

[0052] Furthermore, the steel fiber for concrete reinforcement may have a tensile strength deviation between the coil overlapping portion and the non-overlapping portion of less than ±50 MPa, preferably ±40 MPa or less, and more preferably ±30 MPa or less. A large deviation in the tensile strength of the steel fiber may cause product defects. Furthermore, a small deviation in the tensile strength of the steel fiber can eliminate the problem of the steel fiber coming out of the concrete. Furthermore, the smaller the deviation in the tensile strength of the steel fiber, the more normally the compressive strength can be applied within the concrete, so that concrete containing the steel fiber of the present invention may be more stable against external stresses such as impact.

[0053] Steel fibers for concrete reinforcement can have a work-hardening rate of 405 or more when a total true strain of 4.6 or more, preferably 4.6 to 5.0, is applied during wiredrawing. For example, when a 6.5 mm thick wire is drawn with a 0.55 mm thick steel fiber, the total true strain is 4.93; when a 6.0 mm thick wire is drawn with a 0.55 mm thick steel fiber, the total true strain is 4.78; and when a 5.5 mm thick wire is drawn with a 0.55 mm thick steel fiber, the total true strain can be 4.61. Furthermore, unless a work-hardening rate of 405 or more is satisfied, the desired steel fiber strength cannot be achieved. Here, the work-hardening rate can be calculated with reference to the following formula (1):

[0054] Equation (1): TS = A*Exp(e / 4) + B

[0055] In the above formula (1), TS is the tensile strength, A is the work hardening rate, B is the extrapolated value of the tensile strength when the initial tensile strength Exp(e / 4) is 0, and e is the true strain value.

[0056] The concrete reinforcing steel fibers may have an elongation of 5% or more, preferably 5.8% or more.

[0057] The steel fiber for concrete reinforcement may have a breakage rate of 5.0 times / ton or less, preferably 1.5 times / ton or less. If the breakage rate per ton is too high, the steel fiber may not be suitable for processing in concrete.

[0058] The steel fiber may have a bending strength of 45 MPa or more, preferably 50 MPa or more, when mixed with concrete. The higher the bending strength, the better the low-temperature toughness. The steel fiber according to the present invention may have improved bending strength by designing the alloy composition to be high in Ni. Here, bending strength was measured by repeating 100 cycles of cooling a sample to a low temperature of -20°C, maintaining the temperature for 1 hour, and then maintaining it at a room temperature of 25°C for 1 hour (-20°C x 1 hour → 25°C x 1 hour → -20°C x 1 hour). 3 The load can be applied and measured.

[0059] A method for producing a wire rod for high-strength steel fibers according to the present invention having the above-mentioned alloy composition will be described below.

[0060] The wire rod for high strength steel fiber of the present invention can be manufactured by preparing a billet having the above-mentioned alloy composition, and then subjecting it to a process of reheating, wire rolling, winding and cooling.

[0061] The method for producing a wire rod for steel fibers of the present invention may include the steps of heating a billet containing, by weight, carbon (C): 0.010 to 0.040%, silicon (Si): more than 0% and 0.10% or less, manganese (Mn): 0.10 to 1.50%, nickel (Ni): 0.50 to 1.50%, phosphorus (P): 0.05% or less (including 0%), sulfur (S): 0.05% or less (including 0%), and the remainder being iron (Fe) and unavoidable impurities; rolling the heated billet to obtain a wire rod; coiling the wire rod at a temperature in the range of 880 to 950°C; and cooling the coiled wire rod to 300°C at a rate of 1°C / s or less.

[0062] The reasons for limiting the ranges of the respective alloy compositions have been described above, and each manufacturing step will be described in more detail below.

