Wire rod, steel wire, and method for manufacturing same

Abstract

An ultra-low carbon steel wire rod according to the present invention comprises, by weight%, 0.003% or less (excluding 0) of C, 0.003% or less (excluding 0) of N, 0.01% or less (excluding 0) of Si, 0.05 to 0.15% of Mn, 0.02 to 0.05% of Ti, 0.01 to 0.03% of Nb, and the balance being Fe and unavoidable impurities. When the radius of the wire rod is R, the number of NbC precipitates having a diameter of 10 nm or more observed in a region from the center of a cross section perpendicular to the longitudinal direction to 0.5R is 1,900,000 / ㎟ or more per unit area, and the number of primary TiN precipitates is 2,800,000 / ㎟ or more per unit area.

Description

Wire rod, steel wire and method of manufacturing the same

[0001] The present invention relates to an ultra-low carbon steel wire rod, a steel wire, and a method for manufacturing the same.

[0002] Mechanical structural materials such as reinforcing bars and scaffolding steel plates require fastening through binding when used on-site, and heat-treated wire with a diameter of 0.8 to 1.0 mm is mainly used. Although on-site workers sometimes use semi-automatic equipment to bind the binding wire, they still bind it by hand, so the binding wire must have low strength.

[0003] The binding wire manufacturing process is as follows.

[0004] Cold slabs are used as the base material, torch-cut, reheated, rolled into billets and wire rods, and finished by Stelmore cooling. Wire rods with a diameter of 5.5 mm or less produced in this way have scale removed by mechanical delamination from the wire drawing process, and the diameter is reduced (downsized) through 1 to 2 drawing processes. Finally, to reduce the strength increased by drawing, the wire rod is finished by annealing in the ferrite single-phase region. Although the strength increased by drawing can be significantly reduced through annealing, the strength of the wire rod must be low because if the strength of the wire rod is high, the strength after annealing is also high.

[0005] As mentioned earlier, it is important to keep the strength of the binding wire low, and to achieve this, a soft phase of ferrite must be formed. Additionally, the carbon and nitrogen content, which have the highest solid solution strengthening effect, must be kept as low as possible. However, since decarburization and denitrification treatments during the steelmaking process significantly increase manufacturing costs and there are limits to reducing carbon and nitrogen content, the increase in strength caused by C and N must be suppressed through other elements or changes in the manufacturing process.

[0006] One aspect of the present invention for solving the aforementioned problem is to provide a wire rod, a steel wire, and a method for manufacturing the same, which can reduce strength by suppressing solid solution strengthening caused by residual C and N in the ferrite by forming TiN precipitates and NbC precipitates through the composite addition of Ti and Nb.

[0007] In addition, one aspect of the present invention provides an ultra-low carbon steel wire rod and steel wire that can be applied to reinforcing bars, scaffolding steel plates, mechanical structural products, etc. by lowering the strength of the wire rod and steel wire, and a method for manufacturing the same.

[0008] The technical problems intended to be solved in this document are not limited to those mentioned above, and other technical problems not mentioned will be clearly understood by those skilled in the art to which this invention belongs from the description below.

[0009] To achieve the above objective, an ultra-low carbon steel wire rod according to one embodiment of the present invention comprises, in weight %, C 0.0030% or less (excluding 0), N: 0.0030% or less (excluding 0), Si: 0.01% or less (excluding 0), Mn: 0.05~0.15%, Ti: 0.02~0.05%, Nb: 0.01~0.03%, the remainder being Fe and unavoidable impurities, and when the radius of the wire rod is R, the number of NbC precipitates with a diameter of 10 nm or more observed in the region from the center of the cross-section perpendicular to the longitudinal direction to 0.5R may be 1,900,000 or more per unit area and the number of TiN precipitates may be 2,800,000 or more per unit area.

[0010] The microstructure of the wire according to one embodiment of the present invention may contain ferrite with an area fraction of 99% or more.

[0011] According to one embodiment of the present invention, the wire may have an average grain size of ferrite observed in a region of 0.2R from the center of a cross-section perpendicular to the longitudinal direction of 70㎛ or more.

[0012] According to one embodiment of the present invention, the wire may have a tensile strength of 260 MPa or less and a yield strength of 150 MPa or less.

[0013] A method for manufacturing an ultra-low carbon steel wire rod according to one embodiment of the present invention comprises the steps of: heating a bloom containing, in weight %, C 0.0030% or less (excluding 0), N: 0.0030% or less (excluding 0), Si: 0.01% or less (excluding 0), Mn: 0.05~0.15%, Ti: 0.02~0.05%, Nb: 0.01~0.03%, the remainder being Fe and unavoidable impurities, in a steel billet heating furnace and then rolling the bloom to produce a billet; hot rolling the billet produced; and coiling the hot-rolled wire rod at 830~880℃. The method includes a step of cooling the wound wire rod; wherein the hot rolling step is performed at a wire rod heating furnace outlet temperature of 950 to 1050°C, a finishing rolling (FM) inlet temperature of Ae3-100°C to Ae3-10°C, and a final rolling (RSM) inlet temperature of Ae3-100°C to Ae3-10°C, and the cooling step may be performed by extremely slow cooling at 1°C / s or less at Ae3-800°C or higher, rapid cooling at 20°C / s or more at temperatures above 500°C and below 800°C, and extremely slow cooling at 1°C / s or less at temperatures below 500°C.

