Wire and manufacturing method therefor

Abstract

A wire according to an embodiment of the present invention comprises, based on wt%, 0.60-1.00% of C, 0.1-0.4 of Si, and 0.2-0.6% of Mn, and the remainder of Fe and other impurities, in which a microstructure comprises, by area fraction, 98% or more of pearlite and 2% or less of proeutectoid cementite, and an average lamellar spacing may be 0.230-0.350 ㎛.

Description

Wire rod and its manufacturing method

[0001] The present invention relates to a wire rod and a method for manufacturing the same.

[0002] Steel cords can be used to manufacture steel cords used as tire reinforcement. Steel cords are typically made of high-carbon steel with a carbon content ranging from 0.6 to 1.0% by weight. In addition to carbon, typical steels contain manganese and silicon, and in some cases, small amounts of chromium.

[0003] In order to reduce the isothermal transformation heat treatment from two to one in the conventional steel cord manufacturing process, the lamellar spacing must be widened as much as possible to lower the tensile strength, increase the amount of work hardening that can be accommodated, and keep the work hardening rate low. The lamellar spacing of pearlite is determined by the transformation temperature. The higher the transformation temperature, the coarser the lamellar spacing, lowering the tensile strength, and the lower the transformation temperature, the finer the lamellar spacing, increasing the tensile strength. The microstructure is controlled through continuous cooling after rolling of the wire rod. In this case, since pearlite is generated at various temperatures by continuous cooling, the lamellar spacing of the pearlite is not constant, and the mechanical properties also vary accordingly.

[0004] The present invention aims to provide a wire rod and a method for manufacturing the same, which can reduce the cost of manufacturing steel cord and reduce carbon dioxide emissions due to heat treatment by shortening the isothermal transformation heat treatment, which is usually applied twice, to one time in order to overcome the limitations of processability due to fresh processing in manufacturing steel cord used as automobile tire reinforcement.

[0005] According to one embodiment of the present invention, a wire includes, in weight %, C: 0.60 to 1.00%, Si: 0.10 to 0.40%, and Mn: 0.20 to 0.60%, the remainder being Fe and other impurities, and a microstructure includes, in area fraction, pearlite of 98% or more and proeutectoid cementite of 2% or less, and an average lamellar spacing of 0.230 to 0.350 μm.

[0006] In addition, the tensile strength (TS) of the wire according to one embodiment of the present invention can be controlled according to the value of the following formula:

[0007] TS(MPa)≤69.1+1161.0 x [Ceq]

[0008] Here, [Ceq]=[C]+[Mn] / 6+[Si] / 24, and [C], [Mn], and [Si] represent the weight% of each element.

[0009] In addition, according to one embodiment of the present invention, the wire having a diameter of 1.30 to 5.50 mm may not experience delamination in a range of total strain exceeding 0 and less than 3.

[0010] A method for manufacturing a wire rod according to one embodiment of the present invention comprises the steps of: heating a billet containing, in wt%, C: 0.60 to 1.00%, Si: 0.10 to 0.40%, and Mn: 0.20 to 0.60%, with the remainder being Fe and other impurities, at 1000 to 1250°C; hot-rolling the billet at a temperature of 1000 to 1100°C to prepare a wire rod; coiling the wire rod at a temperature range of 750 to 950°C; and the coiled wire rod having a minimum temperature (T) at the time of pearlite transformation according to a carbon equivalent (Ceq). low ) is T low Controlled to (℃)≥702.3-76.0x[Ceq], and the highest temperature (T high ) is T high (℃)≥715.3-44.9x[Ceq].

[0011] Here, [Ceq]=[C]+[Mn] / 6+[Si] / 24, and [C], [Mn], and [Si] represent the weight% of each element.

[0012] In addition, the method for manufacturing a wire according to one embodiment of the present invention can control the pearlite transformation time to be 12 to 24 seconds.

[0013] In addition, in the method for manufacturing a wire according to one embodiment of the present invention, the average lamellar spacing during the pearlite transformation may be 0.230 to 0.350 μm.

[0014] In addition, in a method for manufacturing a wire according to one embodiment of the present invention, the wire can be subjected to a constant temperature transformation heat treatment once after being fresh processed.

[0015] In addition, in the method for manufacturing a wire according to one embodiment of the present invention, the tensile strength (TS) of the wire during the pearlite transformation can be controlled according to the value of the following formula:

[0016] TS(MPa)≤69.1+1161.0 x [Ceq]

[0017] Here, [Ceq]=[C]+[Mn] / 6+[Si] / 24, and [C], [Mn], and [Si] represent the weight% of each element.

