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%, 0.0025% or less (excluding 0) of C, 0.020% or less (excluding 0) of Si, 0.15% or less (excluding 0) of Mn, 0.015% or less (excluding 0) of P, 0.015% or less (excluding 0) of S, and 0.0040% or less (excluding 0) of N, with the remainder comprising Fe and inevitable impurities, satisfies 0-0.001% of Ti+Nb, has a microstructure containing 99% or more of ferrite in area fraction, and has a cross-sectional diameter of 6 mm or more and less than 8 mm.

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] At work sites where various mechanical structural materials, such as reinforcing bars and scaffolding steel plates, are used, they are stored separately according to their specifications and characteristics. To increase work efficiency, it is proposed to store multiple materials with the same specifications and characteristics as a single bundle.

[0003] Accordingly, low-strength steel wires that deform easily even with minimal force applied by field workers are primarily used to conveniently bind multiple materials together at the work site.

[0004] Low-strength steel wire is typically steel wire with a tensile strength of about 400 MPa to 600 MPa. To produce such low-strength steel wire, the content of carbon and nitrogen, which have a significant solid solution strengthening effect, must be limited. To achieve this, methods such as additionally performing decarburization and denitrification processes during the steel wire manufacturing process, or adding alloying elements such as Ti and Nb to form coarse precipitates such as TiN, TiC, and Ti(C,N), are utilized.

[0005] However, as mentioned above, there is a problem in that the manufacturing cost of steel wire increases significantly when additional decarburization and denitrification processes are performed, and when alloying elements such as Ti are added to form precipitates, a problem may arise in which the strength actually increases due to the precipitation strengthening effect as fine precipitates with small sizes are formed.

[0006] Furthermore, the alloying elements added for precipitate formation are relatively expensive; when utilized in the production of low-strength steel wire, this leads to problems such as reduced price competitiveness due to high manufacturing costs.

[0007] One aspect of the present invention for solving the aforementioned problem is to provide a low-strength ultra-low carbon wire rod, a steel wire, and a method for manufacturing the same, for easily binding mechanical structural materials such as reinforcing bars at a work site.

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

[0009] To achieve the above objective, an ultra-low carbon steel wire rod according to one embodiment of the present invention comprises, in weight%, C: 0.0025% or less (excluding 0), Si: 0.020% or less (excluding 0), Mn: 0.15% or less (excluding 0), P: 0.015% or less (excluding 0), S: 0.015% or less (excluding 0), N: 0.0040% or less (excluding 0), and the remainder being Fe and unavoidable impurities, wherein Ti+Nb satisfies 0 or more and 0.001% or less, the microstructure comprises ferrite with an area fraction of 99% or more, and the cross-sectional diameter may be 6 mm or more and less than 8 mm.

[0010] According to one embodiment of the present invention, the wire may have an average grain size of ferrite of 30 μm or more.

[0011] The wire according to one embodiment of the present invention may have a tensile strength of 320 MPa or more.

[0012] 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 billet satisfying that, in weight percent, C: 0.0025% or less (excluding 0), Si: 0.020% or less (excluding 0), Mn: 0.15% or less (excluding 0), P: 0.015% or less (excluding 0), S: 0.015% or less (excluding 0), N: 0.0040% or less (excluding 0), and the remainder being Fe and unavoidable impurities, wherein Ti+Nb is 0 or more and 0.001% or less, at 1000~1180℃ for 90~200 minutes; hot rolling the heated billet at 950~1100℃ for 80 minutes or more; and coiling the wire rod at 900~980℃. and a step of cooling to 270~330℃ at a cooling rate of 1~20℃ / s after the above-mentioned winding; may be included.

[0013] According to one embodiment of the present invention, the wire may have a cross-sectional diameter of 6 mm or more and less than 8 mm.

[0014] The microstructure of the 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 may be 30㎛ or more.

[0015] A super low carbon steel wire according to one embodiment of the present invention may comprise, in weight%, C: 0.0025% or less (excluding 0), Si: 0.020% or less (excluding 0), Mn: 0.15% or less (excluding 0), P: 0.015% or less (excluding 0), S: 0.015% or less (excluding 0), N: 0.0040% or less (excluding 0), and the remainder being Fe and unavoidable impurities, wherein Ti+Nb satisfies 0 or more and 0.001% or less, and may have a tensile strength of 340 MPa or less and a total elongation of 30% or more.

[0016] The microstructure of the steel wire according to one embodiment of the present invention contains ferrite with an area fraction of 99.9% or more, and the average grain size of the ferrite may be 24㎛ or less.

[0017] A method for manufacturing an ultra-low carbon steel wire according to one embodiment of the present invention comprises the steps of: heating a billet, which satisfies the condition that Ti+Nb is 0 or more and 0.001% or less, and is composed of, in weight%, C: 0.0025% or less (excluding 0), Si: 0.020% or less (excluding 0), Mn: 0.15% or less (excluding 0), P: 0.015% or less (excluding 0), S: 0.015% or less (excluding 0), N: 0.0040% or less (excluding 0), and the remainder being Fe and unavoidable impurities, at 1000~1180℃ for 90~200 minutes; hot rolling the heated billet at 950~1100℃ for 80 minutes or more; coiling the wire rod at 900~980℃; and cooling the wire rod after coiling to 270~330℃ at a cooling rate of 1~20℃ / s. The method may include the step of producing a fresh wire by drawing the cooled wire rod with a total true deformation amount (e) of 4.19 to 4.48; and the step of producing a heat-treated wire by heat-treating the produced fresh wire at Ae3-210℃ or higher and Ae3-110℃ or lower for 1 to 3 hours and then cooling it.