[0014] In one embodiment of the present invention, the wire rod after the hot rolling step may have a number of NbC precipitates with a diameter of 10 nm or more of 1,900,000 per unit area / mm² or more.

[0015] According to one embodiment of the present invention, after the cooling step, the wire may have a number of TiN precipitates of 2,800,000 or more per unit area / mm².

[0016] In one embodiment of the present invention, after the cooling step, the wire rod may have an average grain size of ferrite observed in the region from the center of the cross-section perpendicular to the length direction up to 0.2R, where the radius of the wire rod is R.

[0017] A super low carbon steel wire according to one embodiment of the present invention comprises, in weight %, C 0.0030% or less (excluding 0), N: 0.0030% or less (excluding 0), Si: 0.01% or less (excluding 0), Mn: 0.05~0.15%, Ti: 0.02~0.05%, Nb: 0.01~0.03%, the remainder being Fe and unavoidable impurities, and may have a tensile strength of 320 MPa or less and a yield strength of 200 MPa or less.

[0018] In one embodiment of the present invention, the steel wire may have an average grain size of ferrite observed in the region from the center of the cross-section perpendicular to the length direction up to 0.5R, where the radius of the steel wire is R.

[0019] According to one embodiment of the present invention, the steel wire may have a yield point elongation of 1.0% or less.

[0020] According to the present invention, by forming TiN precipitates and NbC precipitates through the composite addition of Ti and Nb, solid solution strengthening caused by residual C and N in ferrite can be suppressed, thereby securing low-strength wire rods and steel wires. Furthermore, ultra-low carbon steel wire rods and steel wires applicable to reinforcing bars, scaffolding steel plates, machine structural products, etc., and a method for manufacturing the same can be provided.

[0021] The effects obtainable from the present invention are not limited to those mentioned above, and other unmentioned effects will be clearly understood by those skilled in the art to which the present invention belongs from the description below.

[0022] FIG. 1 is a drawing showing a cross-section of a wire according to one embodiment of the present invention.

[0023] FIG. 2 is a diagram showing a method for measuring the number of NbC precipitates and TiN precipitates from a cross-section of a wire rod according to one embodiment of the present invention.

[0024] FIG. 3 is a diagram showing the cooling rate, cover, and air blower usage for each temperature range during the cooling step when manufacturing ultra-low carbon steel wire rod according to one embodiment of the present invention.

[0025] FIG. 4 is a drawing showing an analysis area in a cross-section of a wire rod according to one embodiment of the present invention.

[0026] FIG. 5 is a diagram showing a method for measuring tensile strength using a wire rod or steel wire according to one embodiment of the present invention.

[0027] FIG. 6 is a diagram showing the relationship between yield strength and yield point elongation according to one embodiment of the present invention.

[0028] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in 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 relevant technical field.

[0029] The terms used in this application are used merely to describe specific examples. For this reason, singular expressions include plural expressions unless the context clearly requires them to be singular. Additionally, it should be noted that terms such as “comprising” or “comprising” 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 used to preliminarily exclude the existence of other features, steps, functions, components, or combinations thereof.

[0030] Meanwhile, unless otherwise defined, all terms used in this specification shall be understood to have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Accordingly, unless explicitly defined in this specification, specific terms should not be interpreted in an overly ideal or formal sense. For instance, singular expressions in this specification include plural expressions unless the context clearly indicates an exception.

[0031] Additionally, terms such as "about," "substantially," etc., in this specification are used to mean at or near the stated value when inherent manufacturing and material tolerances are presented in the said sense, and are used to prevent unscrupulous infringers from unfairly exploiting the disclosed content in which precise or absolute values ​​are mentioned to aid in understanding the invention.

[0032] Wire rods and steel wires used for binding, such as reinforcing bars and scaffolding steel plates, require low product strength because they are tied by hand; however, there have been limitations in removing carbon and nitrogen to ensure this low strength. Accordingly, this invention aims to secure low-strength wire rods and steel wires by forming a single ferrite microstructure and suppressing solid solution strengthening caused by C and N. This is achieved by inducing the formation of TiN precipitates and NbC precipitates through the composite addition of Ti and Nb to remove carbon and nitrogen, which are interstitial elements dissolved within the ferrite that increase strength.

[0033] The ultra-low carbon steel wire rod according to one embodiment of the present invention will be described in detail below.

[0034] In the present invention, the "center" (OR) of the wire refers to the intersection point of the major axis and the minor axis of the cross-section perpendicular to the longitudinal direction of the wire, as shown in FIG. 1 (provided that if all axes of the cross-section are the same, the intersection point of the two axes). If the cross-section of the wire is a circle, the distance from the center of the circle to the outermost surface refers to the radius of the wire, and if it is an ellipse, the distance from the center of the ellipse to the outermost surface of the major axis refers to the radius of the major axis.

[0035] In addition, in the present invention, "cross-section of the wire" refers to a surface cut perpendicular to the longitudinal direction of the wire, "center" refers to an area from the center to 0.5R in the cross-section of the wire, and "surface" or "surface area" refers to an area from the center to 0.9R to 1R in the cross-section of the wire.