[0018] In an example of the present invention, delamination may not occur in a range of total strain exceeding 0 and less than 3.

[0019] In addition, the wire rod according to an example of the present invention is subjected to a greater amount of fresh processing (strain ≥2.88) than when performing two conventional isothermal transformation heat treatments, so that the isothermal transformation heat treatment in the subsequent fresh processing can be shortened to one time.

[0020] Figure 1 shows the behavior of the wire over time during stelmore cooling.

[0021] Figure 2 shows the minimum temperature according to the carbon equivalent of the experimental example and the comparative material.

[0022] Figure 3 shows the maximum temperature according to the carbon equivalent of the experimental example and the comparative material.

[0023] Figure 4 shows the transformation time according to the carbon equivalent of the experimental example and the comparative material.

[0024] Figure 5 shows the tensile strength of the wire according to the carbon equivalent of the experimental example and the comparative material.

[0025] Hereinafter, embodiments of the disclosed invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided to sufficiently convey the spirit of the disclosed invention to those skilled in the art. The disclosed invention is not limited to the embodiments presented herein and may be embodied in other forms. To clarify the disclosed invention, the drawings may omit portions irrelevant to the description, and the sizes of components may be slightly exaggerated to facilitate understanding.

[0026] Throughout the specification, whenever a part is said to "include" a component, this does not mean that it excludes other components, but rather that it may include other components, unless otherwise stated.

[0027] Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0028] Hereinafter, the reasons for numerical limitations on the alloy component content in the embodiments of the present invention will be described. Hereinafter, unless otherwise specified, the unit is weight percent.

[0029] The wire according to the present invention contains, in wt%, C: 0.60 to 1.00%, Si: 0.10 to 0.40%, and Mn: 0.20 to 0.60%, with the remainder being Fe and other impurities.

[0030] Below, the reasons for limiting the above alloy composition are explained in detail. Unless otherwise specified, the following component compositions all refer to weight percent.

[0031] The C (carbon) content can be 0.60 to 1.00%.

[0032] C is a key alloying element that constitutes cementite during pearlite transformation, and the lamellar spacing and pearlite equilibrium transformation temperature change depending on the C content. If the carbon content is less than 0.60%, the proeutectoid ferrite fraction increases, making it difficult to secure wire workability. Conversely, if it exceeds 1.00%, the formation and segregation of proeutectoid cementite phases may damage the structural integrity of the core of the wire, which may also result in poor wire workability. Preferably, it may be 0.65 to 0.90%.

[0033] The content of Si (silicon) can be 0.10 to 0.40%.

[0034] Si is mostly dissolved in ferrite during pearlite transformation, and is hardly distributed in cementite, and its diffusion rate is slower than that of carbon. Accordingly, if a large amount of Si is dissolved, the pearlite transformation becomes slow overall. If it is an element that causes a solid solution strengthening effect when dissolved in ferrite, if it is less than 0.10%, the solid solution strengthening effect is insufficient, making it difficult to achieve high strength in the final product, and if it exceeds 0.40%, the ferrite is excessively hardened, reducing the drawability. It may be preferably 0.10 to 0.30%. It may be more preferably 0.10 to 0.25%.

[0035] The content of Mn (manganese) can be 0.20 to 0.60%.

[0036] Mn does not affect the tensile strength of pearlite during isothermal transformation, but in the case of continuous cooling, it can affect the tensile strength change because it can change the transformation initiation temperature by changing the hardenability of the steel under limited cooling capacity. If it is less than 0.20%, it is difficult to expect an appropriate level of hardenability improvement effect, and if it exceeds 0.60%, since it is a high-carbon steel, it segregates in the center together with C, increasing the risk of martensite defects occurring in the center. It can be preferably 0.30 to 0.50%.

[0037] The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment can inevitably be mixed in during the typical manufacturing process, this cannot be ruled out. Since these impurities are readily apparent to anyone skilled in the art of typical manufacturing, their full details are not specifically discussed in this specification.

[0038] According to an example of the present invention, a wire rod may have a microstructure including, in terms of area fraction, 98% or more of pearlite, 98% of proeutectoid cement, and 2% or less of proeutectoid cement. Pearlite is a basic microstructure for drawing, and since colonies rotate in the drawing direction during drawing, it has a form similar to a fibrous reinforcing structure in the form of ferrite and cementite, and drawing workability and an appropriate work hardening rate can be secured during drawing. If pearlite is less than 98%, stress may be concentrated in a non-pearlite portion with different mechanical properties during drawing, which may cause cracks. In particular, if proeutectoid cementite exceeds 2%, proeutectoid cementite may coarsely precipitate at the old austenite grain boundaries, thereby making drawing workability vulnerable.

