Wire rod, steel wire and method for manufacturing same

Abstract

An ultra-low carbon steel wire rod according to the present invention contains, in wt%, 0.003% or less of C, 0.002% or less of N, 0.02% or less of Si, 0.05-0.15% of Mn, and 0.015% or less of S, with the remainder comprising Fe and inevitable impurities. When the radius of the wire rod is R, the microstructure of a region extending 0.2R to 0.45R from the center of a cross-section perpendicular to the longitudinal direction contains, in area%, 60% or more of ferrite having an average grain size of 70 µm or more, with the remainder comprising ferrite having an average grain size of less than 70 µm.

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] After manufacturing a wire rod with a diameter of 5.5 to 6.5 mm using a steel mill slab or billet, the scale is removed by mechanical peeling at a binding wire manufacturer, the diameter is reduced (down-sized) by dry drawing a total of two times, and finally, to reduce the increased strength, the wire is finished by annealing in the ferrite single-phase region.

[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, it was necessary to suppress the increase in strength caused by C and N through other elements or changes in the manufacturing process.

[0006] One aspect of the present invention for solving the aforementioned problem is to provide an ultra-low carbon steel wire rod and steel wire, and a method for manufacturing the same, which can lower the strength of the wire rod and steel wire without the addition of expensive alloys and are applicable to reinforcing bars, scaffolding steel plates, machine structural products, etc.

[0007] The technical problems 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.

[0008] 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: 0.003% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.05~0.15%, S: 0.015% or less (excluding 0), the remainder being Fe and unavoidable impurities, and when the radius of the wire rod is R, the microstructure of the region from 0.2R to 0.45R from the center of the cross-section perpendicular to the longitudinal direction may comprise, in area%, 60% or more of ferrite with an average grain size of 70㎛ or more and the remainder being ferrite with an average grain size of less than 70㎛.

[0009] According to one embodiment of the present invention, the microstructure of the wire rod in a region of 0.2R or less from the center of a cross-section perpendicular to the longitudinal direction may contain 40% or less of ferrite with an average grain size of 30㎛ to 40㎛ in area %, and the remainder may contain ferrite with an average grain size exceeding 40㎛.

[0010] According to one embodiment of the present invention, the microstructure of the wire rod in a region of 0.45R or less from the center of a cross-section perpendicular to the longitudinal direction may comprise 99% or more of ferrite with an average grain size of 70㎛ or more in area %, and the remainder may comprise ferrite with an average grain size of less than 70㎛.

[0011] According to one embodiment of the present invention, the wire rod may contain 70% or more of ferrite with an average grain size of 70㎛ or more in the (001) texture and (011) texture in the rolling (RD) direction, based on the high angle (15°) measured by EBSD, and the remainder may contain ferrite with an average grain size of less than 70㎛.

[0012] According to one embodiment of the present invention, the wire may have an average subcrystal size of 8 μm or less as measured by EBSD in the microstructure of the region from 0.2 R to 0.45 R from the center of the cross-section perpendicular to the longitudinal direction.

[0013] According to one embodiment of the present invention, the yield strength of the wire may be 180 MPa or less.

[0014] The tensile strength of the above wire may be 280 MPa or less.

[0015] 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 steel billet in a furnace and extracting it, wherein the steel billet contains, in weight percent, C: 0.0030% or less (excluding 0), N: 0.003% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.05~0.15%, S: 0.015% or less (excluding 0), and the remainder being Fe and unavoidable impurities; the step of hot wire rod rolling (NTM) and precision rolling (RSM) the steel billet; the step of coiling the rolled wire rod; and the step of cooling the coiled wire rod; wherein the hot wire rod rolling entry temperature and the precision rolling entry temperature are controlled to A3+30℃ or lower for rolling, and the coiling is performed at a temperature of A3+30℃ or lower.

[0016] According to one embodiment of the present invention, the hot wire rolling inlet temperature, the precision rolling inlet temperature, and the coiling temperature can satisfy the following equation (1).

[0017] Equation (1): TS - (0.61 / 3)×NTM inlet temperature - (0.33 / 3)×RSM inlet temperature - (0.27 / 3)×winding temperature + (212.6 / 3) < 0

[0018] (In Equation (1), TS represents the tensile strength of the wire rod, NTM inlet temperature represents the inlet temperature of the hot wire rod rolling mill, and RSM inlet temperature represents the inlet temperature of the precision rolling mill)

[0019] According to one embodiment of the present invention, the hot wire rolling and precision rolling can be performed at A3+30℃ or lower.

[0020] According to one embodiment of the present invention, after the rolling step, the wire rod may have a microstructure in the region from 0.2R to 0.45R from the center of the cross-section perpendicular to the longitudinal direction, where the radius of the wire rod is R, containing 60% or more of ferrite with an average grain size of 70㎛ or more in area %, and the remainder may contain ferrite with an average grain size of less than 70㎛.

[0021] According to one embodiment of the present invention, after the rolling step, the wire rod may have a microstructure in an area of ​​0.2R or less from the center of a cross-section perpendicular to the longitudinal direction, containing 40% or less of ferrite with an average grain size of 30㎛ to 40㎛ in area %, and the remainder may contain ferrite with an average grain size exceeding 40㎛.

[0022] According to one embodiment of the present invention, after the rolling step, the wire rod may contain, in an area %, 99% or more of ferrite with an average grain size of 70㎛ or more from the center of the cross-section perpendicular to the longitudinal direction, and the remainder may contain ferrite with an average grain size of less than 70㎛.