[0036] A ultra-low carbon steel wire rod according to one embodiment of the present invention comprises, in weight %, C 0.0030% or less (excluding 0), N: 0.0030% or less (excluding 0), Si: 0.01% or less (excluding 0), Mn: 0.05~0.15%, Ti: 0.02~0.05%, Nb: 0.01~0.03%, the remainder being Fe and unavoidable impurities.

[0037] The reasons for limiting the compositional range of each alloying element are described below. Unless otherwise noted, units are weight percent.

[0038] The content of C (carbon) may be 0.0030% or less (excluding 0).

[0039] Along with N, C is the element with the greatest solid solution strengthening effect, and adding 0.1% increases strength to the level of 100 MPa. In other words, it is necessary to limit the content of C to achieve low strength. If the content of C exceeds 0.0030%, pearlite is formed in the microstructure, which may fail to meet the target strength of the present invention and may cause wire breakage during drawing. Therefore, it is desirable to maintain the content of C at 0.0030% or less, more preferably at 0.0028% or less, and most preferably at 0.0025% or less.

[0040] The content of N (nitrogen) may be 0.0030% or less (excluding 0).

[0041] N is an element that, along with C, is dissolved in ferrite and significantly increases strength, and adding 0.1% increases strength to the level of about 100 MPa. If the N content exceeds 0.003%, the yield strength may increase significantly. Therefore, it is desirable to maintain the N content at 0.0030% or less, and more preferably at 0.0025%.

[0042] The content of Si (silicon) may be 0.01% or less (excluding 0).

[0043] Si is a ferrite solid solution strengthening element and is a solid solution strengthening element that increases strength to the level of 15 MPa when 0.1% is added. If the Si content is too high, it may be difficult to achieve the target strength of the present invention. In addition, if the Si content is too high, FeSiO4 is formed, which is not advantageous for scale removal during mechanical peeling, and the remaining scale causes wear on the drawing die during drawing processing, so it is important to include as little Si as possible. Therefore, it is desirable to maintain the Si content at 0.01% or less, and more preferably at 0.009% or less.

[0044] The content of Mn (manganese) can be 0.05 to 0.15%.

[0045] Although Mn has a small solid solution strengthening effect, it increases strength to the level of 10 MPa with a 0.1% increase, so it is necessary to control the Mn content. That is, to lower the strength, the Mn content must be maintained at 0.15% or less. If the Mn content is less than 0.05%, surface cracks may occur during continuous casting due to the formation of S grain boundaries, resulting in surface quality defects such as scabs on the wire surface, which can cause wire breakage during drawing processing. Therefore, it is desirable to maintain the Mn content at 0.05~0.15%, and more preferably at 0.07~0.13%.

[0046] The content of Ti (titanium) can be 0.02~0.05%.

[0047] The addition of Ti is necessary to form TiN for the removal of N and C present in the matrix structure. If the Ti content is less than 0.020%, dissolved N remains in the ferrite, making an increase in strength inevitable, and if it exceeds 0.05%, tundish nozzle clogging may occur, leading to an increase in manufacturing costs. Therefore, it is desirable to maintain the Ti content at 0.02~0.05%, and more preferably at 0.025~0.045%.

[0048] The content of Nb (niobium) can be 0.01 to 0.03%.

[0049] Nb is an element that forms Nb-based precipitates, such as carbonitrides of Nb(C, N). If the Nb content is less than 0.01%, precipitate formation may not be sufficient, and if it exceeds 0.03%, the precipitation effect is reduced due to precipitate coarsening, and a variation in strength may be induced. Therefore, it is desirable to maintain the Nb content at 0.01~0.03%, and more preferably at 0.01~0.02%.

[0050] The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any person skilled in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0051] The microstructure of an ultra-low carbon steel wire rod according to one embodiment of the present invention, comprising the alloy composition as described above, may contain ferrite in an area fraction of 99% or more. It is preferable that the microstructure of the wire rod be a ferrite (α) single-phase structure. A ferrite single-phase structure means containing ferrite in an area fraction of 99% or more, or 100% or more.

[0052] When the radius of the wire is denoted as R, the average grain size of the ferrite observed in the region from the center of the cross-section perpendicular to the longitudinal direction to 0.2R may be 70 μm or more. If the grain size of the wire is excessively small, deformation into ferrite is not smooth when stress is applied, resulting in a finer grain size of the wire, and consequently, the yield strength of the wire may increase. Therefore, in the present invention, by coarsening the grain size of the wire to 70 μm or more, it can rapidly deform into ferrite when stress is applied, thereby lowering the yield strength of the wire.

[0053] According to one embodiment of the present invention, the ultra-low carbon steel wire rod may have a number of NbC precipitates with a diameter of 10 nm or more observed in a region from the center of a cross-section perpendicular to the longitudinal direction up to 0.5 R of 1,900,000 or more per unit area.

[0054] In addition, the ultra-low carbon steel wire rod according to one embodiment of the present invention may have a number of TiN precipitates observed in a region of 0.5R from the center of a cross-section perpendicular to the longitudinal direction of 2,800,000 or more per unit area / mm².

[0055] The number of NbC precipitates and TiN precipitates per unit area above is derived by measuring an image at ×20,000 magnification using a TEM (transmission electron microscope) in the region from the center of a cross-section perpendicular to the length direction to 0.5R as shown in Fig. 2 according to the TEM precipitate extraction method (replica), and then measuring the extracted area and converting it into a circle shape.