[0018] The fresh treatment according to one embodiment of the present invention can be performed with a total true deformation amount (e) of 4.19 to 4.48.

[0019] According to one embodiment of the present invention, the fresh wire has a dislocation density of 34,000 μm within the ferrite. 2 It could be more than that.

[0020] According to one embodiment of the present invention, the fresh wire may have a tensile strength of 800 MPa or more.

[0021] The fresh line according to one embodiment of the present invention can satisfy the following formula (1).

[0022] Equation (1): (Tensile strength of the drawn wire) - (Tensile strength of the wire rod) ≥ 470 MPa

[0023] The heat-treated wire according to one embodiment of the present invention may have a tensile strength of 340 MPa or less.

[0024] The heat treatment line according to one embodiment of the present invention can satisfy the following equation (2).

[0025] Equation (2): (Tensile strength of heat-treated wire) - (Tensile strength of wire rod) ≤ 10 MPa

[0026] The heat treatment line according to one embodiment of the present invention can satisfy the following equation (3).

[0027] Equation (3): (Tensile strength of fresh wire) - (Tensile strength of heat-treated wire) ≥ 470 MPa

[0028] According to the present invention, the convenience of field workers can be improved by providing a steel wire that has excellent ductility while having a strength that is about 70 MPa or lower than that of a conventional low-carbon steel wire having a tensile strength of about 400 to 600 MPa.

[0029] In addition, the manufacturing process can be simplified by omitting decarburization and carbonitriding processes during the wire rod manufacturing process, and the total manufacturing cost can be effectively reduced by not adding expensive alloying elements such as titanium, thereby improving the price competitiveness of the product.

[0030] Furthermore, by controlling the cross-sectional diameter and drawing amount of the wire rod during the manufacturing process of the steel wire, the dislocation density inside the wire rod is increased, thereby reducing the heat treatment time required to lower the tensile strength of the steel wire to 340 MPa or less, which can increase the production efficiency of the steel wire.

[0031] 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.

[0032] FIG. 1 is a drawing showing a cross-section of a wire according to one embodiment of the present invention.

[0033] FIG. 2 is a drawing showing a method for measuring tensile strength using a wire rod, a fresh wire, and a heat-treated wire according to one embodiment of the present invention.

[0034] FIG. 3 is a diagram showing a method for measuring dislocation density in fresh wire ferrite according to one embodiment of the present invention.

[0035] FIG. 4 is a diagram showing the relationship between tensile strength, yield strength, and total elongation of a steel wire according to one embodiment of the present invention.

[0036] FIG. 5 is a diagram showing a cross-sectional view of the microstructure of a wire according to one embodiment of the present invention.

[0037] FIG. 6 is a cross-sectional view of the microstructure of a steel wire according to one embodiment of the present invention.

[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] Unless otherwise specifically stated in this specification, the % indicating the content of each element is based on weight.

[0043] 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.

[0044] In addition, in the present invention, "cross-section of the wire" refers to a surface cut perpendicular to the longitudinal direction of the wire, "center" refers to an area from the center to 0.5R in the cross-section of the wire, and "surface" or "surface area" refers to an area from the center to 0.9R to 1R in the cross-section of the wire.

[0045] Seonjae

[0046] First, the ultra-low carbon steel wire rod according to the present invention will be described.

[0047] An ultra-low carbon steel wire rod according to one embodiment of the present invention comprises, in weight%, C: 0.0025% or less (excluding 0), Si: 0.020% or less (excluding 0), Mn: 0.15% or less (excluding 0), P: 0.015% or less (excluding 0), S: 0.015% or less (excluding 0), N: 0.0040% or less (excluding 0), and the remainder being Fe and unavoidable impurities, wherein Ti+Nb satisfies 0 or more and 0.001% or less.

[0048] Hereinafter, the reason for the numerical limitation of the alloy component content in the embodiments of the present invention will be explained.

[0049] C (Carbon): 0.0025% or less (excluding 0)

[0050] C is an element that can most effectively increase strength, and a 0.1% increase in C results in a strength increase of 100 MPa. Since it is difficult to use the product if the C content exceeds 0.0025% because it exceeds the target strength of the present invention, it is desirable to control the C content to 0.0025% or less, more preferably to 0.0010~0.0025%, and most preferably to 0.0015~0.0020%.