[0023] According to one embodiment of the present invention, after the rolling step, the wire rod may contain 70% or more of ferrite with an average grain size of 70㎛ or more in the (001) texture and (011) texture in the rolling (RD) direction based on the high angle (15°) measured by EBSD, and the remainder may contain ferrite with an average grain size of less than 70㎛.

[0024] According to one embodiment of the present invention, after the winding step, the wire may satisfy a binding ability index of 0 or less as indicated by the following formula (2).

[0025] Equation (2): TS - 0.287 × winding temperature + 0.547 × FGS - 55.56

[0026] (In Equation (2), TS represents the tensile strength of the wire and FGS represents the average grain size of the wire)

[0027] A super low carbon steel wire according to one embodiment of the present invention comprises C: 0.0030% or less (excluding 0), N: 0.003% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.05~0.15%, S: 0.015% or less (excluding 0), the remainder being Fe and unavoidable impurities, and may comprise 80% or more of ferrite having an average grain size of 70㎛ or more in the (001) texture and (011) texture in the rolling (RD) direction based on the high angle (15°) measured by EBSD, and the remainder being ferrite having an average grain size of less than 70㎛.

[0028] According to one embodiment of the present invention, the steel wire may have a tensile strength of 340 MPa or less, a yield strength of 190 MPa or less, a twisting frequency of 80 times or more, and an elongation of 40% or more.

[0029] According to the present invention, the strength of the wire rod and steel wire can be lowered without the addition of expensive alloys, thereby providing an ultra-low carbon steel wire rod, steel wire, and a method for manufacturing the same that can be applied to reinforcing bars, scaffolding steel plates, machine structural products, etc.

[0030] 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.

[0031] FIG. 1 is a drawing showing a cross-section of a wire according to one embodiment of the present invention.

[0032] FIG. 2 is a diagram showing a method for measuring tensile strength using a wire rod according to one embodiment of the present invention.

[0033] FIG. 3 is a diagram showing the relationship between tensile strength and yield strength according to one embodiment of the present invention.

[0034] FIG. 4 is a diagram showing the microstructure of the region from 0.2R to 0.45R from the center of a cross-section perpendicular to the longitudinal direction of a wire according to one embodiment of the present invention.

[0035] Figure 5 is a diagram showing the microstructure of the region from 0.2R to 0.45R from the center of the cross-section perpendicular to the longitudinal direction of the wire of Comparative Example 1.

[0036] FIG. 6 is a diagram showing the microstructure of a cross-section of a wire rod according to one embodiment of the present invention.

[0037] Figure 7 is a diagram showing the microstructure of the cross-section of the wire of Comparative Example 1.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] When pouring concrete, heat-treated wires with a diameter of about 0.8 mm or less are used to fix reinforcing bars. Since field workers fix them manually, the yield strength and tensile strength must be low to minimize hand fatigue.

[0043] Binding wire is produced by using cold slabs of IF steel from a steel mill as the base material, torch cutting, reheating, billet rolling, and wire rod rolling, followed by Stelmore cooling to finish. The 5.5–6.5 mm wire rods produced at this stage are mechanically delaminated at a drawing spinneret to remove scale, and then drawn (diameter reduction) to produce 0.8–0.9 mm diameter wire. Next, the wire undergoes annealing heat treatment (dislocation reduction, recrystallization) to lower strength, and is then straightened and cut to produce the final product. Although the strength increased by drawing can be significantly reduced through annealing heat treatment, the yield strength after annealing is also high if the wire rod yield strength is high; therefore, the wire rod yield strength must fundamentally be low to be used for binding wire applications.

[0044] The best way to reduce yield strength is to remove alloying elements that cause solid solution strengthening within the ferrite. When dissolved within ferrite, interstitial elements contribute more significantly to strength increase than substitutional elements. In other words, to lower the yield strength of binding wire, it is necessary to reduce the amount of alloying elements dissolved within the ferrite, and in particular, the content of C and N, which have a significant solid solution strengthening effect, must be minimized.

[0045] Generally, to reduce C and N, decarburization and denitrification must be performed during the steelmaking process; however, since this significantly increases manufacturing costs, the strengthening effect is suppressed by adding precipitate / carbide-forming elements such as Ti and Nb. This is because the addition of Ti forms TiN (1420°C) and Ti4C2S2 (1220°C) precipitates, which can remove N and C from the ferrite. While the combined addition of Nb can lead to the formation of NbC, there is a problem of increased manufacturing costs due to NbC being an expensive element.

[0046] However, if a billet manufacturing process involving continuous casting bloom is included, it is impossible to suppress solid solution strengthening through Ti. Ti4C2S2 precipitates at 1225°C, but because the temperature of the billet heating furnace is high at 1230°C, Ti4C2S2 is re-dissolved. This means that N solid solution strengthening is suppressed, but C solid solution strengthening is maintained.

[0047] Meanwhile, another method to effectively lower yield strength is to coarsen the grain size. This is because deformation occurs quickly when stress is applied if the grain size is large. However, since wire rod rolling is fast and rolling is performed above the recrystallization temperature, the grain size must be fine.

[0048] Accordingly, the present invention aims to provide ultra-low carbon steel wire rods and steel wires capable of lowering yield strength without using expensive elements such as Ti and Nb by controlling manufacturing processes such as the wire rod heating furnace and rolling temperature.

[0049] The ultra-low carbon steel wire rod according to one embodiment of the present invention will be described in detail below.

[0050] 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.