[0056] If the diameter of the above NbC precipitates is less than 10 nm, or if the number of NbC precipitates with a diameter of 10 nm or more is less than 1,900,000 per unit area / mm², the tensile strength of the wire may increase, making it difficult to achieve the low strength intended in the present invention.

[0057] In addition, if the number of TiN precipitates is less than 2,800,000 per unit area / mm², the strength of the wire may increase due to the retention of solid solution N in the ferrite.

[0058] If the strength is too high, it may be difficult to use it as a binding wire or annealed wire for binding reinforcing bars, scaffolding steel plates, etc., which are intended in the present invention. Accordingly, the ultra-low carbon steel wire according to one embodiment of the present invention may have a tensile strength of 260 MPa or less and a yield strength of 150 MPa or less.

[0059] Hereinafter, a method for manufacturing an ultra-low carbon steel wire rod according to one embodiment of the present invention will be described.

[0060] A method for manufacturing an ultra-low carbon steel wire rod according to one embodiment of the present invention may include the steps of: heating a bloom containing, in weight %, C 0.0030% or less (excluding 0), N: 0.0030% or less (excluding 0), Si: 0.01% or less (excluding 0), Mn: 0.05~0.15%, Ti: 0.02~0.05%, Nb: 0.01~0.03%, the remainder being Fe and unavoidable impurities in a steel billet heating furnace and then rolling the bloom to produce a billet; hot rolling the billet produced; coiling the hot-rolled wire rod at 830~880℃; and cooling the coiled wire rod.

[0061] The reason for limiting the component range of each alloy composition above may be the same as described above, and each manufacturing step will be explained in more detail below.

[0062] First, a bloom having the compositional components described above is prepared.

[0063] Subsequently, the bloom can be manufactured into a billet by maintaining it in a steel billet heating furnace at a temperature of 1180 to 1250°C for at least 200 minutes and then rolling it into a steel billet. If the steel billet heating furnace temperature is below 1180°C, operational defects may occur due to roll load during steel billet rolling, and if it exceeds 1250°C, partial re-dissolution may occur. Therefore, it is desirable to maintain the steel billet heating furnace temperature at 1180 to 1250°C, and more preferably at 1150 to 1200°C. A billet can be manufactured through steel billet rolling while maintaining it at the above steel billet heating furnace temperature for at least 200 minutes, preferably 240 to 300 minutes.

[0064] Next, the billet manufactured above is rolled into a wire rod.

[0065] Specifically, the billet can be heated to a temperature of 950 to 1050°C in a wire rod heating furnace to enable rolling, and then hot rolling consisting sequentially of rough rolling (RM), intermediate rough rolling (IRM), intermediate finishing rolling (IFM), finishing rolling (FM), and final rolling (RSM) can be performed to produce a wire rod of a desired size.

[0066] The re-dissolution temperature of NbC precipitates is 900℃, and the formation and growth of NbC precipitates must proceed during the rolling stage.

[0067] To this end, the present invention can perform hot rolling by controlling the extraction temperature of the billet extracted from the wire rod heating furnace to 950~1050℃ and the inlet temperature at which the billet enters the finishing rolling (FM) and final rolling (RMS) to Ae3-100℃ to Ae3-10℃.

[0068] That is, Nb is added to the ultra-low carbon steel according to one embodiment of the present invention, and the Nb combines with C to precipitate as NbC, thereby removing C within the ferrite structure. However, the re-dissolution temperature of the NbC precipitate is 900°C, and the temperature of the rolling stage must be controlled to be below the NbC precipitate re-dissolution temperature so that the NbC precipitate is formed and grows without being re-dissolved. Accordingly, in the present invention, the inlet temperature entering the finishing rolling (FM) and final rolling (RSM) during wire rod rolling is controlled to Ae3-100°C to Ae3-10°C, which is below the temperature at which NbC is re-dissolved, thereby preventing the re-dissolution of NbC precipitates and effectively suppressing the solid solution strengthening effect caused by C.

[0069] After undergoing the above wire rolling process, a winding step can be performed.

[0070] Since it is necessary to lower the strength through ferrite grain growth and to improve scale peelability, a scale thickness of 5㎛ or more is required, it is desirable to maintain the coiling temperature at a high level of 830~880℃.

[0071] If the winding temperature is below 830℃, the grain size of the wire rod becomes small, and the average grain size may be as small as 25㎛. If the temperature exceeds 880℃, the average grain size of the ferrite may be increased to 40㎛ or more, but winding defects such as ring indentation may occur due to the high temperature, making it difficult to manufacture the wire rod. Therefore, it is preferable that the above winding be performed at a temperature of 830 to 880℃, and more preferably at a temperature of 840 to 870℃.

[0072] After the above winding, a cooling step is performed.

[0073] It is preferable to perform multi-stage cooling to induce the growth of NbC precipitates and prevent an increase in strength due to the formation of TiC precipitates.

[0074] Specifically, as shown in FIG. 3, the multi-stage cooling can be performed by extremely slow cooling at 1°C / s or less to induce the growth of NbC precipitates at Ae3-800°C or higher, rapid cooling at 20°C / s or more to prevent an increase in strength due to the formation of TiC precipitates at temperatures above 500°C and below 800°C, and then extremely slow cooling at 1°C / s or less at temperatures below 500°C.