[0063] After producing a billet having the above-described alloy composition, it is maintained at a furnace temperature of 1,000 to 1,250°C for 90 to 120 minutes for normalizing and austenite formation, followed by rolling. Maintaining the temperature below 1,000°C can result in a problem of long charging time, while maintaining the temperature above 1,250°C can increase the furnace load. Therefore, it is desirable to control the temperature between 1,000 and 1,250°C. Furthermore, maintaining the temperature for less than 90 minutes can make it difficult to form central austenite, and maintaining the temperature for more than 120 minutes can cause coarse grain growth. Therefore, maintaining the temperature for 90 to 120 minutes is desirable.

[0064] The heated billet is rolled under normal rolling conditions, that is, hot rolling consisting of rough rolling, intermediate rough rolling / finish rolling, and finish rolling is performed sequentially on the heated billet to produce a wire rod.

[0065] To form an oxide scale on the wire surface to the appropriate thickness, the coiling temperature is controlled at 880-950°C using a water-cooled table. To use the wire as a mechanically peeled product, a scale thickness of 8-20 μm is required. If the coiling temperature is below 880°C, the scale may form at a thickness of less than 8 μm, which does not meet the appropriate thickness. On the other hand, if the coiling temperature exceeds 950°C, the scale may form at a thickness of 20 μm or less, which meets the appropriate thickness range, but there is a possibility of poor coiling shape, so it is desirable to control it below that level. Additional capital investment may be required to resolve poor coiling shape.

[0066] The coiled wire is then cooled to 300°C from the coiling temperature on a Stelmor cooling table at a rate of 1°C / s or less. Because overlapping sections exist within the Stelmor ring, tensile strength deviations occur compared to non-overlapping sections. If the cooling rate exceeds 1°C / s, the tensile strength deviations of the wire and steel fiber can be ±40 MPa or more. Since there is no material homogenization process between the wire and wire-drawing processes, the tensile strength deviations of the wire are reduced, which also improves the quality of the steel fiber. To suppress tensile strength deviations on a Stelmor cooling table, it is recommended to cover the cooling table with a cover to minimize airflow and control the cooling rate to 1°C / s or less.

[0067] The produced wire rod for steel fibers may contain polygonal ferrite as a microstructure and may have a tensile strength of 340 MPa or more.

[0068] The produced wire rod for steel fibers may have a tensile strength deviation between the coil overlapping portion and the non-overlapping portion of less than ±40 MPa, preferably ±30 MPa or less, and more preferably ±25 MPa or less.

[0069] The produced wire rod for steel fiber may have a scale formed on the surface thereof with a thickness of 8 to 20 μm.

[0070] A method for producing the concrete reinforcing steel fiber of the present invention having the above-mentioned alloy composition will be described below.

[0071] The concrete reinforcing steel fiber of the present invention can be produced by subjecting the above-mentioned wire rod for steel fiber to dry wire drawing and wet wire drawing.

[0072] The method for manufacturing steel fibers for concrete reinforcement according to the present invention may include a step of manufacturing steel fibers by dry wire drawing and wet wire drawing the wire manufactured by the method for manufacturing steel fiber wire at a total true strain of 4.6 or more, preferably 4.6 to 5.0. Wire drawing at a high true strain can reduce tensile strength deviation within the product.

[0073] The manufactured steel fiber had 4x10 void defects with a size of 5 μm or more generated at the grain boundaries. 7 pieces / mm 2 It can be the following:

[0074] The produced steel fibers may have a tensile strength of 1,220 MPa or more, a work hardening rate of 405 or more, and an elongation rate of 5% or more.

[0075] The produced steel fibers have a breakage rate of 1.5 times / ton or less and a bending strength of 45 MPa or more when mixed with concrete.

[0076] The produced steel fiber may have a tensile strength deviation between the coil overlapping portion and the non-overlapping portion of less than ±50 MPa, preferably ±40 MPa or less, and more preferably ±30 MPa or less.

[0077] The present invention will be described in more detail with reference to the following examples. However, the following embodiments are intended to more specifically describe the present invention, and the scope of the present invention is not limited to the following embodiments.