[0051] 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" refers to an area from the center to 0.9R to 1R in the cross-section of the wire.

[0052] 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: 0.003% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.05~0.15%, S: 0.015% or less (excluding 0), the remainder being Fe and unavoidable impurities.

[0053] The reasons for limiting the compositional range of each alloying element are explained below. Unless otherwise noted, units are weight percent.

[0054] The content of C (carbon) may be 0.0030% or less (excluding 0).

[0055] 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 not satisfy 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 0.0025% or less, and most preferably at 0.0022% or less.

[0056] The content of N (nitrogen) may be 0.003% or less (excluding 0).

[0057] N is an element that, along with C, is dissolved in ferrite and significantly increases strength; 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 0.003% or less, more preferably at 0.0025%, and most preferably at 0.002%.

[0058] The content of Si (silicon) may be 0.02% or less (excluding 0).

[0059] Si is a ferrite solid solution strengthening element, and adding 0.1% increases strength to the level of 15 MPa. 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 stripping, 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 0.02% or less, and more preferably at 0.01% or less.

[0060] The content of Mn (manganese) can be 0.05~0.15%.

[0061] Mn is also a ferrite solid solution strengthening element, and a 0.1% increase increases strength to the level of 10 MPa. In other words, to reduce 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. Therefore, it is desirable to maintain the Mn content at 0.05~0.15%, and more preferably at 0.07~0.13%.

[0062] The content of S (sulfur) may be 0.015% or less (excluding 0).

[0063] S is an element that is inevitably added during the steel manufacturing process and has the problem of reducing workability by forming MnS inclusions at grain boundaries. Therefore, it is desirable to maintain the S content at 0.015% or less, and more preferably at 0.010% or less.

[0064] 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.

[0065] 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 99% or more of ferrite in area %. It is preferable that the microstructure of the wire rod be a ferrite (α) single-phase structure. A ferrite single-phase structure means containing 99% or more, or 100%, of ferrite in area %.

[0066] In one embodiment of the present invention, the ultra-low carbon steel wire rod may have a microstructure in the region from 0.2R to 0.45R from the center of a cross-section perpendicular to the longitudinal direction, where the radius of the wire rod is R, containing 60% or more of ferrite with an average grain size of 70㎛ or more in area %, and the remainder may contain ferrite with an average grain size of less than 70㎛.

[0067] The area fraction of ferrite refers to the value measured at ×200 magnification using EBSD in the microstructure of the region from 0.2R to 0.45R from the center of the cross-section perpendicular to the longitudinal direction of the wire (Step size: 0.2㎜, tolerance angle: 15°).

[0068] In addition, the ultra-low carbon steel wire rod according to one embodiment of the present invention may have a microstructure in a region of 0.2R or less from the center of a cross-section perpendicular to the longitudinal direction, in area %, comprising 40% or less of ferrite with an average grain size of 30㎛ to 40㎛ and the remainder comprising ferrite with an average grain size exceeding 40㎛.

[0069] Preferably, the ultra-low carbon steel wire rod according to one embodiment of the present invention may have a microstructure in a region of 0.45R or less from the center of a cross-section perpendicular to the longitudinal direction, comprising 99% or more of ferrite with an average grain size of 70㎛ or more in area %, and the remainder comprising ferrite with an average grain size of less than 70㎛.

[0070] More preferably, the ultra-low carbon steel wire rod according to one embodiment of the present invention may contain 100% of ferrite with an average grain size of 70㎛ or more in an area % of the microstructure of a region of 0.45R or less from the center of a cross-section perpendicular to the longitudinal direction.

[0071] That is, the ultra-low carbon steel wire rod according to one embodiment of the present invention must contain at least 60% of coarse ferrite with an average grain size of 70 μm or more in its microstructure to satisfy a yield strength of 180 MPa or less, and the yield strength of the steel wire targeted by the present invention can also be achieved only by using a wire rod that satisfies the above yield strength. Therefore, it is most preferable for the ultra-low carbon steel wire rod of the present invention to contain 100% of coarse ferrite with an average grain size of 70 μm or more in its microstructure. However, since the ultra-low carbon steel wire rod of the present invention can achieve the yield strength targeted by the present invention even when fine ferrite with an average grain size of 30 to 40 μm is 40%, the ultra-low carbon steel wire rod of the present invention may contain 60% or more of coarse ferrite with an average grain size of 70 μm and 40% or less of fine ferrite with an average grain size of 30 to 40 μm.

[0072] In addition, the ultra-low carbon steel wire rod according to one embodiment of the present invention may contain 70% or more of ferrite with an average grain size of 70㎛ or more in the (001) texture and (011) texture in the rolling (RD) direction, based on the high angle (15°) measured by EBSD, and the remainder may contain ferrite with an average grain size of less than 70㎛.

[0073] Ferrite in the (001) texture and (011) texture in the rolling direction refers to the (001) plane and (011) texture formed perpendicular to the rolling direction observed using EBSD (Step size: 0.2 mm, tolerance angle: 15°).

[0074] In the rolling (RD) direction, if the ferrite with an average grain size of 70 μm or more in the (001) texture and (011) texture is less than 70%, the torsional properties may be reduced.

[0075] In addition, the ultra-low carbon steel wire rod according to one embodiment of the present invention may have an average subcrystalline size of 8 μm or less as measured by EBSD in the microstructure of the region from 0.2 R to 0.45 R from the center of the cross-section perpendicular to the longitudinal direction.

