Wire rod, steel wire, and manufacturing methods therefor

Abstract

An ultra-low-carbon steel wire rod according to the present invention comprises: 0.004% or less of C+N, 0.02% or less of Si, 0.1% or less of Mn, 0.005-0.015% of Ti, 0.02-0.05% of Cr, 0.02-0.06% of Mo, and the remainder of Fe and inevitable impurities, wherein assuming that the radius of the wire rod is R, TiN precipitates having an average size of 40 nm or more, among precipitates observed in a region to 0.9R from the center of a cross section perpendicular to the longitudinal direction, are included in an area fraction of 85% or more.

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, 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 secure low-strength wire rods and steel wires by suppressing solid solution strengthening by N through the addition of Ti and preventing solid solution strengthening by C through the formation of Cr and Mo clusters through the annealing heat treatment of the wire rod.

[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+N: 0.004% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.1% or less (excluding 0), Ti: 0.005~0.015%, Cr: 0.02~0.05%, Mo: 0.02~0.06%, the remainder being Fe and unavoidable impurities, and when the radius of the wire rod is R, it may contain TiN precipitates with an average size of 40 nm or more in an area fraction of 85% or more among precipitates observed in the region from the center of the cross-section perpendicular to the longitudinal direction to 0.9R.

[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 ferrite grain size of 30 μm or more observed in the region from the center of the cross-section perpendicular to the longitudinal direction up to 0.9 R.

[0012] The wire according to one embodiment of the present invention may have a tensile strength of 280 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: manufacturing a billet by maintaining a bloom containing, in weight%, C+N: 0.004% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.1% or less (excluding 0), Ti: 0.005~0.015%, Cr: 0.02~0.05%, Mo: 0.02~0.06%, the remainder being Fe and unavoidable impurities, in a steel billet heating furnace at a temperature of 1150~1200°C for at least 240 minutes; maintaining the manufactured billet in a wire rod heating furnace at a temperature of 1100~1200°C for at least 80 minutes and then hot rolling; and coiling the manufactured wire rod at a temperature of 880~950°C. and may include the step of cooling the wound wire to 300°C at a cooling rate of 5°C / s or less.

[0014] According to one embodiment of the present invention, the wire rod after the hot rolling step may contain TiN precipitates with an average size of 40 nm or more in an area fraction of 85% or more among precipitates observed in the region from the center of the cross-section perpendicular to the longitudinal direction up to 0.9 R, where the radius of the wire rod is R.

[0015] According to one embodiment of the present invention, the microstructure of the wire after the winding step may contain ferrite with an area fraction of 99% or more.

[0016] According to one embodiment of the present invention, after the winding step, the wire may have an average ferrite grain size of 30 μm or more observed in the region from the center of the cross-section perpendicular to the longitudinal direction up to 0.9 R.

[0017] According to one embodiment of the present invention, after the cooling step, the wire may have a tensile strength of 280 MPa or less.

[0018] A super-low carbon steel wire according to one embodiment of the present invention comprises, in weight%, C+N: 0.004% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.1% or less (excluding 0), Ti: 0.005~0.015%, Cr: 0.02~0.05%, Mo: 0.02~0.06%, the remainder being Fe and unavoidable impurities, and may contain Cr clusters and Mo clusters with an average size of 50~150 nm observed in the region from the center of the cross-section perpendicular to the longitudinal direction to 0.9R' when the radius of the steel wire is R', with an area fraction of 70% or more.

[0019] The steel wire according to one embodiment of the present invention may have a tensile strength of 340 MPa or less.

[0020] According to one embodiment of the present invention, the steel wire may have a yield point elongation of 1.0% or less.

[0021] A method for manufacturing an ultra-low carbon steel wire according to one embodiment of the present invention comprises the steps of: manufacturing a billet by maintaining a bloom, comprising, in weight%, C+N: 0.004% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.1% or less (excluding 0), Ti: 0.005~0.015%, Cr: 0.02~0.05%, Mo: 0.02~0.06%, the remainder being Fe and unavoidable impurities, in a steel billet heating furnace at a temperature of 1150~1200°C for at least 240 minutes; maintaining the manufactured billet in a wire rod heating furnace at a temperature of 1100~1200°C for at least 80 minutes and then hot rolling; and coiling the manufactured wire rod at a temperature of 880~950°C. The method may include the step of cooling the wound wire to 300°C at a cooling rate of 5°C / s or less; the step of drawing the cooled wire; and the step of cooling the drawn wire after annealing it at a temperature of 550 to 650°C for at least 2 hours.