[0051] Si (Silicon): 0.020% or less (excluding 0)

[0052] Silicon is a solid solution strengthening element for ferrite, typically increasing tensile strength to 14 MPa to 16 MPa when added at a concentration of 0.1%. It is known to exist in ferrite and segregate at grain boundaries due to its low solubility in cementite. As previously explained, to manufacture wire rods and steel wires with a tensile strength of 400 MPa or less, it is desirable that the silicon content not exceed 0.020 wt%. Furthermore, as the silicon content increases, compounds such as FeSiO4 (ferrosilicate) are formed, increasing the likelihood that a thick oxide film with strong bonding forces will form on the surface. In this case, difficulties arise during the mechanical peeling process to remove scale when manufacturing steel wire using the wire rod. Additionally, the remaining scale that is not completely removed causes wear on the die during drawing, so it is important to keep the silicon content as low as possible. For the reasons mentioned above, it is desirable to control the Si content to 0.020% or less, more preferably to 0.005~0.020%, and most preferably to 0.010~0.015%.

[0053] Mn (Manganese): 0.15% or less (excluding 0)

[0054] Although the solid solution strengthening effect of Mn is less than that of Si, a 0.1% increase in tensile strength increases by about 10 MPa. In the present invention, it is preferable to control the Mn content to 0.15% or less, more preferably to 0.05~0.15%, and most preferably to 0.08~0.12%.

[0055] P(Phenomenon): 0.015% or less (excluding 0)

[0056] The solid solution strengthening effect of P shows a strength increase of approximately 80 MPa for every 0.1% increase, so the strength increase effect due to solid solution strengthening is very significant. To achieve the target strength of the present invention, it is desirable to maintain the P content at 0.015% or less, and if the P content exceeds this, the risk of wire breakage during fresh processing increases due to intergranular embrittlement. Therefore, it is desirable to control the P content to 0.015% or less, more preferably to 0~0.010 wt%, and most preferably to 0~0.008 wt%.

[0057] S (Sulfur): 0.015% or less (excluding 0)

[0058] S is an element that is inevitably added during the steel manufacturing process, and it has the problem of reducing workability by forming MnS inclusions at grain boundaries. Therefore, it is desirable to control the S content to 0.015% or less, more preferably to 0 to 0.010 wt%, and most preferably to 0 to 0.008 wt%.

[0059] N (Nitrogen): 0.0040 wt% or less (excluding 0)

[0060] N is an element that increases strength significantly by fixing to dislocations during the annealing heat treatment process of low-carbon steel wire, and adding 0.1% N results in a strength improvement effect of approximately 100 MPa. If the N content exceeds 0.0040%, the recovery rate after heat treatment slows down, which can lead to an excessive increase in the tensile strength of the steel wire. Therefore, it is desirable to control the N content to 0.0040% or less, more preferably to 0.0005~0.0040%, and most preferably to 0.0010~0.0035%.

[0061] At this time, the ultra-low carbon wire according to the present invention is configured so as not to add expensive alloying elements such as titanium (Ti) and niobium (Nb) in order to limit the carbon and nitrogen content, unlike the alloy composition of conventional low-strength wires. To this end, in the alloy composition described above, the content of titanium (Ti) and niobium (Nb) is limited to 0 to 100 ppm or less, respectively.

[0062] In addition, it is desirable to limit the content of the elements so that the total sum of the titanium (Ti) and niobium (Nb) content is 0 or more and 0.001% or less, more preferably 0 or more and 0.0008% or less, and most preferably 0 or more and 0.0005% or less, thereby effectively reducing the manufacturing cost of the wire rod.

[0063] 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 conventional steel manufacturing process, they cannot be excluded. As these impurities are known to any skilled person in the conventional steel manufacturing process, all details thereof are not specifically mentioned in this specification.

[0064] An ultra-low carbon steel wire according to one embodiment of the present invention may have a tensile strength of 320 MPa or more.

[0065] 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 intended in the present invention. Accordingly, the ultra-low carbon steel wire according to one embodiment of the present invention may have a tensile strength of 320 MPa or more, preferably 320 to 400 MPa, and more preferably 340 to 380 MPa.

[0066] The microstructure of an ultra-low carbon steel wire rod according to one embodiment of the present invention may contain ferrite in an area fraction of 99% or more. It is preferable that the microstructure of the wire rod be a single-phase ferrite (a) structure. A single-phase ferrite structure means containing ferrite in an area fraction of 99% or more, or 100% of the area fraction.

[0067] 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 may be 30 μm or more, preferably 40 μm or more. If the average grain size of the ferrite 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 average grain size of the ferrite to reduce the density of grain boundaries.

[0068] In addition, the ultra-low carbon steel wire according to one embodiment of the present invention may have a cross-sectional diameter of 6 mm or more and less than 8 mm.

[0069] In the present invention, in order to manufacture low-strength wire rods and steel wires, the cross-sectional diameter (wire diameter) of the wire rod is controlled to be 6 mm or more and less than 8 mm. This is intended to effectively lower the tensile strength of the steel wire by controlling the cross-sectional diameter of the wire rod, unlike conventional methods for manufacturing low-strength steel wires which have involved adding separate alloying elements or performing decarburization / denitrification processes to control the carbon and nitrogen content. This will be described later.

[0070] steel wire

[0071] Next, an ultra-low carbon steel wire according to one embodiment of the present invention may comprise, in weight percent, C: 0.0025% or less (excluding 0), Si: 0.020% or less (excluding 0), Mn: 0.15% or less (excluding 0), P: 0.015% or less (excluding 0), S: 0.015% or less (excluding 0), N: 0.0040% or less (excluding 0), the remainder being Fe and unavoidable impurities.