[0076] The average subcrystal size refers to the value observed using EBSD in the region from 0.2R to 0.45R from the center of the cross-section perpendicular to the longitudinal direction (Step size: 0.2㎜, tolerance angle: 5°).

[0077] When stress is applied to the region where ferrite is formed and deformation occurs, the sub-grains formed within the ferrite act as seeds, causing them to grow in a direction that lowers energy when deformation ends, that is, in a direction that increases the grain size. At this time, the size of the ferrite grains in the microstructure of the wire rod may vary depending on the size of the sub-grains acting as seeds. When the average sub-grain size measured in the microstructure of the 0.2R to 0.45R range is 8㎛ or less, coarse ferrite of 70㎛ or more is formed, thereby increasing the grain size of the ferrite to a level sufficient to achieve the yield strength of the wire rod intended in the present invention.

[0078] 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 is the purpose of this invention. In particular, the strength increased by the wire drawing process by the manufacturer can be significantly reduced through annealing heat treatment. However, if the yield strength of the wire rod is high, it remains high even after annealing heat treatment; therefore, the yield strength of the wire rod must be fundamentally low to be suitable for use as a binding wire.

[0079] Accordingly, the ultra-low carbon steel wire rod according to one embodiment of the present invention may have a tensile strength of 280 MPa or less and a yield strength of 180 MPa or less.

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

[0081] 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 steel billet containing, in weight percent, C: 0.0030% or less (excluding 0), N: 0.003% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.05~0.15%, S: 0.015% or less (excluding 0), the remainder being Fe and unavoidable impurities, in a heating furnace and then extracting it; the steps of hot wire rod rolling (NTM) and precision rolling (RSM) of the steel billet; the step of coiling the rolled wire rod; and the step of cooling the coiled wire rod.

[0082] 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.

[0083] First, a bloom having the compositional components described above is prepared.

[0084] 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.

[0085] Next, the billet manufactured above is rolled into a wire rod with a cross-sectional diameter of 5.5 to 6.5 mm.

[0086] The above wire rod rolling process can be performed by maintaining the wire rod in a heating furnace at a temperature of 1000 to 1200°C for at least 80 minutes. If the temperature of the heating furnace is below 1000°C, the temperature may be too low in the leading rolling section, such as rough rolling, which may cause severe roll wear; if it exceeds 1200°C, it may be difficult to achieve the yield strength targeted in the present invention, and there may be a problem of increased costs due to large scale loss, and ductility may also decrease as AGS increases.

[0087] More specifically, the wire rod rolling process can be performed as follows.

[0088] The above-mentioned billet is subjected to hot wire rolling (NTM) and precision rolling (RSM) in a wire rod heating furnace maintained at a temperature of 1000 to 1200°C for 80 minutes or more, wherein the inlet temperature of the hot wire rolling (NTM) and the inlet temperature of the precision rolling (RSM) can each be controlled to A3+30°C or lower.

[0089] The A3 temperature varies depending on the carbon content, and since fine ferrite may be formed if the A3 temperature is exceeded, it is desirable to control the rolling process of the wire rod to A3 or lower. However, in the present invention, since the yield strength intended in the present invention can be achieved when coarse ferrite of 70㎛ or more is formed at a fraction of 60% or more, the temperature of the wire rod rolling process of the present invention can be controlled to A3+30℃ or lower.

[0090] In addition, even if deformation occurs during hot wire rod rolling, fine grains may appear in the center of the wire rod and coarse grains on the surface due to surface cooling before entering precision rolling. Therefore, in the present invention, by controlling the temperature to be lower than A3+30℃ from the entry temperature of the hot wire rod rolling process during the wire rod rolling process, coarse ferrite can be formed not only on the surface but also in the center.

[0091] That is, in the present invention, the input temperature of the hot wire rod rolling (NTM) and the input temperature of the precision rolling (RSM) in the wire rod rolling process are controlled to A3+30℃ or lower, and by performing the hot wire rod rolling and precision rolling at A3+30℃ or lower, the yield strength of the wire rod targeted in the present invention can be satisfied by securing a fraction of 60% or more of coarse ferrite of 70㎛ or more.

[0092] The wire rod that has undergone the above wire rod rolling process may undergo a winding step to reduce the strength of the wire rod through an increase in grain size and to reduce the strength variation within the coil.

[0093] It is preferable that the above-mentioned coiling temperature be performed at a temperature of A3+30℃ or lower. If the coiling temperature exceeds A3+30℃, the grain size of the wire becomes smaller, and the fraction of fine ferrite with an average grain size of 30 to 40㎛ increases, making it difficult to obtain a microstructure containing 60% or more of coarse ferrite with an average grain size of 70㎛ or more, and consequently, it may be difficult to satisfy the target yield strength of the present invention.

[0094] After winding as described above, a cooling step is performed.

[0095] To reduce variations in strength within the coil, it is best to proceed with cooling as slowly as possible.

[0096] The above cooling can be performed without applying a blower (air) in the Stelmore cooling unit, while maintaining the lowest conveyor speed and covering the unit. At this time, the cooling can be performed up to 600°C while maintaining a cooling rate of 0.6 to 0.7°C / s.

[0097] If the above cooling rate is less than 0.6℃ / s, equipment investment becomes necessary due to the limit of the conveying speed, and if it exceeds 0.7℃ / s, the variation in tensile strength within the coil may increase. Therefore, it is desirable to perform the above cooling at a speed of 0.6 to 0.7℃ / s.

[0098] In the above method for manufacturing ultra-low carbon steel wire rods, the hot wire rod rolling inlet temperature, the precision rolling inlet temperature and the coiling temperature can satisfy the following equation (1).