[0022] According to one embodiment of the present invention, when the radius of the steel wire is denoted as R' after the cooling step following the annealing heat treatment, Cr clusters and Mo clusters with an average size of 50 to 150 nm observed in the region from the center of the cross-section perpendicular to the length direction to 0.9R' may be included in an area fraction of 70% or more.

[0023] According to one embodiment of the present invention, after the cooling step following the annealing heat treatment, the steel wire may have a tensile strength of 340 MPa or less.

[0024] According to the present invention, a wire rod, a steel wire, and a method for manufacturing the same can be provided, which suppress solid solution strengthening by N through the addition of Ti and prevent solid solution strengthening by C through the formation of Cr and Mo clusters via the annealing heat treatment of the wire rod. Furthermore, since low-strength wire rods and steel wires can be secured, ultra-low carbon steel wire rods, steel wires, and a method for manufacturing the same can be provided, which are applicable to reinforcing bars, scaffolding steel plates, mechanical structural products, etc.

[0025] 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.

[0026] FIG. 1 is a drawing showing a cross-section of a wire according to one embodiment of the present invention.

[0027] FIG. 2 is a diagram showing a method for measuring the average size of TiN precipitates in a cross-section of a wire rod according to one embodiment of the present invention.

[0028] FIG. 3 is a drawing showing an analysis area in a cross-section of a wire rod according to one embodiment of the present invention.

[0029] FIG. 4 is a drawing showing a method for measuring tensile strength using a wire rod or steel wire according to one embodiment of the present invention.

[0030] FIG. 5 is a diagram showing the relationship between the yield strength and yield point elongation of a steel wire according to one embodiment of the present invention.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] Since wire rods and steel wires used for binding, such as reinforcing bars and scaffolding steel plates, are tied by hand by workers, low strength is essential for the products. Accordingly, in order to manufacture low-strength products, the present invention aims to secure low-strength wire rods and steel wires by controlling the C+N content of the wire rod microstructure to 0.004% or less as ferrite, inducing the formation of TiN through the addition of Ti to remove interstitial elements (carbon, nitrogen) that are dissolved in the ferrite and increase strength, thereby suppressing solid solution strengthening by N, and controlling the annealing heat treatment process after drawing to induce the formation of Cr and Mo clusters within the ferrite structure, thereby suppressing solid solution strengthening by C.

[0036] The ultra-low carbon steel wire rod according to one embodiment of the present invention will be described in detail below.

[0037] In the present invention, the term "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. Furthermore, in the present invention, the term "center (OR')" of the steel wire can be interpreted as having the same meaning as the center of the wire.

[0038] 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.

[0039] A ultra-low carbon steel wire rod according to one embodiment of the present invention comprises, in weight%, C+N: 0.004% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.1% or less (excluding 0), Ti: 0.005~0.015%, Cr: 0.02~0.05%, Mo: 0.02~0.06%, the remainder being Fe and unavoidable impurities.

[0040] The reasons for limiting the compositional range of each alloying element are described below. Unless otherwise noted, units are weight percent.

[0041] The C+N content may be 0.004% or less (excluding 0).

[0042] C and N are interstitial elements that are dissolved in ferrite and significantly increase strength. Since the addition of 0.1% of both C and N improves strength to the level of 100 MPa, it is necessary to manage the total content of C+N. If the content of C+N exceeds 0.004%, it may be difficult to achieve the target strength of the present invention. Therefore, it is desirable to maintain the content of C+N at 0.004% or less, and it is desirable that the content of C+N includes at least 0.001% of C and at least 0.001% of N.

[0043] The Si content may be 0.02% or less (excluding 0).

[0044] Si is a ferrite 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 die during wire drawing, 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 0.015% or less.

[0045] The Mn content may be 0.1% or less (excluding 0).

[0046] Although Mn has a less solid solution strengthening effect compared to Si, it increases strength to the level of 10 MPa with a 0.1% increase. In other words, to lower the strength, the Mn content must be kept low. Since it may be difficult to achieve the target strength of the present invention when the Mn content exceeds 0.1%, the Mn content is maintained at 0.1% or less, preferably 0.09%.

[0047] The Ti content may be 0.005~0.015%.