[0075] In the temperature range of Ae3-800℃ or higher, to induce the growth of NbC precipitates, ultra-slow cooling is performed with the cover closed while maintaining the lowest conveyor speed, without applying a blower (air) in the Stelmor cooling zone. At this time, cooling to 800℃ can be performed while maintaining the cooling rate at 1℃ / s or less. If the above cooling rate exceeds 1℃ / s, the growth of NbC precipitates does not proceed smoothly, making it difficult to obtain the strength reduction effect caused by the formation of NbC precipitates.

[0076] Next, in the temperature range of over 500℃ and under 800℃, rapid cooling can be performed at a cooling rate of 20℃ / s or higher by applying 100% blower without covering to prevent an increase in strength due to the formation of TiC precipitates. If the cooling rate is less than 20℃ / s, the strength of the wire rod may increase due to the formation of TiC precipitates.

[0077] Finally, in the temperature range below 500℃, the process can be finished by ultra-slow cooling at 1℃ / s or less with the cover closed and without applying blower (air) in the Stelmore cooling zone, while maintaining the lowest conveyor speed. If the cooling speed exceeds 1℃ / s, the variation in tensile strength within the coil may increase.

[0078] An ultra-low carbon steel wire rod according to one embodiment of the present invention, manufactured by controlling the hot rolling and cooling processes in the manner described above, may have at least 1,900,000 NbC precipitates with a diameter of 10 nm or more per unit area and at least 2,800,000 TiN precipitates per unit area.

[0079] In addition, the ultra-low carbon steel wire rod according to one embodiment of the present invention may have an average grain size of ferrite observed in the region from the center of the cross-section perpendicular to the longitudinal direction of the wire rod up to 0.2R of 70㎛ or more, a tensile strength of 260MPa or less, and a yield strength of 150MPa or less.

[0080] Hereinafter, an ultra-low carbon steel wire according to one embodiment of the present invention will be described.

[0081] A super low carbon steel wire according to one embodiment of the present invention may contain, in weight %, C 0.0030% or less (excluding 0), N: 0.0030% or less (excluding 0), Si: 0.01% or less (excluding 0), Mn: 0.05~0.15%, Ti: 0.02~0.05%, Nb: 0.01~0.03%, the remainder being Fe and unavoidable impurities.

[0082] The above ultra-low carbon steel wire can be manufactured into a wire by mechanically peeling the wire rod, drawing it into a wire, and then performing an annealing heat treatment to produce a heat-treated wire.

[0083] Specifically, a scale layer exists on the surface of the wire, and a process to remove the scale from the surface of the wire can be performed through a descaler that peels off the scale before the wire passes through a wire drawing die for wire drawing.

[0084] After mechanically peeling the above wire, the diameter of the wire can be reduced by drawing the wire to a total true deformation amount (e) of 3.62 (0.9 mm wire diameter) to 3.98 (0.75 mm wire diameter), preferably 3.73. At this time, the wire drawing process can be performed using a WC (tungsten carbide) die, etc., with a reduction amount of about 10 to 20% per pass, and the number of passes of the wire drawing process can be appropriately adjusted to one or more times in consideration of cost.

[0085] The above-mentioned wire drawing process is a process that reduces the diameter of the wire rod. Although the strength of the wire rod can be increased by the wire drawing process, the strength of the wire rod is lowered through a subsequent annealing heat treatment process, thereby enabling the production of a steel wire having the target strength of the present invention.

[0086] The temperature or time during the above annealing heat treatment can be appropriately adjusted considering aspects such as grain size, strength, and cost, and preferably, the above-mentioned wire can be cooled after annealing heat treatment at A3-260℃ or higher and A3-160℃ or lower for at least 2 hours.

[0087] If the above annealing heat treatment temperature is below A3-260℃, the tensile strength is high, which may lead to increased hand fatigue when used as binding wire; if it exceeds A3-160℃, it may be difficult to use as a product due to accelerated oxidation and localized corrosion.

[0088] The ultra-low carbon steel wire according to one embodiment of the present invention as described above may have a tensile strength of 320 MPa or less, preferably 315 MPa or less.

[0089] In addition, the ultra-low carbon steel wire according to one embodiment of the present invention may have a yield strength of 200 MPa or less, preferably 190 MPa or less.

[0090] If the tensile strength and yield strength of the above ultra-low carbon steel wire are too high, the strength may be too great, making it difficult to apply it as a binding wire or annealed wire for binding reinforcing bars, scaffolding steel plates, etc., which is the purpose of the present invention.

[0091] In addition, the ultra-low carbon steel wire of the present invention can reduce yield point elongation to 1.0% or less despite having low yield strength.

[0092] Yield point elongation is a phenomenon that generally occurs during strip winding. While the amount of deformation increases during the initial strip winding, the radius of curvature becomes minimum and the amount of deformation becomes maximum at the moment the strip is completely wound onto the mandrel of the winder. As a result, the yield point decreases, and this decrease is applied to the steel plate as supersaturated deformation energy. This supersaturated deformation energy causes the occurrence of tack defects. However, the ultra-low carbon steel wire of the present invention can reduce the yield point elongation to 1.0% or less despite having low yield strength, thereby reducing the occurrence of tack defects. Preferably, the yield point elongation may be 0%.