[0099] Equation (1): TS - (0.61 / 3)×NTM inlet temperature - (0.33 / 3)×RSM inlet temperature - (0.27 / 3)×winding temperature + (212.6 / 3) < 0

[0100] (In Equation (1), TS represents the tensile strength of the wire rod, NTM inlet temperature represents the inlet temperature of the hot wire rod rolling mill, and RSM inlet temperature represents the inlet temperature of the precision rolling mill)

[0101] The meaning of the value of Equation (1) above being greater than 0 is that the rolling temperature is high and the coiling temperature is high. When the rolling temperature and coiling temperature are high, rolling occurs in the austenite single-phase region, resulting in fine grains. Therefore, in the present invention, by controlling the value of Equation (1) above to be less than 0, coarse-grained ferrite can be formed, making it possible to secure low-strength wire rods and steel wires as intended in the present invention.

[0102] In addition, after the above winding step, the wire can satisfy a binding ability index of 0 or less as indicated by the following formula (2).

[0103] Equation (2): TS - 0.287 × winding temperature + 0.547 × FGS - 55.56

[0104] (In Equation (2), TS represents the tensile strength of the wire and FGS represents the average grain size of the wire)

[0105] The reason yield strength is important when binding reinforcing bars is that field workers bind the heat-treated wires by hand. If the yield strength is high, binding must be done at a correspondingly high stress, and since the yield strength and tensile strength increase as the binding capacity index increases, the binding capacity index expressed by the above equation (2) must be low. That is, the binding capacity index expressed by the above equation (2) must satisfy 0 or less so that hand fatigue for binding during work is reduced and workability can be improved.

[0106] In an embodiment of the present invention, an ultra-low carbon steel wire rod manufactured by the above method may have a microstructure in the region from 0.2R to 0.45R from the center of a cross-section perpendicular to the longitudinal direction, where the radius of the wire rod is R, containing 60% or more of ferrite with an average grain size of 70㎛ or more in area %, and the remainder may contain ferrite with an average grain size of less than 70㎛.

[0107] In addition, the ultra-low carbon steel wire rod of the present invention may have a microstructure in a region of 0.2R or less from the center of a cross-section perpendicular to the longitudinal direction, in area %, containing 40% or less of ferrite with an average grain size of 30㎛ to 40㎛, and the remainder may contain ferrite with an average grain size exceeding 40㎛.

[0108] Preferably, the ultra-low carbon steel wire rod of the present invention may have a microstructure in a region of 0.45R or less from the center of a cross-section perpendicular to the longitudinal direction, comprising 99% or more of ferrite with an average grain size of 70㎛ or more in area %, and the remainder comprising ferrite with an average grain size of less than 70㎛.

[0109] According to one embodiment of the present invention, the ultra-low carbon steel wire rod may contain 70% or more of ferrite with an average grain size of 70㎛ or more in the (001) texture and (011) texture in the rolling (RD) direction, based on the high angle (15°) measured by EBSD, and the remainder may contain ferrite with an average grain size of less than 70㎛.

[0110] Hereinafter, an ultra-low carbon steel wire according to one embodiment of the present invention will be described.

[0111] A super low carbon steel wire according to one embodiment of the present invention may contain, in weight percent, C: 0.0030% or less (excluding 0), N: 0.003% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.05~0.15%, S: 0.015% or less (excluding 0), the remainder being Fe and unavoidable impurities.

[0112] 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.

[0113] 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.

[0114] After mechanically peeling the wire, the diameter of the wire can be reduced by drawing the wire so that the total true deformation amount (e) is 3.73 to 3.86, preferably 3.73. At this time, the wire drawing 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 times the wire drawing can be appropriately adjusted to one or more times considering the cost aspect.

[0115] 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.

[0116] During the annealing heat treatment below, the temperature or time can be appropriately adjusted by considering aspects such as grain size, strength, and cost.

[0117] Preferably, the above-mentioned fresh wire can be cooled after annealing heat treatment at A3-260℃ or higher and A3-160℃ or lower for at least 2 hours.

[0118] If the above heating temperature is below A3-260℃, the tensile strength is high, which may cause increased hand fatigue when used as a binding wire; if it exceeds A3-160℃, it may be difficult to use as a product due to accelerated oxidation and localized corrosion.

[0119] According to one embodiment of the present invention as described above, the ultra-low carbon steel wire may contain 80% or more of ferrite with an average grain size of 70㎛ or more in the (001) texture and (011) texture in the rolling (RD) direction, based on the high angle (15°) measured by EBSD, and the remainder may contain ferrite with an average grain size of less than 70㎛.

[0120] In addition, the ultra-low carbon steel wire according to one embodiment of the present invention may have a tensile strength of 340 MPa or less, a yield strength of 190 MPa or less, a twisting frequency of 80 times or more, and an elongation of 40% or more.

[0121] 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.

[0122] Examples

[0123] A continuous casting bloom was produced by test casting according to the alloy composition shown in Table 1 below, and after maintaining it at 1180~1250℃ for more than 200 minutes, it was rolled into a steel billet of 160 mm × 160 mm. After rolling the billet into a wire rod and coiling it, it was extremely slow cooled at a cooling rate of 0.6~0.7℃ / s in a Stelmore cooling zone without blower air, covered with a cover, and finished at a temperature of 600℃ when it exited the cover.

[0124] The above wire rod heating furnace temperature, precision rolling (RSM) inlet temperature, coiling temperature, and cooling rate are shown in Table 2 below.