[0048] The addition of Ti is necessary to form TiN for the removal of N present in the matrix structure. If the Ti content is less than 0.005%, dissolved N remains in the ferrite, making an increase in strength inevitable, and if it exceeds 0.015%, the strength may increase due to the retention of Ti in the ferrite. Therefore, it is desirable to maintain the Ti content at 0.005~0.015%, and more preferably at 0.010~0.015%.

[0049] The Cr content may be 0.02~0.05%.

[0050] Cr is an element that inhibits solid solution strengthening by C by forming Cr clusters and suppressing the diffusion of C into grain boundaries. If the Cr content is less than 0.02%, it may be difficult to achieve the target strength of the present invention, and if it exceeds 0.05%, the yield strength may increase. Therefore, it is desirable to maintain the Cr content at 0.02~0.05%, and more preferably at 0.03~0.05%.

[0051] The Mo content can be 0.02~0.06%.

[0052] Mo is an element that inhibits solid solution strengthening by C by forming Mo clusters and suppressing the diffusion of C into grain boundaries. If the Mo content is less than 0.02%, it may be difficult to achieve the target strength of the present invention, and if it exceeds 0.06%, the product price may increase significantly and the yield strength may increase. Therefore, it is desirable to maintain the Mo content at 0.02~0.06%, and more preferably at 0.03~0.06%.

[0053] 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.

[0054] 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.

[0055] 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 up to 0.9R of the wire may be 30㎛ or more, and preferably 40㎛ or more. If the grain size of the wire is excessively small, the influence of grain boundaries hindering the movement of domain walls increases, which may lead to an increase in coercivity. Therefore, it is desirable to increase the grain size of the wire to reduce the density of grain boundaries.

[0056] In one embodiment of the present invention, the ultra-low carbon steel wire may have an area fraction of 85% or more of TiN precipitates with an average size of 40 nm or more observed in the region from the center of the cross-section perpendicular to the longitudinal direction to 0.9 R (R is the radius of the wire).

[0057] The average size of the TiN precipitate is derived by measuring the TEM (transmission electron microscope) at ×100,000 magnification in the 0.9R region from the center of the cross-section perpendicular to the longitudinal direction 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.

[0058] If the size of the TiN precipitates is less than 40 nm or the area fraction is less than 85%, an increase in strength due to precipitation strengthening may occur. Therefore, it is desirable that the size of the TiN precipitates be 40 nm or larger, and that the area fraction of the precipitates be 85% or larger, particularly 87% or larger.

[0059] If the tensile 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 the purpose of the present invention. Therefore, it is preferable that the ultra-low carbon steel wire rod according to one embodiment of the present invention has a tensile strength of 280 MPa or less.

[0060] Hereinafter, a method for manufacturing an ultra-low carbon steel wire rod according to one embodiment of the present invention will be described.

[0061] A method for manufacturing an ultra-low carbon steel wire rod according to one embodiment of the present invention comprises the steps of: manufacturing a billet by maintaining a bloom containing, in weight%, C+N: 0.004% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.1% or less (excluding 0), Ti: 0.005~0.015%, Cr: 0.02~0.05%, Mo: 0.02~0.06%, the remainder being Fe and unavoidable impurities, in a steel billet heating furnace at a temperature of 1150~1200°C for at least 240 minutes; maintaining the manufactured billet in a wire rod heating furnace at a temperature of 1100~1200°C for at least 80 minutes and then hot rolling; and coiling the manufactured wire rod at a temperature of 880~950°C. and may include the step of cooling the wound wire to 300°C at a cooling rate of 5°C / s or less.

[0062] 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.

[0063] First, a bloom having the compositional components described above is prepared.

[0064] Subsequently, the bloom can be manufactured into a billet by maintaining it at a temperature of 1150 to 1200°C in a steel billet heating furnace for at least 240 minutes and then rolling it into a steel billet. If the temperature of the steel billet heating furnace is below 1150°C, operational defects may occur due to roll load during steel billet rolling, and if it exceeds 1200°C, partial re-dissolution may occur. Therefore, it is desirable to maintain the temperature of the steel billet heating furnace at 1150 to 1200°C, and more preferably at 1160 to 1180°C. A billet can be manufactured through steel billet rolling while maintaining it at the above temperature for at least 240 minutes, preferably 240 to 300 minutes.

[0065] Next, the above-mentioned billet can be maintained at 1100~1200℃ for at least 80 minutes in a wire rod heating furnace, and then hot-rolled to produce a wire rod.