[0072] Similar to the wire rod described above, in the alloy composition described above, the content of titanium (Ti) and niobium (Nb) is limited to 0 to 100 ppm or less, and at the same time, the total sum of the titanium (Ti) and niobium (Nb) content is limited to 0 or more and 0.001% or less, thereby reducing the manufacturing cost of the steel wire.

[0073] The microstructure of an ultra-low carbon steel wire according to one embodiment of the present invention may contain ferrite with an area fraction of 99.9% or more.

[0074] The above steel wire may have an average grain size of ferrite in its microstructure of 24 μm or less, preferably 10 to 24 μm, and more preferably 15 to 20 μm.

[0075] The tensile strength of the ultra-low carbon steel wire according to one embodiment of the present invention may be 340 MPa or less, preferably 330 MPa or less, and more preferably 250 to 330 MPa.

[0076] Meanwhile, the above steel wire may have a total elongation of 30% or more. The ultra-low carbon steel wire according to one embodiment of the present invention does not exhibit deterioration of ductility characteristics to a degree similar to that of conventional low-strength steel wire, which has an elongation of approximately 30%.

[0077] The ultra-low carbon steel wire of the present invention as described above not only has a tensile strength that is at least 70 MPa lower than that of conventional low-strength steel wire, but also exhibits superior physical properties compared to existing low-strength steel wires because its ductility is not degraded. Furthermore, it can effectively reduce total manufacturing costs by not adding expensive alloying elements during the manufacturing process and by omitting the decarburization / denitrification process, thereby having excellent price competitiveness.

[0078] Method for manufacturing wire rods and steel wires

[0079] Hereinafter, an example of a method for manufacturing wire rods and steel wires according to the present invention will be described. However, the method for manufacturing wire rods and steel wires according to the present invention is not necessarily limited thereto.

[0080] A method for manufacturing an ultra-low carbon steel wire rod according to an embodiment of the present invention comprises, in weight percent, a step of heating a billet satisfying that Ti+Nb is 0 or more and 0.001% or less, comprising C: 0.0025% or less (excluding 0), Si: 0.020% or less (excluding 0), Mn: 0.15% or less (excluding 0), P: 0.015% or less (excluding 0), S: 0.015% or less (excluding 0), N: 0.0040% or less (excluding 0), and the remainder being Fe and unavoidable impurities, at 1000~1180℃ for 90~200 minutes; a step of hot rolling the heated billet at 950~1100℃ for 80 minutes or more; and a step of coiling the wire rod at 900~980℃. and a step of cooling to 270 to 330℃ at a cooling rate of 1 to 20℃ / s after the above-mentioned winding; may be included.

[0081] That is, the ultra-low carbon steel wire rod according to the present invention can be manufactured by preparing a billet of the alloy composition described above and then passing it through the processes of heating and holding, wire rod rolling, coiling, and cooling.

[0082] To examine this in detail, first, the heating of the billet is a process of charging it into a heating furnace that produces wire rods and heating and maintaining it at a constant temperature, which can be performed by maintaining it at 1000~1180℃ for 90~200 minutes.

[0083] The above temperature range is an austenite single-phase region. If the temperature exceeds 1180°C, there is a risk that the austenite grains will be formed coarsely, and if it is below 1000°C, the heating and holding process may take too long, which may reduce productivity. Therefore, the heating temperature of the billet is preferably 1000 to 1180°C, more preferably 1050 to 1170°C, and most preferably 1070 to 1150°C.

[0084] In addition, if the time maintained in the above-mentioned temperature range is less than 90 minutes, residual carbons, etc. in the billet may not be sufficiently dissolved, and if the above-mentioned temperature range is maintained for a long time, productivity may be significantly reduced, so it is desirable to limit the upper limit of the heating and holding time to 200 minutes, and more preferably to 90 minutes to 120 minutes.

[0085] At this time, the alloy composition of the billet may be limited to a content of titanium (Ti) and niobium (Nb) of 0 to 100 ppm or less, and together with this, the total sum of the titanium (Ti) and niobium (Nb) contents may be 0 or more and 0.001% or less, preferably 0 or more and 0.0008% or less, and more preferably 0 or more and 0.0005% or less.

[0086] A billet heated and maintained under the above-described conditions is rolled into a wire rod to produce a wire rod with a cross-sectional diameter of 6 mm or more and less than 8 mm. Preferably, the cross-sectional diameter of the wire rod is 6.2 to 7.8 mm, and more preferably 6.5 to 7.5 mm.

[0087] The above wire rod rolling process can be performed according to a conventional hot rolling process, and preferably, it is performed at a temperature of 950 to 1100°C for 80 minutes or more.

[0088] The above hot-rolled wire rod may undergo a winding step at 900~980℃ to reduce the strength of the wire rod through an increase in grain size and to reduce the strength variation within the coil.

[0089] Since it is necessary to reduce strength through ferrite grain growth and the wire manufacturer removes scale by mechanical peeling, it is desirable to maintain a high winding temperature.