[0125] The units in Table 1 below are weight %.

[0126] CSiMnSMn / SNA3 Equilibrium Temperature (°C) Example 1 0.00 2 20.0 2 0.1 00.00 5 1 1.6 7 0.00 30 870 Example 2 20.0 2 20.0 2 100.00 5 1 1.6 7 0.00 30 870 Comparative Example 1 0.00 2 20.0 2 100.00 5 1 1.6 7 0.00 30 870 Comparative Example 2 20.0 2 20.0 2 100.00 5 1 1.6 7 0.00 30 870 Comparative Example 3 0.00 35 0.0 20. 100.00511.670.0030858Comparative Example 40.00220.300.100.00511.670.0030867Comparative Example 50.00220.020.170.00519.840.0030868Comparative Example 60.00220.020.050.0055.830.0030869Comparative Example 70.00220.020.100.00511.670.0030870

[0127] Heating furnace temperature (°C) Precision rolling (RSM) entry side temperature (°C) Coiling temperature (°C) Cooling rate (C / s) Example 1 10358658680.68 Example 2 10388688970.67 Comparative Example 1 10408929150.66 Comparative Example 2 10408629110.69 Comparative Example 3 10358668650.69 Comparative Example 4 10358678600.68 Comparative Example 5 10368708600.67 Comparative Example 6 1034865860no data Comparative Example 7 10389128680.68

[0128] The microstructure of the wire rod manufactured above, the average ferrite grain size in the microstructure from 0.2R to 0.45R from the center of the wire rod cross-section, the average ferrite grain size in the microstructure below 0.2R, the fraction of fine ferrite with an average grain size of less than 40㎛, tensile strength, yield strength, and the occurrence of cobble were measured, and the results are shown in Tables 3 and 4 below. The average ferrite grain size in the microstructure was measured using EBSD for the microstructure in the region from 0.2R to 0.45R from the center of the cross-section perpendicular to the longitudinal direction, where R is the wire rod radius, and for the average ferrite grain size in the microstructure below 0.2R. At this time, the step size was set to 0.2 mm, the tolerance angle to 15°, and the magnification to ×200.

[0129] The fraction of fine ferrite with an average grain size of less than 40 μm was expressed by calculating the area of ​​ferrite with an average grain size of less than 40 μm from the microstructure of the cross-section center to the 0.45 R region of the wire rod.

[0130] Tensile strength was measured using an Instron 8862, and the tensile speed was set to 20 mm / min. As shown in Figure 2, 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.

[0131] Yield strength refers to the maximum strength before strength decreases after elastic deformation, and was measured as shown in Fig. 3.

[0132] Whether cobble occurred was evaluated by checking whether jamming or cutting occurred during rolling when rolling a 2-ton wire rod.

[0133] Microstructure 0.2~0.45R Ferrite Average grain size (㎛) 0.2R or less Ferrite Average grain size (㎛) Less than 40㎛ Ferrite fraction (%) Example 1 Ferrite 83780 Example 2 Ferrite 813430 Comparative Example 1 Ferrite 3432100 Comparative Example 2 Ferrite 843360 Comparative Example 3 Ferrite + Pearlite 45440 Comparative Example 4 Ferrite 8600 Comparative Example 5 Ferrite 7900 Comparative Example 6 Ferrite no data no data no data Comparative Example 7 Ferrite 3232100

[0134] Tensile Strength (TS,MPa) Yield Strength (YS,MPa) TS-YS(MPa) Cobble Occurrence Example 1 26 4.6 154.1 110.5 No occurrence Example 2 27 6.0 16 6.1 109.9 No occurrence Comparative Example 1 33 0.8 184.8 146.0 No occurrence Comparative Example 2 30 8.2 17 7.6 130.6 No occurrence Comparative Example 3 34 5.7 192.3 153.4 No occurrence Comparative Example 4 33 7.0 180.7 156.3 No occurrence Comparative Example 5 33 1.0 191.2 139.8 No occurrence Comparative Example 6 No data No data No data Occurrence Comparative Example 7 32 7.8 198.7 130.6 No occurrence

[0135] The scale was removed from the above-manufactured wire rod through mechanical peeling at the processing plant, and a wire was produced with a total true deformation (e) of 3.73 (e = 2 × ln(initial diameter / final diameter)) through first and second drawing, followed by annealing heat treatment in an annealing furnace at 670°C for 2 hours and cooling to produce a heat-treated wire. The occurrence of wire breakage during processing of the above-manufactured wire, as well as the yield strength, number of twists, and binding ability index of the heat-treated wire, are shown in Table 5 below.

[0136] The number of twists of the heat treatment wire refers to the number of times a test specimen with a length of 100D (where D is the diameter) is fixed and rotated in the same direction until fracture occurs.