[0066] For example, the above-mentioned billet can be manufactured into a wire rod of a desired size by performing hot rolling sequentially consisting of rough rolling, intermediate rough rolling / finish rolling, and finish rolling.

[0067] The above hot rolling can be performed at 1100~1200℃ for 80 minutes or more, and during finish rolling, a phase transformation from austenite to ferrite begins, the austenite completely disappears, and the microstructure can consist only of ferrite.

[0068] The above hot-rolled wire rod may undergo a winding step at 880~950℃ to reduce the strength of the wire rod through an increase in grain size and to reduce the strength variation within the coil.

[0069] Since it is necessary to reduce strength through ferrite grain growth and the binding wire manufacturer removes scale by mechanical peeling, it is desirable to maintain a high winding temperature.

[0070] If the winding temperature is below 880℃, the average grain size of the ferrite becomes small, approximately 25㎛. Additionally, if the temperature exceeds 950℃, the average grain size of the ferrite can be increased to 40㎛ or more, but winding defects such as ring indentation due to the high temperature may occur, making it difficult to manufacture the wire rod. Therefore, the above winding step is preferably performed at a temperature of 880 to 950℃, and more preferably at a temperature of 890 to 920℃.

[0071] After the above winding, a cooling step is performed.

[0072] To reduce variations in strength within the coil, it is best to proceed with cooling as slowly as possible.

[0073] The above cooling can be performed in a Stelmore cooling unit without applying a blower (air), while maintaining the conveyor speed at the lowest level and covering the unit. At this time, the cooling can be performed up to 300°C while maintaining the cooling speed at 5.0°C / s or less.

[0074] If the above cooling rate exceeds 5.0℃ / s, the tensile strength variation within the coil may increase to 90MPa or more. Therefore, it is desirable to perform the cooling at a rate of 5.0℃ / s or less, and more preferably at a rate of 3.0℃ / s or less.

[0075] In an embodiment of the present invention, the ultra-low carbon steel wire rod manufactured by the above method may have an area fraction of 85% or more of TiN precipitates with an average size of 40 nm or more among the precipitates observed in the region from the center of the cross-section perpendicular to the longitudinal direction up to 0.9 R, where the radius of the wire rod is R.

[0076] The microstructure of an ultra-low carbon steel wire rod according to one embodiment of the present invention may contain ferrite with an area fraction of 99% or more, and the average grain size of the ferrite observed in the region from the center of the cross-section perpendicular to the longitudinal direction up to 0.9R may be 30㎛ or more.

[0077] In addition, the ultra-low carbon steel wire according to one embodiment of the present invention may have a tensile strength of 280 MPa or less.

[0078] Hereinafter, an ultra-low carbon steel wire according to one embodiment of the present invention will be described.

[0079] A super low carbon steel wire according to one embodiment of the present invention may comprise, in weight%, C+N: 0.004% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.1% or less (excluding 0), Ti: 0.005~0.015%, Cr: 0.02~0.05%, Mo: 0.02~0.06%, the remainder being Fe and unavoidable impurities.

[0080] Cr clusters and Mo clusters may be formed by annealing heat treatment during the manufacture of the above ultra-low carbon steel wire. The strength of the steel wire may be increased by the formation of fine Cr and Mo carbides. Therefore, it is desirable that the above ultra-low carbon steel wire contains Cr clusters and Mo clusters with an average size of 50 to 150 nm, observed in the region from the center of the cross-section perpendicular to the longitudinal direction to 0.9R' where the radius of the steel wire is R', with an area fraction of 70% or more.

[0081] When the average size of the above Cr clusters and Mo clusters is less than 50 nm, the strength may increase, and when it exceeds 150 nm, the Cr clusters and Mo clusters present in the steel may increase the working temperature and manufacturing costs, and degrade processability. Therefore, the average size of the Cr clusters and Mo clusters is preferably 50 to 150 nm, and more preferably 80 to 150 nm.

[0082] In addition, if the area fraction of Cr clusters and Mo clusters with an average size of 50 to 150 nm is less than 70%, the solid solution strengthening effect caused by C in the ferrite structure resulting from the formation of Cr clusters and Mo clusters, which is the objective of the present invention, cannot be suppressed. Therefore, it is desirable that the area fraction of Cr clusters and Mo clusters be 70% or more, and more preferably 72% or more.