[0090] If the winding temperature is less than 900℃, the scale thickness is low at 8㎛ or less, and the average grain size of the ferrite is also small at 25㎛. In addition, if the temperature exceeds 980℃, 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 900~980℃, and more preferably at a temperature of 940~960℃.

[0091] After the above winding, a cooling step is performed.

[0092] The above cooling can be performed with the cover closed without applying a blower (air) in the Stelmore cooling zone. At this time, the cooling rate can be maintained at 1 to 20°C / s and the temperature can be maintained at 270 to 330°C until entry into the reforming tube.

[0093] If the above cooling rate is less than 1℃ / s, equipment investment becomes necessary due to the limitations of the conveyor speed, and if it exceeds 20℃ / s, the variation in tensile strength within the coil may increase. Therefore, it is desirable to perform the cooling at a speed of 1 to 20℃ / s, more preferably at 1 to 15℃ / s, and most preferably at 5 to 12℃ / s. In addition, if the cooling temperature is less than 270℃ or exceeds 330℃, jamming or sagging may occur when entering the reforming tube, causing the coil shape to become non-uniform.

[0094] The present invention can manufacture an ultra-low carbon steel wire having target properties using an ultra-low carbon steel wire produced by the manufacturing method described above.

[0095] A method for manufacturing an ultra-low carbon steel wire according to one embodiment of the present invention may include: a step of drawing the ultra-low carbon steel wire rod; and a step of heating and then cooling the drawn wire rod.

[0096] First, a wire can be produced by mechanically peeling and drawing the ultra-low carbon steel wire produced by the method described above.

[0097] The above wire has a scale layer on its surface, and a process to remove the scale from the surface of the wire can be performed using a descaler that peels off the scale before the wire passes through a wire drawing die for wire drawing.

[0098] In the present invention, in order to lower the strength under the same conditions as before without changing the heat treatment temperature and time during the manufacturing stage of ultra-low carbon steel wire, the dislocation density within the ferrite is increased, and the increase in dislocation density can be achieved by increasing the total amount of processing. However, since the final product diameter of the ultra-low carbon steel wire of the present invention is fixed, the increase in the total amount of processing can be achieved by increasing the wire rod diameter. To this end, it is desirable to process the wire rod by controlling the total true deformation amount (e) of the wire rod during the drawing process to 4.19 or more, preferably 4.19 to 4.48.

[0099] As described above, when the total true strain (e) is controlled to be 4.19 or higher, the dislocation density within the ferrite of the wire microstructure is 34,000 μm 2 This allows for securing the ideal, and subsequently, the tensile strength of the heat-treated wire can be reduced to 340 MPa or less.

[0100] The dislocation density within the ferrite of the above-mentioned fresh wire microstructure is 34,000 μm 2 Ideally, 50,000 µm 2 Ideally, 80,000 µm 2 That is the case.

[0101] When the total true deformation (e) of the wire rod during drawing is less than 4.19, it may be difficult to meet the target strength of the steel wire due to the increase in dislocation density within the ferrite. In addition, if it exceeds 4.48, the amount of deformation is too large, and chevron cracks, which are V-shaped defects, may occur inside the material during drawing, making drawing impossible.

[0102] The present invention, in manufacturing a steel wire using a wire rod with a cross-sectional diameter controlled to be 6 mm or more and less than 8 mm as described above, can effectively reduce the tensile strength of the steel wire by controlling the total true deformation of the wire rod during the drawing process to satisfy 4.19 or more, preferably 4.19 to 4.48.

[0103] The wire drawn according to the above total deformation amount may have a tensile strength of 800 MPa or more and may satisfy the following equation (1).

[0104] Equation (1): (Tensile strength of the drawn wire) - (Tensile strength of the wire rod) ≥ 470 MPa

[0105] In this way, increasing the total true strain of the wire increases the dislocation density within the wire with a limited diameter, and consequently, recrystallization proceeds rapidly during the process of manufacturing steel wire by annealing the wire, causing the internal structure to stabilize quickly and allowing for a rapid reduction in tensile strength compared to conventional low-carbon steel wire.

[0106] In addition, through the above configuration, the low-carbon steel wire according to one embodiment of the present invention can improve the price competitiveness of the product by reducing energy costs required for steel wire manufacturing and effectively reducing total manufacturing costs, by increasing the dislocation density inside the wire rod by controlling the cross-sectional diameter and drawing deformation amount of the low-carbon wire rod to a preset range and thereby reducing the strength during annealing in the steel wire manufacturing process more rapidly than in the conventional method.

[0107] After the above-mentioned fresh treatment, a step of heat-treating and cooling the fresh material can be performed. Specifically, a heat-treated wire can be manufactured by heat-treating the fresh material at Ae3-210℃ or higher and Ae3-110℃ or lower for 1 to 3 hours and then cooling it.

[0108] The process of cooling after the above heat treatment is a process for recovering and recrystallizing the dislocation of the material. If the heat treatment temperature is below Ae3-210℃, additional time is required for the recovery and recrystallization of the dislocation, which may increase manufacturing costs. If it exceeds Ae3-110℃, the recovery and recrystallization speeds are fast, but manufacturing costs are incurred due to the additional input of electricity, so it is desirable to control it to a lower temperature.