[0093] The present invention will be explained in more detail below through the following examples. However, the following examples are merely illustrative of the present invention, and the scope of the present invention is not limited thereto.

[0094] Examples

[0095] As shown in Table 1 below, a test casting was performed using a base composition system of 0.0025C-0.0030N-0.01Si-0.1Mn by weight% with varying Ti and Nb contents to produce a continuous casting bloom. The product was maintained at a steel billet heating furnace temperature of 1170°C for 240 minutes, after which it was rolled to produce a billet of 160 mm × 160 mm. The billet was maintained in a wire rod heating furnace for 220 minutes to enable rolling, and then extracted at a wire rod heating furnace extraction temperature of 950°C. Subsequently, after undergoing rough rolling (RM), intermediate rough rolling (IRM), and intermediate finishing rolling (IFM), the product was cooled by pre-cooling before entering the finishing mill (FM), at which time the inlet temperature of the finishing mill was Ae3-80°C. After passing through the finishing rolling mill, cooling water was applied to control the inlet temperature of the final rolling mill (RSM) to Ae3-70℃ to finish rolling. The rolled wire rod was coiled at 860℃ and then subjected to multi-stage cooling in a Stelmore cooling zone. First, it was extremely slow cooled up to 800℃ with a cover and without blower air at a cooling rate of 0.7℃ / s, then rapidly cooled up to 500℃ with 100% airflow without a cover and at a cooling rate of 22℃ / s, and finally, below 500℃, it was finished by extremely slow cooling with a cover and at a cooling rate of 0.8℃ / s without blower air.

[0096] The above wire rod heating furnace exit temperature, finishing rolling (FM) inlet temperature, final rolling (RSM) inlet temperature, coiling temperature, and cooling rates for each multi-stage cooling temperature range are shown in Table 2 below.

[0097] Classification C (wt%) N (wt%) Si (wt%) Mn (wt%) Ti (wt%) Nb (wt%) Ti / N (atomic ratio) Nb / C (atomic ratio) Example 1 0.00 250.00 300.0 10.100.0200.0 191.95 0.98 Comparative Example 1 0.00 250.00 300.0 10.100 0.0 190.00 0.98 Comparative Example 2 0.00 250.00 300.0 10.100.0200 1.95 0.00 Comparative Example 3 0.00 250.00 30 0.010.100.0150.0191.460.98 Comparative Example 40.00250.00300.010.100.0200.0401.952.07 Comparative Example 50.00250.00300.010.100.0200.0191.950.98 Comparative Example 60.00250.00300.010.100.0200.0191.950.98 Comparative Example 70.00250.00300.010.100.0200.0191.950.98

[0098] Classification Wire Rod Heating Furnace Outlet Temperature (°C) FM Inlet Temperature (°C) RSM Inlet Temperature (°C) Winding Temperature (°C) 800°C or higher Cooling Rate (°C / s) 500~800°C Cooling Rate (°C / s) 500°C Cooling Rate (°C / s) Example 1 950 820 830 8600.68220.82 Comparative Example 1 950 820 830 8600.68220.82 Comparative Example 2 950 820 830 8600.68220.82 Comparative Example 3 950 820 830 8600.68220.82 Comparative Example 4 950 820 830 8600.68220.82 Comparative Example 5 950 920 910 8600.68220.82 Comparative Example 6 950 820 830 86021220.82 Comparative Example 7 950 820 830 8600.680.70.82

[0099] The microstructure of the wire rod prepared above, the average ferrite grain size in the microstructure, the number of TiN, TiC, and NbC particles per unit area, tensile strength, yield strength, the increase in yield strength during the sintering performance evaluation, and the presence or absence of yield point elongation were observed, and the results are shown in Table 3 below. The microstructure of the wire rod, the average ferrite grain size in the microstructure, and the number of TiN, TiC, and NbC particles per unit area were determined from the analysis area shown in Fig. 4 within the region extending 0.5R from the center of the cross-section perpendicular to the longitudinal direction of the wire rod. The average ferrite grain size in the microstructure was measured using EBSD, based on the average ferrite grain size observed in the region extending 0.2R from the center of the cross-section perpendicular to the longitudinal direction, where R is the radius of the wire rod. The specimens were polished and finished with silica gel, and cross-sectional measurements were taken at ×100 magnification (Step size: 0.2 mm, tolerance angle: 15°).

[0100] The number of TiN, TiC, and NbC per unit area ( / mm²) was determined by preparing test specimens using the TEM precipitate extraction method (replica) as shown in Fig. 2, and by obtaining a large number of images at ×20,000 magnification in the analysis area shown in Fig. 4 to measure the area fraction for each precipitate.

[0101] Tensile strength was measured using an Instron 8862, and the tensile speed was set to 20 mm / min. As shown in Figure 5, a 5.5 mm diameter wire was cut into 300 mm lengths and subjected to a tensile test. To verify the yield point, a prestrain of 2% was applied, and the wire was straightened and subjected to a tensile test until fracture occurred.

[0102] Yield strength and yield point elongation were measured as shown in Fig. 6 (in Fig. 6, YS represents yield strength and TS represents tensile strength). Yield strength refers to the maximum strength before strength decreases after elastic deformation, and yield point elongation refers to the region where elongation continues even though stress decreases slightly after passing the yield point.