[0078] [Example]

[0079] <Wire alloy composition, manufacturing and physical property evaluation>

[0080] In this example, steel having the alloy composition shown in Table 1 below was produced in an electric furnace, and then cast under normal conditions to form a 160 x 160 mm steel sheet. 2 Continuously cast billet material was produced. The billet material was maintained at a furnace temperature of 1,050°C for 90 minutes, and then rolled under normal conditions. After finish rolling, it was cooled on a water cooling rack, controlled at the coiling temperature listed in Table 2 below, and cooled to 300°C on a Stelmor cooling rack at the cooling rate listed in Table 2 below to produce wire rod.

[0081] The tensile strength (TS), tensile strength deviation, oxide scale thickness on the surface, and fine structure fraction of the manufactured wire rod were measured, and the results are shown in Table 2 below.

[0082] The tensile test was conducted in accordance with ISO 6892-1 standard, with a cross head speed of 20 m / min. The sampled wire was cut into 12 consecutive pieces with a length of 400 mm, and the tensile strength was measured, after which the average and deviation were confirmed.

[0083] To measure the thickness of the surface scale, a 1 cm length was cut from the tensile test specimen using a microcutter, and the cross section was polished to a mirror finish. The scale thickness was measured from different positions on the cross section using an optical microscope, and the average thickness was confirmed.

[0084] The microstructure fraction was measured by photographing at 200x magnification using an optical microscope, and the area fraction was determined for a total of 10 photographs, and then the average value was determined.

[0085] [Table 1]

[0086] [Table 2]

[0087] Tables 1 and 2 show the test compositions and mechanical properties of the wires. Example 1 has a typical composition of 0.021C-0.08Si-0.2Mn-0.99Ni (wt%). The coiling temperature was 908°C, with a cooling rate of 0.8°C / s. Figure 1 shows a microstructure photograph of the center of the wire from Example 1, which confirms that the wire microstructure is polygonal ferrite. Figure 2, J-mat Pro, confirms that only ferrite forms even at fast cooling rates. The wire tensile strength of Example 1 was 360 MPa, and the scale thickness was 13.2 μm. Example 2 to Example 4 were based on Example 1, but the carbon, manganese, or nickel content was adjusted within the ranges of the present invention. These results demonstrate that the wires met the requirements of a tensile strength of 340 MPa or more and a scale thickness of 8 to 20 μm.

[0088] On the other hand, Comparative Example 1 satisfies the alloy composition range according to the present invention, but the coiling temperature was lowered to 830°C, and the scale thickness was formed at 3.5 μm, which was significantly reduced compared to Inventive Example 1.

[0089] Comparative Example 2 had the same alloy composition as Invention Example 1, but the cooling rate was increased to 7.5°C / s, resulting in a large tensile strength deviation of ±48 MPa.

[0090] Comparative Examples 3 to 6 are cases where the alloy composition range according to the present invention is not satisfied.

[0091] <Production and evaluation of steel fibers>

[0092] The wire rods were dry-drawn and processed to produce steel fibers, and the tensile strength (TS), tensile strength deviation at overlapping portions, work hardening rate (A), elongation rate, number of voids with a size of 5 μm or more, and wire breakage rate per ton were measured and shown in Table 3 below.

[0093] The tensile test was performed in accordance with ISO 6892-1, with a cross head speed of 50 m / min. The test specimens were 300 mm long and cut into 20 pieces. The tensile strength was measured and the average and deviation were confirmed.

[0094] The number of void defects was confirmed using a scanning electron microscope, and photographs were taken at 1000x magnification.

[0095] The wire breakage rate per ton refers to the number of times that wire drawing is interrupted due to wire breakage during drawing, and is calculated by counting the number of times for a total of 100 tons of wire supplied during the work and then averaging the results.

[0096] Also, 20 kg of steel fibers manufactured according to Tables 1 and 2 above and 135 kg of concrete were mixed to prepare a test specimen (rectangular hexahedron) measuring 3,400 mm in length, 1,200 mm in width, and 200 mm in thickness.