[0109] In addition, the heat treatment time is 1 to 3 hours, which is much shorter than the heat treatment time for manufacturing conventional low-carbon steel wire, so energy costs can be significantly reduced compared to existing methods, and accordingly, the manufacturing cost of steel wire can also be effectively reduced.

[0110] The heat-treated wire manufactured by cooling after heat treatment as described above can have a tensile strength of about 340 MPa or less, which is at least 70 MPa lower than conventional low-strength steel wire that typically has a tensile strength of about 400 to 600 MPa.

[0111] In addition, the above heat treatment line can satisfy the following equations (2) and (3).

[0112] Equation (2): (Tensile strength of heat-treated wire) - (Tensile strength of wire rod) ≤ 10 MPa

[0113] Equation (3): (Tensile strength of fresh wire) - (Tensile strength of heat-treated wire) ≥ 470 MPa

[0114] 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.

[0115] Examples

[0116] A billet having the alloy composition of Table 1 below was produced, and hot-rolled by holding it in a wire rod heating furnace at 1170°C for 90 minutes to produce rolled wire rods with different cross-sectional diameters of 5.5 to 8.0 mm. Next, the wire rod was coiled at a coiling temperature of 950°C, and then finished by cooling it at a cooling rate of 15°C / s in a Stelmore cooling zone with a cover and without blowing air.

[0117] Table 1 below shows the composition, cross-sectional diameter, and tensile strength for each of the wires manufactured above.

[0118] The cross-sectional diameter was measured by determining the length of the surface cut perpendicular to the length direction of the wire (wire diameter).

[0119] 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 400 mm lengths and subjected to a tensile test. To verify the yield point, a prestrain of 2% was applied to straighten the wire, and the tensile test was performed until fracture occurred.

[0120] Classification Alloy Composition (Weight%) Cross-sectional Diameter (mm) Tensile Strength (MPa) CNTiSiMnSP Example 1 0.00 250.00 400.0 20.15 0.00 50.01 56.5329 Example 2 0.00 250.00 400.0 20.15 0.00 50.01 57324 Example 3 0.00 250.00 400.0 20.15 0.00 50.0157.5327 Comparative Example 10.0020.0020.030.020.10.0050.0155.5280 Comparative Example 20.00250.00400.020.150.0050.0155.5331 Comparative Example 30.00250.00400.020.150.0050.0158325

[0121] Looking at Table 1 above, it can be seen that Examples 1 to 3 and Comparative Examples 2 and 3 have the same alloy composition, whereas Comparative Example 1 has a different C, N, and Mn composition and a different alloy composition due to the addition of Ti. When examining the cross-sectional diameters of the wire rods of each example, it was confirmed that Examples 1 to 3 have cross-sectional diameters of 6.5 mm to 7.5 mm, which correspond to a preset range, whereas Comparative Examples 1 to 3 have cross-sectional diameters of 5.5 mm and 8 mm.

[0122] In addition, when examining the tensile strength of each example, it was confirmed that Examples 1 to 3 and Comparative Examples 2 and 3 exhibited a tensile strength of 320 MPa or higher, whereas Comparative Example 1 exhibited a low tensile strength of 280 MPa due to differences in alloy element content and wire cross-sectional diameter.

[0123] After peeling off the scale from the above-mentioned wire rods, a wire was produced by drawing the wire with the total true deformation amount (e) of Table 2 below, and then heat-treated for 2 hours at Ae3-210℃ or higher and Ae3-110℃ or lower, followed by cooling to produce a heat-treated wire (steel wire).

[0124] The total true strain, tensile strength, and dislocation density in ferrite of the above-manufactured wire, and the average grain size (FGH), tensile strength, and total elongation of the heat-treated wire are shown in Table 2 below.

[0125] The tensile strength of the fresh wire and heat-treated wire was measured using the same method as for the wire rod.

[0126] The dislocation density within the ferrite of the fresh wire was analyzed using a transmission electron microscope (TEM) as shown in Fig. 3. The test specimen was prepared by joining 0.8 mm wires, punching them to a diameter of 3 mm, and polishing them to a thickness of 80 μm or less. The specimen was then electrolytically etched using an ASTM E340 aqueous solution at a voltage of 1.5 A until a hole was formed. After the hole was formed, a TEM photograph was taken at 100,000x magnification, and the dislocation density was calculated as the area fraction.

[0127] The average ferrite grain size of the heat-treated wire was measured using EBSD 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 wire rod. The specimens were polished and finished with silica gel, and cross-sectional measurements were taken at ×100 magnification (Step size: 0.2 mm, tolerance angle: 15°).

[0128] The total elongation of the heat-treated wire was measured as shown in Figure 4.

[0129] Classification Total True Strain of Wire (e) Wire Tensile Strength (MPa) Dislocation Density in Wire Ferrite (㎛) 2 ) Heat-treated wire FGH (um) Heat-treated wire Tensile strength (MPa) Total elongation (%) Example 1 4.19 80 134,150 24 327 36 Example 2 4.34 850 87,830 23 325 37 Example 3 4.48 890 83,110 22 320 36 Comparative Example 1 3.86 710 11,850 26 328 36 Comparative Example 2 3.86 750 11,870 25 42 129 Comparative Example 3 4.61 Single wire 82,990 Unmeasurable Unmeasurable Unmeasurable

[0130] As shown in Table 2 above, each wire was drawn with a different total true deformation amount, Examples 1 to 3 were processed to a degree of 4.19 to 4.48 corresponding to a preset range, and Comparative Examples 1 and 2 were processed to a degree of 3.86.