[0103] For the evaluation of bake hardening, a rod specimen with a wire length of 30 cm was used. The specimen was straightened by applying 2% prestrain, and the tensile test was stopped after applying 2% prestrain again. Next, the specimen was maintained at 170°C for 20 minutes in a nitrogen atmosphere and then water-cooled, and the specimen was subjected to a tensile test in the same testing machine until fracture (tensile speed 20 m / m). At this time, the value of 2% prestrain was set as the lower yield (reference point), and the value obtained after heat treatment at 170°C for 20 minutes was set as the upper yield (point at which the yield point occurs), and the result was calculated by calculating 'upper yield - lower yield'.

[0104] Microstructure Grain Size (㎛) Number of TiN ( / mm²) Number of TiC ( / mm²) Number of NbC (>10 nm / mm²) Tensile Strength (MPa) Yield Strength (MPa) Increase in Yield Strength during Searing Performance Evaluation Yield Point Elongation Example 1 Ferrite 7 2,800,000 50 1,900,000 25 21 45 2 No occurrence Comparative Example 1 Ferrite 70 00 2,000,000 28 21 89 6 5 Occurrence Comparative Example 2 Ferrite 67 2,900,000 800 28 31 90 6 7 Occurrence Comparative Example 3 Ferrite 66 2,850,000 60 2,000, 000 294 1973 Non-occurring Comparative Example 4 Ferrite 70 2,800,000 50 2,100,000 296 1981 Non-occurring Comparative Example 5 Ferrite 32 2,700,000 40 500,000 320 2141 Non-occurring Comparative Example 6 Ferrite 58 2,800,000 100 1,400,000 288 2112 Non-occurring Comparative Example 7 Ferrite 71 2,700,000 800,000 2,000,000 284 1904 Non-occurring

[0105] Scale was removed from the above-mentioned wire rod through mechanical peeling in a wire drawing machine, and the wire was drawn to 0.8 mm using a dry drawing machine. After heat-treating the wire in an annealing furnace at 710°C for 2 hours, the wire was cooled in the furnace to produce a steel wire. The tensile strength, yield strength, and elongation of the above-mentioned steel wire were measured and are shown in Table 4 below. The tensile strength and yield strength of the steel wire were measured using the same method as for the wire rod, and the yield point elongation refers to the section in Fig. 6 where the stress decreases slightly after passing the yield point but elongation continues to occur.

[0106] Classification Tensile Strength (MPa) Yield Strength (MPa) Elongation (%) Example 1 31 219 338 Comparative Example 1 35 72 19 27 Comparative Example 2 34 32 13 32 Comparative Example 3 35 42 19 33 Comparative Example 4 35 622 134 Comparative Example 5 38 02 36 31 Comparative Example 6 34 82 16 32 Comparative Example 7 34 42 13 35

[0107] As shown in Tables 3 and 4 above, the following could be confirmed. Example 1 and Comparative Example 1 above are examples showing the effects of Ti addition.

[0108] Example 1 was prepared by adding 0.02% Ti (Ti / N atomic ratio of 1.95), and Comparative Example 1 was prepared by not adding Ti. Both Example 1 and Comparative Example 1 were manufactured using the same manufacturing process except for the presence or absence of Ti addition. Both had a ferrite microstructure and similar grain sizes of approximately 70 μm. However, regarding the number of TiNs formed depending on the presence or absence of Ti addition, in the case of Example 1, approximately 2.8 million TiNs were formed per unit area ( / mm²) regardless of TiN size, whereas no TiNs were observed at all in Comparative Example 1. Additionally, almost no TiC was observed in Example 1, while NbCs with a size of 10 nm or larger were observed at a level of approximately 1.9 million per unit area. In addition, the tensile strength of the wire rod in Example 1 was 252 MPa and the yield strength was 145 MPa, which were very low, whereas the tensile strength of Comparative Example 1 was 282 MPa and the yield strength was 189 MPa, which were very high, indicating that it was unsuitable for use in binding. Furthermore, yield point elongation was observed in Comparative Example 1, and as a result of the bake performance evaluation (maintaining at 170℃ for 2 hours), it was confirmed that the yield strength increased by 65 MPa. Moreover, it was confirmed that the tensile strength and yield strength of the steel wire produced by drawing and heat-treating the wire rod of Comparative Example 1 were also very high, at 357 MPa and 219 MPa, respectively.

[0109] The above Example 1 and Comparative Example 2 are examples showing the effects of Nb addition.

[0110] Example 1 above is a case where 0.02% of Nb was added, and Comparative Example 2 is a case where Nb was not added. In the case of Comparative Example 2, just like Comparative Example 1 which did not add Ti, Nb was not added so that NbC precipitates were not formed, and thus an increase in tensile strength due to C was observed, and an increase in yield strength was observed during the sintering performance evaluation. From these results, it was found that the strength of wire rods and steel wires can be reduced by forming NbC precipitates through the addition of Nb.