[0083] The above ultra-low carbon steel wire may have a tensile strength of 340 MPa or less, preferably 330 MPa or less, and most preferably 300 MPa or less.

[0084] If the tensile strength of the above ultra-low carbon steel wire is 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.

[0085] In addition, the ultra-low carbon steel wire of the present invention can reduce the yield point elongation to 1.0% or less.

[0086] 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, thereby reducing the occurrence of tack defects. Preferably, the yield point elongation may be 0%.

[0087] Hereinafter, a method for manufacturing an ultra-low carbon steel wire according to one embodiment of the present invention will be described.

[0088] A method for manufacturing an ultra-low carbon steel wire according to one embodiment of the present invention comprises the steps of: manufacturing a billet by maintaining a bloom containing, in weight%, C+N: 0.004% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.1% or less (excluding 0), Ti: 0.005~0.015%, Cr: 0.02~0.05%, Mo: 0.02~0.06%, the remainder being Fe and unavoidable impurities, in a steel billet heating furnace at a temperature of 1150~1200°C for at least 240 minutes; maintaining the manufactured billet in a wire rod heating furnace at a temperature of 1100~1200°C for at least 80 minutes and then hot rolling; and coiling the manufactured wire rod at a temperature of 880~950°C. The method may include the step of cooling the wound wire to 300°C at a cooling rate of 5°C / s or less; the step of drawing the cooled wire; and the step of cooling the drawn wire after annealing it at a temperature of 550 to 650°C for at least 2 hours.

[0089] The manufacturing of the above wire rod may be the same as the method for manufacturing an ultra-low carbon steel wire rod according to one embodiment of the present invention described above.

[0090] The drawing of the above wire rod can be performed by drawing the wire rod after mechanical peeling with a total true deformation amount (e) of 3.73 to 0.86, preferably 3.73. Specifically, the drawn wire rod can be cooled after annealing heat treatment at a temperature of 550 to 650°C for at least 2 hours.

[0091] After the above drawing step, a step of annealing followed by cooling may be performed. During drawing, ferrite has a fiber structure elongated in the drawing direction. Additionally, due to the high dislocation density within the ferrite, the strength is high and the ductility is very low. Therefore, the annealing process must be included.

[0092] If the annealing heat treatment temperature is below 550°C, recovery is slow, making it difficult to secure the tensile strength intended by the present invention; if it exceeds 650°C, Cr clusters and Mo clusters may not be formed. Therefore, it is preferable to control the annealing heat treatment temperature to 550–650°C, more preferably to 550–630°C, and most preferably to 580–620°C. Furthermore, if the annealing heat treatment is performed for less than 2 hours, the formation of Cr clusters and Mo clusters intended by the present invention may not be sufficient. Therefore, it is preferable to perform the annealing heat treatment for 2 hours or more.

[0093] The ultra-low carbon steel wire according to one embodiment of the present invention, manufactured by annealing heat treatment as described above, contains Cr clusters and Mo clusters with an average size of 50 to 150 nm observed in the region from the center of the cross-section perpendicular to the length direction to 0.9R' when the radius of the steel wire is R', with an area fraction of 70% or more, and may have a tensile strength of 340 MPa or less.

[0094] 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.

[0095] Examples

[0096] As shown in Table 1 below, a continuous casting bloom was produced by test casting based on 0.002(C+N)-0.02Si-0.1Mn-0.015Ti-0.05Cr-0.06Mo by weight%, and a billet was manufactured by maintaining the steel billet in a heating furnace at 1170°C for 240 minutes and then rolling the steel billet. Next, the wire rod was hot-rolled after maintaining the wire rod in a heating furnace at 1130°C for 85 minutes, and then coiled at 900°C. Subsequently, the coiled wire rod was cooled to 300°C at a cooling rate of 3°C / s to manufacture a wire rod with a diameter of 5.5 mm.

[0097] Classification C+NSiMnTiCrMo Steel Bille Heating Furnace Temperature (°C) Wire Rod Heating Furnace Temperature (°C) Winding Temperature (°C) Cooling Rate (°C / s) Wire Rod 10.00 40.0 20.10.0 150.0 50.0 61 170 1130 900 3 Wire Rod 20.00 40.0 20.10.0 150.0 50.0 61 170 1130 900 3

[0098] The ferrite grain size, tensile strength, and TiN area fraction with an average size of 40 nm or more of the wire rod prepared above are shown in Table 2 below. The ferrite grain size and TiN area fraction of the wire rod were determined from the analysis area shown in Fig. 3 within the region extending to 0.9R from the center of the cross-section perpendicular to the longitudinal direction of the wire rod. The ferrite grain size of the wire rod was measured using EBSD. 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°).