[0131] At this time, Comparative Example 3 was processed to a degree of 4.61, but a chevron crack occurred during the drawing process, resulting in a breakage. It is presumed that the breakage occurred because the ductility became insufficient due to excessive drawing.

[0132] As shown in Table 2 above, in the case of Examples 1 to 3, where the total true strain (e) of the wire was controlled to be 4.19 or higher, preferably 4.19 to 4.48, the dislocation density in the wire ferrite was 34,150 μm each. 2 , 87,830㎛ 2 , 83,110 µm 2It was confirmed that the total true strain was higher compared to Comparative Examples 1 and 2, which had a total true strain of less than 4.19. From these results, it was found that the dislocation density within the ferrite can be increased by increasing the total true strain.

[0133] Table 3 below shows the difference in tensile strength of the wire rod at each stage of the steel wire manufacturing process.

[0134] Classification Tensile Strength Difference (MPa) (Fresh Wire - Wire Rod) (Fresh Wire - Heat-Treated Wire) (Heat-Treated Wire - Wire Rod) Example 1 472474-2 Example 2 5265251 Example 3 563570-7 Comparative Example 1 43038248 Comparative Example 2 41932990 Comparative Example 3 Unmeasurable Unmeasurable Unmeasurable

[0135] Referring to Tables 1 to 3 above, it can be seen that Comparative Example 1 forms coarse precipitates such as TiN precipitates and Ti4C2S2 by adding Ti for decarburization and denitrification effects, similar to conventional low-strength steel wire. The tensile strength of the low-strength wire rod of Comparative Example 1 produced in this manner is 280 MPa, and it can be seen that the tensile strength of the drawn wire is 710 MPa, which is an increase of 430 MPa compared to the wire rod, when processed by applying a total drawing amount of 3.86 from the binding wire. In addition, after heat treatment, the tensile strength of the heat-treated wire is 328 MPa, the average ferrite grain size is 26 μm, and the total elongation is 36%, and the decrease in tensile strength due to heat treatment was confirmed to be 382 MPa.

[0136] Compared to Comparative Example 1, in the case of Examples 1 to 3 according to the present invention, the wire cross-sectional diameter was controlled to be 6.5 mm to 7.5 mm and the total true deformation during drawing was controlled to be 4.19 to 4.48. Since no nitride or precipitate elements were added in Examples 1 to 3, it was confirmed that the tensile strength of the wire was approximately 330 MPa, which is higher than that of Comparative Example 1. The tensile strength of the drawn wires of Examples 1 to 3 was 801 to 890 MPa, which is an increase of 472 MPa or more compared to the wire. When heat-treated under the same conditions as Comparative Example 1, the tensile strength of the heat-treated wire was 327 to 320 MPa, which is almost similar to Comparative Example 1, and it was confirmed that the average ferrite grain size (FGH) was 22 to 24 μm, which is slightly smaller than that of Comparative Example 1. In addition, it was confirmed that the total elongation was also 36% or more, similar to Comparative Example 1 with added Ti.

[0137] Comparative Example 2 has the same composition as Examples 1 to 3, but the wire cross-sectional diameter is 5.5 mm, so the total true deformation during drawing is 3.86, which is small, and the tensile strength of the drawn wire is 331 MPa, which is similar to Examples 1 to 3. However, the tensile strength of the heat-treated wire after heat treatment is 421 MPa, which is too high to be used as a binding wire. This appears to be the result of the tensile strength decreasing slowly even when heat treatment is performed under the same conditions, as the recovery and recrystallization rates of dislocations are slow due to the relatively low dislocation density.

[0138] Table 4 below shows the change in tensile strength of steel wire according to the heat treatment time for each wire rod under the same temperature conditions.

[0139] Classification Tensile Strength (MPa) According to Annealing Time 30 min 1 hour 1.5 hours 2 hours 3 hours 5 hours Example 3 5 9 9 3 4 1 3 2 8 3 2 0 2 4 2 1 7 8 Comparative Example 1 7 0 5 5 1 1 4 0 4 3 2 8 3 0 1 2 5 9 Comparative Example 2 7 4 6 5 8 8 4 5 3 4 2 1 3 8 5 3 2 2 Comparative Example 3 Unmeasurable Unmeasurable Unmeasurable Unmeasurable Unmeasurable

[0140] As shown in Table 4 above, it was found that Example 3 could effectively lower tensile strength with a much shorter annealing time compared to Comparative Examples 2 and 3. Figures 5 and 6 are photographs showing the microstructure of an ultra-low carbon steel wire rod and a steel wire (heat-treated wire) according to one embodiment of the present invention.