[0111] Comparative Example 3 above represents a case where Ti was under-added at 0.015%, and Comparative Example 4 represents a case where Nb was over-added at 0.04%. Strength increased with the under-addition of Ti. On the other hand, when Ti is over-added, its high oxidizing power causes it to clog the tundish nozzle, which lowers the continuous casting rate and increases manufacturing costs. Furthermore, since only two continuous castings are actually possible, this can be considered a greater problem than the increase in strength; therefore, the over-addition of Ti should be avoided. Meanwhile, when Nb was over-added, the number of NbC particles per unit area increased to 2.1 million, but the wire rod strength increased, indicating that the over-addition of Nb should also be avoided.

[0112] Comparative Example 5 above is an example in which the temperature was raised high, as in the production of general mild steel, without controlling the wire rod heating furnace exit temperature, FM, and RSM inlet temperatures during annual rolling. It was confirmed that the ferrite grain size was 32 μm, which was finer compared to Example 1, and the number of NbC particles larger than 10 nm was 500,000 per unit area. As a result, the strength of the wire rod and the strength of the steel wire of Comparative Example 5 increased significantly compared to Example 1, making it unsuitable for use as a binding wire.

[0113] Comparative Examples 6 and 7 above are examples showing the effect of cooling rate in the Stelmore cooling zone. In Comparative Example 6, rapid cooling at a temperature of 800°C or higher resulted in inefficient NbC growth, and it was confirmed that the number of NbC particles larger than 10 nm was 1.4 million, which was significantly lower than in Example 1; this was considered a factor that increased the strength of the wire rod and steel wire. In addition, in Comparative Example 7, ultra-slow cooling at 0.7°C / s was performed without rapid cooling in the 500–800°C range, resulting in the formation of fine TiC (800,000 particles per unit area), and it was confirmed that this increased the strength of the wire rod and steel wire.

[0114] Although embodiments of the invention disclosed above have been illustrated and described, the disclosed invention is not limited to the specific embodiments described above, and various modifications may be made by those skilled in the art to which the disclosed invention belongs without departing from the essence claimed in the claims.

Claims

1. A wire rod comprising, in weight %, C 0.003% or less (excluding 0), N: 0.003% or less (excluding 0), Si: 0.01% or less (excluding 0), Mn: 0.05~0.15%, Ti: 0.02~0.05%, Nb: 0.01~0.03%, and the remainder being Fe and unavoidable impurities, wherein, when the radius of the wire rod is R, the number of NbC precipitates with a diameter of 10 nm or more observed in the region from the center of the cross-section perpendicular to the longitudinal direction up to 0.5R is 1,900,000 or more per unit area and the number of TiN precipitates is 2,800,000 or more per unit area.

2. In Paragraph 1, The microstructure of the above wire contains ferrite with an area fraction of 99% or more.

3. In Paragraph 1, The above wire is a wire in which the average grain size of ferrite observed in the region from the center of the cross-section perpendicular to the longitudinal direction up to 0.2R is 70㎛ or more.

4. In Paragraph 1, The above wire is a wire having a tensile strength of 260 MPa or less and a yield strength of 150 MPa or less.

5. A step of manufacturing a billet by heating a bloom containing, in weight %, C 0.0030% or less (excluding 0), N: 0.0030% or less (excluding 0), Si: 0.01% or less (excluding 0), Mn: 0.05~0.15%, Ti: 0.02~0.05%, Nb: 0.01~0.03%, the remainder Fe and unavoidable impurities, in a steel billet heating furnace and then rolling the bloom into a steel billet; a step of hot rolling the manufactured billet; a step of coiling the hot-rolled wire rod at 830~880℃; and a step of cooling the coiled wire rod; comprising, The above hot rolling step is performed at a wire rod heating furnace outlet temperature of 950 to 1050°C, a finishing rolling (FM) inlet temperature of Ae3-100°C to Ae3-10°C, and a final rolling (RSM) inlet temperature of Ae3-100°C to Ae3-10°C, and A method for manufacturing a wire rod, wherein the cooling step is performed by ultra-slow cooling at 1℃ / s or less at Ae3-800℃ or higher, rapid cooling at 20℃ / s or more at temperatures above 500℃ and below 800℃, and ultra-slow cooling at 1℃ / s or less at temperatures below 500℃.

6. In Paragraph 5, A method for manufacturing a wire rod in which, after the above hot rolling step, the number of NbC precipitates with a diameter of 10 nm or more is 1,900,000 or more per unit area / mm².

7. In Paragraph 5, A method for manufacturing a wire rod in which, after the cooling step above, the number of TiN precipitates is 2,800,000 or more per unit area / mm².

8. In Paragraph 5, A method for manufacturing a wire rod in which, after the cooling step, the average grain size of ferrite observed in the region from the center of the cross-section perpendicular to the length direction up to 0.2R, where R is the radius of the wire rod, is 70㎛ or more.

9. In weight %, containing C 0.003% or less (excluding 0), N: 0.003% or less (excluding 0), Si: 0.01% or less (excluding 0), Mn: 0.05~0.15%, Ti: 0.02~0.05%, Nb: 0.01~0.03%, and the remainder being Fe and unavoidable impurities, The tensile strength is 320 MPa or less, and Steel wire with a yield strength of 200 MPa or less.

10. In Paragraph 9, The above steel wire is a steel wire in which the average grain size of ferrite observed in the region from the center of the cross-section perpendicular to the length direction up to 0.5R, where the radius is R, is 17㎛ or more.

11. In Paragraph 9, The above steel wire is a steel wire having a yield point elongation of 1.0% or less.

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