[0099] The tensile strength of the wire was measured using an Instron 8862, and the tensile speed was set to 20 mm / min. As shown in Figure 4, a wire with a diameter of 5.5 mm 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.

[0100] As shown in Fig. 2, the area fraction of TiN was measured by preparing a test specimen using the TEM precipitate extraction method (replica) and obtaining a large number of images at ×50,000 magnification in the analysis area shown in Fig. 3, thereby measuring the area fraction of TiN precipitates with an average size of 40 nm or more.

[0101] Classification Ferrite Grain Size (㎛) Tensile Strength (MPa) Area Fraction of TiN with Average Size of 40 nm or Larger (%) Wire Rod 1 3 1 3 0 2 8 6 Wire Rod 2 3 3 3 0 8 8 7

[0102] As shown in Table 2 above, it was confirmed that the microstructure of wire rods 1 and 2 manufactured in the present invention consisted entirely of ferrite, the grain size of the ferrite was 31 to 33 μm, and the tensile strength was 278 MPa or less. Using the wire rods 1 and 2 manufactured above, scale was removed by mechanical peeling from the drawn wire, and dry drawing was performed until the total true deformation (e) was 3.73 (0.85 mm). Then, as shown in Table 3 below, steel wire was manufactured by annealing heat treatment at different temperatures and then furnace cooling.

[0103] The tensile strength, yield point elongation, and area fractions of Cr clusters and Mo clusters with an average size of 80 to 150 nm of the steel wire manufactured above are shown in Table 3 below.

[0104] Tensile strength was measured using the same method as the tensile strength of the wire rod, and the occurrence of yield point elongation was measured as shown in Fig. 5 (in Fig. 5, YS represents yield strength and TS represents tensile strength).

[0105] The area fractions of Cr clusters and Mo clusters with an average size of 80–150 nm were measured using Atom probe tomography (analytical instrument) and observed in the region (500 nm × 500 nm × 500 nm) from the center of the cross-section perpendicular to the longitudinal direction of the steel wire to 0.9 R' (R' is the radius of the steel wire).

[0106] Classification Wire Rod Used Annealing Heat Treatment Temperature (°C) Annealing Heat Treatment Time (hr) Tensile Strength (MPa) Yield Point Elongation Area Fraction (%) of Cr clusters and Mo clusters with average size of 80~150 nm Example 1 Wire Rod 15 50 23 21 No occurrence 72 Example 2 Wire Rod 26 00 23 12 No occurrence 84 Comparative Example 1 Wire Rod 14 50 23 48 Occurred 27 Comparative Example 2 Wire Rod 27 00 23 45 Occurred 16

[0107] As shown in Table 3 above, Examples 1 and 2 were annealed at temperatures of 550°C and 600°C, respectively, after drawing. It was confirmed that as the annealing temperature increased, the strength decreased, and the tensile strengths of the steel wires produced in Examples 1 and 2 were 321 MPa and 312 MPa, respectively. These results can be attributed to the influence of the bonding between clusters and C resulting from the formation of Cr clusters and Mo clusters at the corresponding temperatures. Since smaller Cr clusters and Mo clusters increase strength, it was found that the effect of strength reduction was excellent when more than 70% of Cr clusters and Mo clusters with an average size of 80–150 nm were present in the region from the center of the cross-section perpendicular to the longitudinal direction of the steel wire to 0.9 R'.

[0108] On the other hand, Comparative Examples 1 and 2 were annealed at 450°C and 700°C, respectively, and showed a high tensile strength of 345 MPa or higher, which is presumed to be the result of insufficient formation of coarse-sized Cr clusters and Mo clusters. These results can also be indirectly confirmed by the presence or absence of yield point elongation; yield point elongation was observed in Comparative Examples 1 and 2, whereas yield point elongation was not observed in Examples 1 and 2.

[0109] 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+N: 0.004% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.1% or less (excluding 0), Ti: 0.005~0.015%, Cr: 0.02~0.05%, Mo: 0.02~0.06%, the remainder being Fe and unavoidable impurities, A wire rod containing TiN precipitates with an average size of 40 nm or more in an area fraction of 85% or more, among precipitates observed in the region from the center of the cross-section perpendicular to the longitudinal direction up to 0.9 R, where the radius of the wire rod is R.