[0141] Figure 5 is a photograph showing the cross-section of the microstructure of the wire rod of Example 1, and it was confirmed that the wire rod of Example 1 contains ferrite with an average grain size of 30 μm or more in an area fraction of 99% or more. In addition, Figure 6 is a photograph showing the cross-section of the microstructure of the steel wire of Example 1, and it was confirmed that the steel wire (heat-treated wire) of Example 1 contains ferrite with an average grain size of 24 μm or less in an area fraction of 99.9% or more.

[0142] 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: 0.0025% or less (excluding 0), Si: 0.020% or less (excluding 0), Mn: 0.15% or less (excluding 0), P: 0.015% or less (excluding 0), S: 0.015% or less (excluding 0), N: 0.0040% or less (excluding 0), and the remainder being Fe and unavoidable impurities, satisfying that Ti+Nb is 0 or more and 0.001% or less, and The microstructure contains ferrite with an area fraction of 99% or more, and Wire with a cross-sectional diameter of 6 mm or more and less than 8 mm.

2. In Paragraph 1, The above wire is a wire having an average grain size of ferrite of 30㎛ or more.

3. In Paragraph 1, The above wire is a wire with a tensile strength of 320 MPa or higher.

4. A step of heating a billet, comprising, in weight%, C: 0.0025% or less (excluding 0), Si: 0.020% or less (excluding 0), Mn: 0.15% or less (excluding 0), P: 0.015% or less (excluding 0), S: 0.015% or less (excluding 0), N: 0.0040% or less (excluding 0), and the remainder being Fe and unavoidable impurities, satisfying the condition that Ti+Nb is 0 or more and 0.001% or less, at 1000~1180℃ for 90~200 minutes; A step of hot rolling the above heated billet at 950~1100℃ for 80 minutes or more; A step of winding the above wire at 900~980℃; and A method for manufacturing a wire rod comprising the step of cooling to 270~330℃ at a cooling rate of 1~20℃ / s after the above-mentioned winding.

5. In Paragraph 4, The above wire is a method for manufacturing a wire with a cross-sectional diameter of 6 mm or more and less than 8 mm.

6. In Paragraph 4, A method for manufacturing a wire rod in which the microstructure of the wire rod comprises ferrite with an area fraction of 99% or more and the average grain size of the ferrite is 30㎛ or more.

7. In wt%, C: 0.0025% or less (excluding 0), Si: 0.020% or less (excluding 0), Mn: 0.15% or less (excluding 0), P: 0.015% or less (excluding 0), S: 0.015% or less (excluding 0), N: 0.0040% or less (excluding 0), and the remainder being Fe and unavoidable impurities, satisfying that Ti+Nb is 0 or more and 0.001% or less, and The tensile strength is 340 MPa or less, and Ultra-low carbon steel wire with a total elongation of 30% or more.

8. In Paragraph 7, The microstructure of the above steel wire contains ferrite with an area fraction of 99.9% or more, and the average grain size of the ferrite is 24㎛ or less.

9. A step of heating a billet comprising, in wt%, C: 0.0025% or less (excluding 0), Si: 0.020% or less (excluding 0), Mn: 0.15% or less (excluding 0), P: 0.015% or less (excluding 0), S: 0.015% or less (excluding 0), N: 0.0040% or less (excluding 0), and the remainder being Fe and unavoidable impurities, satisfying the condition that Ti+Nb is 0 or more and 0.001% or less, at 1000~1180℃ for 90~200 minutes; A step of hot rolling the above heated billet at 950~1100℃ for 80 minutes or more; A step of winding the above wire at 900~980℃; A step of cooling to 270~330℃ at a cooling rate of 1~20℃ / s after the above-mentioned winding; A step of manufacturing a fresh wire by drawing the above-mentioned cooled wire with a total true deformation amount (e) of 4.19 to 4.48; and A method for manufacturing a steel wire comprising the step of manufacturing a heat-treated wire by heat-treating the above-mentioned fresh wire at Ae3-210℃ or higher and Ae3-110℃ or lower for 1 to 3 hours and then cooling it.

10. In Paragraph 9, A method for manufacturing steel wire in which the above-mentioned fresh treatment is performed with a total true deformation amount (e) of 4.19 to 4.

48.

11. In Paragraph 10, The above-mentioned fresh wire has a dislocation density of 34,000 μm within the ferrite. 2 Method for manufacturing steel wire with an ideal profile.

12. In Paragraph 9, The above-mentioned wire is a method for manufacturing steel wire having a tensile strength of 800 MPa or more.

13. In Paragraph 9, The above-mentioned wire is a method for manufacturing a steel wire that satisfies the following formula (1). Equation (1): (Tensile strength of the drawn wire) - (Tensile strength of the wire rod) ≥ 470 MPa 14. In Paragraph 9, The above heat-treated wire is a method for manufacturing steel wire having a tensile strength of 340 MPa or less.

15. In Paragraph 9, The above heat-treated wire is a method for manufacturing steel wire that satisfies the following equation (2). Equation (2): (Tensile strength of heat-treated wire) - (Tensile strength of wire rod) ≤ 10 MPa 16. In Paragraph 9, The above heat-treated wire is a method for manufacturing steel wire that satisfies the following equation (3). Equation (3): (Tensile strength of fresh wire) - (Tensile strength of heat-treated wire) ≥ 470 MPa

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