[0137] Classification Fresh wire Heat treated wire Processing Interrupted wire YS (MPa) Twist (times) Type (2) Binding ability indicator Example 1 No occurrence 172.186 -2.1 Example 2 No occurrence 185.1820.0 Comparative Example 1 No occurrence 211.86728.2 Comparative Example 2 No occurrence 194.66929.6 Comparative Example 3 Break wire no data no data 62.5 Comparative Example 4 Break wire no data no data 73.9 Comparative Example 5 Break wire no data no data 64.7 Comparative Example 6 no data no data no data Comparative Example 7 No occurrence 199.76840.6

[0138] As shown in Tables 3 to 5 above, in the case of Example 1, in which a 0.0022C-0.02Si-0.10Mn-0.002B-0.003N (wt.%) composition system (A3 equilibrium temperature = 870℃) was used and the precision rolling entry temperature and coiling temperature were maintained at A3+30℃ or lower during wire rod rolling, the microstructure was ferrite. As shown in Fig. 4, the average grain size of ferrite in the microstructure of 0.2R to 0.45R from the center of the wire rod cross-section was 83㎛, and the average grain size of ferrite in the microstructure of 0.2R or less at the center was 78㎛, confirming that no fine ferrite was formed. In addition, the tensile strength of the wire in Example 1 was 264.6 MPa and the yield strength was 154.1 MPa, which is very low, and the scale thickness was 8.8 μm, indicating that no breakage occurred when the wire was scaled off and drawn. In addition, the yield strength of the heat-treated wire was 172.1 MPa, which is very low, and the torsional characteristics were also good at 86 turns, and the binding ability index of Equation (2) was also confirmed to be -2.1.

[0139] In contrast, Comparative Example 1 is a case where water is not applied in the water cooling zone after precision rolling (RSM), causing the temperature to increase and the coiling temperature to significantly exceed A3℃. In the case of Comparative Example 1, as shown in Fig. 5, the average grain size of ferrite in the microstructure at 0.2R to 0.45R from the center of the wire rod cross-section was 34㎛, which is a 2.5-fold decrease, and it was confirmed that the tensile strength and yield strength of Comparative Example 1 were high. The binding ability index of Equation (2) was also 28.2, which did not meet the standard. In the case of the heat-treated wire using the wire rod of Comparative Example 1, the yield strength was high at 211.8MPa, and from these results, it was found that when the yield strength of the wire rod is high, the yield strength of the heat-treated wire after annealing is also high.

[0140] In addition, from Comparative Example 1, it was confirmed that when the wire strength is high, the strength of the heat-treated wire after annealing heat treatment is high.

[0141] Example 2 is a case where the precision rolling (RSM) entry temperature is A3°C or lower and the coiling temperature is A3+30°C or lower, and it was confirmed that the average grain size of ferrite in the microstructure of 0.2R to 0.45R from the center of the wire cross-section increased significantly. However, it was confirmed that fine ferrite of 30 to 40 μm size was formed in the microstructure of 0.2R or lower in the center.

[0142] Figures 6 and 7 respectively show the microstructures of the wires of Example 2 and Comparative Example 1. As shown in Figures 6 and 7, Comparative Example 1, which has high strength, has fine grains of less than 40 μm throughout its microstructure, whereas Example 2 has fine grains of less than 40 μm in the center, but coarse grains in other parts of the microstructure.

[0143] Meanwhile, in the case of Example 2, it was found that the yield strength and tensile strength increased significantly when the ferrite with fine grains of less than 40 μm was included in a fraction of 30%, whereas in the case of Comparative Example 2, the ferrite with fine grains of less than 40 μm was included in a fraction of 60%. From these results, it was found that the yield strength and tensile strength of the wire rod increase when the fraction of fine ferrite grains of less than 40 μm is 40% or more.

[0144] Comparative Example 3 is an example intended to show the change in mechanical properties of wire rods according to the C content and whether wire breakage occurs during drawing processing. An increase in C content forms a pearlite (cementite) structure that increases strength, which is accompanied by an increase in strength due to the formation of hard cementite in addition to solid solution strengthening by C.

[0145] The carbon content of Example 1 was low at 0.0022%, while that of Comparative Example 3 was high at 0.0045%, which is a carbon content at which pearlite can be formed. It was confirmed that, despite the same rolling and cooling conditions, the tensile strength and yield strength of Comparative Example 3 were very high at 345.7 MPa and 192.3 MPa, respectively. In addition, in the case of Comparative Example 3, wire breakage occurred during processing in the wire drawing process, which was presumed to be due to pearlite acting as a crack-inducing site for wire breakage during processing. Furthermore, in the case of Comparative Example 3, the binding ability index of Equation (2) was also very high at 62.5, indicating poor workability.

[0146] Comparative Example 4 is a case where the Si content was added at 0.3%, and compared to Example 1, the tensile strength and yield strength were very high, and the bonding ability index of Equation (2) was also very high at 73.9, indicating poor workability. In addition, in the case of Comparative Example 4, wire breakage occurred during processing in the wire drawing process, which was found to be due to die defects such as breakage caused by the high bonding strength between Fe2SiO4 (pyralite) and the Fe matrix.

[0147] Comparative Example 5 was a case where Mn was added at a content of 0.17%, and it was found that it was unsuitable for use because wire breakage occurred during processing in the fresh yarn.

[0148] Mn combines with S in the molten steel to exist as Mn / S, but if the Mn content is too low, S exists at the grain boundaries and degrades the surface quality of the wire rod. In the case of Comparative Example 6, the Mn / S ratio was lower than 6, so it was found to be unsuitable for use as a product due to quality issues.

[0149] In addition, Comparative Example 7 is a case where the precision rolling (RSM) temperature significantly exceeds A3+30℃, and only fine ferrite with an average grain size of less than 40㎛ exists, resulting in high tensile strength and yield strength of the wire rod, and it was confirmed that the yield strength of the heat-treated wire using the wire rod of Comparative Example 7 was high at 199.7MPa.