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 having an average ferrite grain size of 30 μm or more observed in the region from the center of the cross-section perpendicular to the longitudinal direction up to 0.9 R.

4. In Paragraph 1, The above wire is a wire with a tensile strength of 280 MPa or less.

5. A step of manufacturing a billet by maintaining a bloom containing, in wt%, C+N: 0.004% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.1% or less (excluding 0), Ti: 0.005~0.015%, Cr: 0.02~0.05%, Mo: 0.02~0.06%, and the remainder being Fe and unavoidable impurities, in a steel billet heating furnace at a temperature of 1150~1200℃ for at least 240 minutes, and then rolling the bloom into a steel billet; A step of maintaining the above-mentioned manufactured billet in a wire rod heating furnace at a temperature of 1100~1200℃ for at least 80 minutes and then hot-rolling it; A step of winding the above-mentioned manufactured wire at a temperature of 880 to 950°C; and A method for manufacturing a wire rod comprising the step of cooling the wound wire rod to 300°C at a cooling rate of 5°C / s or less.

6. In Paragraph 5, A method for manufacturing a wire rod in which, after the above-mentioned hot rolling step, the wire rod contains TiN precipitates with an average size of 40 nm or more in an area fraction of 85% or more among precipitates observed in a region from the center of a cross-section perpendicular to the longitudinal direction up to 0.9 R, where the radius of the wire rod is R.

7. In Paragraph 5, A method for manufacturing a wire rod in which the microstructure of the wire rod after the above-mentioned winding step contains ferrite with an area fraction of 99% or more.

8. In Paragraph 5, A method for manufacturing a wire rod in which, after the above-mentioned winding step, the average ferrite grain size observed in the region from the center of the cross-section perpendicular to the longitudinal direction up to 0.9R is 30㎛ or more.

9. In Paragraph 5, A method for manufacturing a wire rod having a tensile strength of 280 MPa or less, after the cooling step above.

10. In wt%, containing C+N: 0.004% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.1% or less (excluding 0), Ti: 0.005~0.015%, Cr: 0.02~0.05%, Mo: 0.02~0.06%, and the remainder being Fe and unavoidable impurities, A steel wire containing Cr clusters and Mo clusters with an average size of 50–150 nm and an area fraction of 70% or more, observed in the region from the center of the cross-section perpendicular to the longitudinal direction to 0.9R', where R' is the radius of the steel wire.

11. In Paragraph 10, The above steel wire is a steel wire with a tensile strength of 340 MPa or less.

12. In Paragraph 10, The above steel wire is a steel wire having a yield point elongation of 1.0% or less.

13. A step of manufacturing a billet by maintaining a bloom containing, in wt%, C+N: 0.004% or less (excluding 0), Si: 0.02% or less (excluding 0), Mn: 0.1% or less (excluding 0), Ti: 0.005~0.015%, Cr: 0.02~0.05%, Mo: 0.02~0.06%, and the remainder being Fe and unavoidable impurities, in a steel billet heating furnace at a temperature of 1150~1200℃ for at least 240 minutes, and then rolling the bloom into a steel billet; A step of maintaining the above-mentioned manufactured billet in a wire rod heating furnace at a temperature of 1100~1200℃ for at least 80 minutes and then hot-rolling it; A step of winding the above-mentioned manufactured wire at a temperature of 880 to 950°C; A step of cooling the above-mentioned wound wire to 300℃ at a cooling rate of 5℃ / s or less; The step of drawing the above-mentioned cooled wire; and A method for manufacturing steel wire comprising the step of cooling the above-mentioned fresh wire after annealing heat treatment at a temperature of 550 to 650°C for at least 2 hours.

14. In Paragraph 13, A method for manufacturing a steel wire comprising Cr clusters and Mo clusters with an average size of 50 to 150 nm and an area fraction of 70% or more, observed in the region from the center of a cross-section perpendicular to the length direction to 0.9R', where R' is the radius of the steel wire after the above-mentioned annealing heat treatment and cooling step.

15. In Paragraph 13, A method for manufacturing a steel wire having a tensile strength of 340 MPa or less, after the step of cooling following the above-mentioned annealing heat treatment.

Images