[0097] In addition, after the above specimen was cycle tested (-20°C x 1 hour → 25°C x 1 hour → -20°C x 1 hour → ...; repeated 100 times), a node was used in the center to apply 130 kg / mm 3 A load was applied to measure the bending strength, and the results are shown in Table 3 below.

[0098] [Table 3]

[0099] As shown in Table 3 above, the tensile strength of the wire rod of Example 1 at the time of steel fiber production was 1,250 MPa, and the elongation rate was 8.5%. The work hardening rate (A) was calculated using the following formula (1), which was 410. Formula (1): TS = A*Exp(e / 4) + B

[0100] In the above formula (1), TS is the tensile strength, A is the work hardening rate, B is the extrapolated value of the tensile strength when the initial tensile strength Exp(e / 4) is 0, and e is the true strain value.

[0101] FIG. 3 is an SEM photograph of the cross section of the steel fiber of Example 1. Referring to this, the steel fiber of Example 1 has 2*10 defects such as voids having a size of 5 μm or more. 7 pieces / mm 2 At this time, the wire breakage rate per ton was a good 0.5 times.

[0102] On the other hand, it was confirmed that the bending strength of concrete produced by blending steel fibers of Example 1 was 55 MPa.

[0103] Inventive Example 2 is a steel containing 0.011% carbon, and although the numerical values ​​for wire breakage etc. are similar to those of Inventive Example 1, the strength is reduced and the elongation rate is slightly increased.

[0104] Inventive Example 3, the steel contains 1.02% Mn, and does not have significant break lines. Compared to Inventive Example 1, the strength is increased, and although the elongation rate is slightly reduced, the concrete flexural strength is good.

[0105] Inventive Example 4 has an alloy composition with a Ni content of 0.51%, which is lower than that of Inventive Example 1. As a result, physical properties such as tensile strength and elongation are somewhat reduced, but it can be confirmed that the physical properties targeted by the present invention are achieved.

[0106] Furthermore, conventional steel fibers could not achieve a bending strength of 45 MPa or more unless approximately 25 kg of steel fibers were mixed per 135 kg of concrete. On the other hand, the steel fibers of embodiments 1 to 4 can achieve a bending strength of 45 MPa or more even when approximately 20 kg of steel fibers are mixed per 135 kg of concrete, demonstrating that excellent effects can be achieved while reducing the amount of steel fibers used.

[0107] On the other hand, Comparative Examples 1 to 6 show the differences from Invention Example 1 when the coiling temperature, Stelmor cooling rate, C content, Si content, Mn content, or Ni content is changed, with Invention Example 1 being used as the reference.

[0108] In Comparative Example 1, the coiling temperature during wire production was lowered to 830°C, and it was confirmed that the number of wire breakages per ton was 8.2, which was significantly inferior to Invention Examples 1 to 4. Specifically, in the case of Comparative Example 1, wire drawing was possible, but the wire scale thickness was formed as too thin at 3.5 μm, and wire breakage easily occurred during processing.

[0109] Comparative Example 2 is related to the Stelmor cooling rate, and the cooling rate during wire production was significantly increased to 7.5°C / s compared to Example 1. The tensile strength was similar to Example 1, but the tensile strength deviation of the product after wiredrawing was significantly increased to ±65 MPa, and the number of defects such as voids with a size of 5 μm or more also significantly increased.

[0110] In Comparative Example 3, the carbon content was increased to 0.045%, and a large amount of pearlite or cementite was formed at the grain boundaries, making wire drawing impossible and significantly increasing the wire breakage rate.

[0111] In Comparative Example 4, the Si content was increased to 0.32%, and the high Si content caused a scale of Fe2SiO4 composition to form on the wire surface, resulting in a poor breakage rate per ton.

[0112] In Comparative Example 5, the Mn content was increased to 1.59%, and it was confirmed that the high Mn content caused central segregation to form, preventing wire drawing.