[0150] 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. In wt%, C: 0.0030% or less (excluding 0), N: 0.003% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.05~0.15%, S: 0.015% or less (excluding 0), the remainder being Fe and unavoidable impurities, A wire rod in which, when the radius of the wire rod is R, the microstructure of the region from 0.2R to 0.45R from the center of the cross-section perpendicular to the longitudinal direction contains, in area %, 60% or more of ferrite with an average grain size of 70㎛ or more, and the remainder is ferrite with an average grain size of less than 70㎛.

2. In Paragraph 1, The above wire rod comprises a microstructure of a region of 0.2R or less from the center of a cross-section perpendicular to the longitudinal direction, wherein, in area %, it contains 40% or less of ferrite with an average grain size of 30㎛ to 40㎛ and the remainder contains ferrite with an average grain size exceeding 40㎛.

3. In Paragraph 1, The above wire is a wire in which the microstructure of the region of 0.45R or less from the center of the cross-section perpendicular to the longitudinal direction comprises 99% or more of ferrite with an average grain size of 70㎛ or more in area %, and the remainder comprises ferrite with an average grain size of less than 70㎛.

4. In Paragraph 1, The above wire rod comprises 70% or more of ferrite with an average grain size of 70㎛ or more in the (001) texture and (011) texture in the rolling (RD) direction, based on the high angle (15°) measured by EBSD, and the remainder comprises ferrite with an average grain size of less than 70㎛.

5. In Paragraph 1, The above wire is a wire having an average subcrystal size of 8㎛ or less as measured by EBSD in the microstructure of the region from 0.2R to 0.45R from the center of the cross-section perpendicular to the longitudinal direction.

6. In Paragraph 1, A wire rod having a yield strength of 180 MPa or less.

7. In Paragraph 1, A wire rod having a tensile strength of 280 MPa or less.

8. A step of heating and extracting a steel billet containing, in weight%, C: 0.0030% or less (excluding 0), N: 0.003% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.05~0.15%, S: 0.015% or less (excluding 0), and the remainder being Fe and unavoidable impurities, in a heating furnace; a step of hot wire rod rolling (NTM) and precision rolling (RSM) of the steel billet; a step of coiling the rolled wire rod; and a step of cooling the coiled wire rod; comprising The above hot wire rod rolling inlet temperature and precision rolling inlet temperature are controlled to A3+30℃ or lower for rolling, and A method for manufacturing a wire rod, comprising performing the above-mentioned winding at a temperature of A3+30℃ or lower.

9. In Paragraph 8, The above hot wire rod rolling inlet temperature, precision rolling inlet temperature and coiling temperature satisfy the following equation (1) for the method of manufacturing wire rod. Equation (1): TS - (0.61 / 3)×NTM inlet temperature - (0.33 / 3)×RSM inlet temperature - (0.27 / 3)×winding temperature + (212.6 / 3) < 0 (In Equation (1), TS represents the tensile strength of the wire rod, NTM inlet temperature represents the inlet temperature of the hot wire rod rolling mill, and RSM inlet temperature represents the inlet temperature of the precision rolling mill) 10. In Paragraph 8, The above hot wire rolling and precision rolling are a method for manufacturing wire rods performed at A3+30℃ or lower.

11. In Paragraph 8, A method for manufacturing a wire rod after the above rolling step, wherein, when the radius of the wire rod is R, the microstructure of the region from 0.2R to 0.45R from the center of the cross-section perpendicular to the longitudinal direction contains 60% or more of ferrite with an average grain size of 70㎛ or more in area %, and the remainder contains ferrite with an average grain size of less than 70㎛.

12. In Paragraph 11, A method for manufacturing a wire rod in which, after the above rolling step, the microstructure of the wire rod in a region of 0.2R or less from the center of the cross-section perpendicular to the longitudinal direction contains 40% or less of ferrite with an average grain size of 30㎛ to 40㎛ in area %, and the remainder contains ferrite with an average grain size exceeding 40㎛.

13. In Paragraph 11, A method for manufacturing a wire rod in which, after the above rolling step, the microstructure of the wire rod in a region of 0.45R or less from the center of the cross-section perpendicular to the longitudinal direction comprises 99% or more of ferrite with an average grain size of 70㎛ or more in area %, and the remainder comprises ferrite with an average grain size of less than 70㎛.

14. In Paragraph 11, A method for manufacturing a wire rod in which, after the above-mentioned rolling step, the wire rod comprises 70% or more of ferrite with an average grain size of 70㎛ or more in the (001) texture and (011) texture in the rolling (RD) direction based on the high angle (15°) measured by EBSD, and the remainder comprises ferrite with an average grain size of less than 70㎛.

15. In Paragraph 8, A method for manufacturing a wire rod in which, after the above-mentioned winding step, the wire rod satisfies a binding ability index of 0 or less as indicated by the following formula (2). Equation (2): TS - 0.287 × winding temperature + 0.547 × FGS - 55.56 (In Equation (2), TS represents the tensile strength of the wire and FGS represents the average grain size of the wire) 16. In wt%, containing C: 0.0030% or less (excluding 0), N: 0.003% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.05~0.15%, S: 0.015% or less (excluding 0), and the remainder being Fe and unavoidable impurities, A steel wire comprising 80% or more of ferrite with an average grain size of 70 μm or more in the (001) texture and (011) texture in the rolling (RD) direction, based on a high angle (15°) measured by EBSD, and the remainder being ferrite with an average grain size of less than 70 μm.

17. In Paragraph 16, The above steel wire is, The tensile strength is 340 MPa or less, and The yield strength is 190 MPa or less, and The number of twists is 80 or more, and Steel wire with an elongation of 40% or more.

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