[0113] In Comparative Example 6, the Ni content was increased to 1.61%, and Ni, which increases hardenability and forms a relatively hard low-temperature structure, caused chevron uniformity during cold drawing, making wiredrawing impossible. In addition, the wire breakage rate per ton was 7, making it unsuitable for processing. Figure 4 is an SEM photograph of the cross section of the steel fiber in Comparative Example 6, and as can be seen from this, void defects were also found to be 6 x 10 7 pieces / mm 2 It can be seen that a large amount of

[0114] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and it will be understood by those skilled in the art that various changes and modifications can be made without departing from the concept and scope of the claims set forth below.

Claims

1. The alloy contains, by weight, carbon (C): 0.010 to 0.040%, silicon (Si): more than 0% to 0.10% or less, manganese (Mn): 0.10 to 1.50%, nickel (Ni): 0.50 to 1.50%, phosphorus (P): 0.05% or less (including 0%), sulfur (S): 0.05% or less (including 0%), and the remainder consisting of iron (Fe) and unavoidable impurities, Including those having a scale formed on the surface with a thickness of 8 to 20 μm, A wire rod for steel fibers, characterized in that the tensile strength deviation is less than ±40 MPa.

2. 2. The wire rod for steel fibers according to claim 1, wherein the wire rod has a tensile strength of 340 MPa or more.

3. 2. The wire rod for steel fibers according to claim 1, characterized in that it contains polygonal ferrite as a microstructure.

4. The alloy contains, by weight, carbon (C): 0.010 to 0.040%, silicon (Si): more than 0% to 0.10% or less, manganese (Mn): 0.10 to 1.50%, nickel (Ni): 0.50 to 1.50%, phosphorus (P): 0.05% or less (including 0%), sulfur (S): 0.05% or less (including 0%), and the remainder consisting of iron (Fe) and unavoidable impurities, The number of void defects with a size of 5 μm or more occurring at the grain boundary is 4 × 10 ７ pieces / mm ２ A steel fiber for concrete reinforcement, characterized in that:

5. The tensile strength is 1,220 MPa or more, 5. The steel fiber for concrete reinforcement according to claim 4, characterized in that the work hardening rate is 405 or more.

6. 5. The steel fiber for concrete reinforcement according to claim 4, characterized in that the elongation is 5% or more.

7. 5. The steel fiber for concrete reinforcement according to claim 4, characterized in that the rate of breakage is 1.5 times / ton or less.

8. 5. The steel fiber for concrete reinforcement according to claim 4, characterized in that the tensile strength deviation is less than ±50 MPa.

9. a step of heating a billet containing, by weight, carbon (C): 0.010 to 0.040%, silicon (Si): more than 0% and 0.10% or less, manganese (Mn): 0.10 to 1.50%, nickel (Ni): 0.50 to 1.50%, phosphorus (P): 0.05% or less (including 0%), sulfur (S): 0.05% or less (including 0%), and the remainder being iron (Fe) and unavoidable impurities; rolling the heated billet to provide a wire rod; Winding the wire at a temperature in the range of 880 to 950°C; and a step of cooling the wound wire to 300° C. at a rate of 1° C. / s or less.

10. 10. The method for manufacturing a wire rod for steel fiber according to claim 9, wherein the billet is heated at a furnace temperature of 1,000 to 1,250° C. for 90 to 120 minutes.

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

12. The method includes a step of producing steel fibers by dry drawing and wet drawing the wire rod according to any one of claims 1 to 3 at a true strain of 4.6 or more, A method for producing steel fibers for concrete reinforcement, characterized in that the wire breakage rate during drawing is 1.5 times / ton or less.

13. The steel fiber has a grain boundary void defect count of 4x10 with a size of 5 μm or more. ７ pieces / mm ２ 13. The method for producing steel fibers for concrete reinforcement according to claim 12, wherein:

14. The steel fibers are The tensile strength is 1,220 MPa or more, The tensile strength deviation is less than ±50 MPa, The work hardening rate is 405 or more, The stretching ratio is 5% or more. A method for producing the steel fiber for concrete reinforcement according to claim 12.

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