[0039] According to an example of the present invention, the wire may have an average lamella spacing of 0.230 to 0.350 μm. If the lamella spacing is coarsened to 0.230 μm or more, the amount of work hardening during drawing processing decreases, thereby reducing the tensile strength that can ultimately be achieved.

[0040] According to an example of the present invention, the wire may have an average lamellar spacing of 0.230 to 0.350 μm. More preferably, it may be 0.240 to 0.340 μm. When the average lamellar spacing exceeds 0.350 μm, the number of dislocations piled up at the ferrite / cementite interface becomes excessive, which increases pressure and makes it easy for fracture to occur.

[0041] Therefore, it is recommended to control it within 0.350㎛ because the fresh processing limit is rather reduced. On the other hand, if the average lamellar spacing is smaller than 0.230㎛, the strength of pearlite increases and the amount of work hardening increases, so that excessive load is applied during fresh processing, which also reduces the fresh processing limit. Therefore, when the average lamellar spacing is controlled within the above range according to the present invention, the total amount of fresh processing area that can be imparted to the wire rod can be maximized, and it is possible to manufacture the final steel cord from the wire rod with only one isothermal transformation heat treatment.

[0042] According to one example of the present invention, the tensile strength (TS) of the wire can be controlled according to the value of the following formula.

[0043] TS(MPa)≤69.1+1161.0 x [Ceq]

[0044] Here, [Ceq] = [C] + [Mn] / 6 + [Si] / 24, and [C], [Mn], and [Si] represent the weight percent of each element. By controlling the tensile strength (TS), the fresh processability can be improved at an appropriate tensile strength level.

[0045] According to an example of the present invention, a wire having a diameter of 1.30 to 5.50 mm may not experience delamination in a range of a total strain of more than 0 and less than 3.

[0046] Hereinafter, a method for manufacturing a wire according to the present invention will be described in detail.

[0047] A method for manufacturing a wire rod according to an example of the present invention comprises the steps of: heating a billet containing, in wt%, C: 0.60 to 1.00%, Si: 0.10 to 0.40%, and Mn: 0.20 to 0.60%, with the remainder being Fe and other impurities, at 1000 to 1250°C; hot-rolling the billet at a temperature of 1000 to 1100°C to prepare a wire rod; coiling the wire rod at a temperature range of 750 to 950°C; and the coiled wire rod has a minimum temperature (T) at the time of pearlite transformation according to a carbon equivalent (Ceq). low ) is T low Controlled to (℃)≥702.3-76.0x[Ceq], and the highest temperature (T high ) is T high (℃)≥715.3-44.9x[Ceq].

[0048] Here, [Ceq] = [C] + [Mn] / 6 + [Si] / 24, and [C], [Mn], and [Si] represent the weight percent of each element. Each step is described in detail below.

[0049] Step of heating the billet

[0050] The above heating step can be performed at a temperature of 1000°C to 1250°C for 5 hours or more, taking carbon diffusion into account. If the temperature is below 1000°C, the time required for carbon diffusion increases, resulting in reduced productivity. On the other hand, if the temperature exceeds 1250°C, the time required for carbon diffusion decreases, but the excessively high temperature causes scale to thicken, resulting in significant loss, and a sharp increase in energy consumption, which leads to increased productivity and manufacturing costs.

[0051] Step of hot rolling billets to produce wire rods

[0052] As described above, the prepared billet can be subjected to wire rolling and then cooled to produce wire. At this time, the temperature during the wire rolling is preferably 1000 to 1100°C. If the temperature is lower than 1000°C, flaws may occur on the wire surface due to the rolling load, making subsequent drawing difficult. On the other hand, if the temperature exceeds 1100°C, the size of the austenite crystal grains may increase, potentially reducing strength and ductility. After hot rolling, it is preferable to cool the billet to the temperature range required for the subsequent coiling process using a conventional water-cooling method. The diameter of the wire to be produced may, for example, be 1.30 to 5.50 mm.

[0053] Steps to wind the wire

[0054] Scale removal for the production of eco-friendly products is accomplished through mechanical exfoliation. The appropriate scale thickness varies depending on the mechanical exfoliation method, but is approximately 10 um on average. If the scale thickness is less than this, mechanical exfoliation is insufficient and it remains on the wire surface, causing dies to break during drawing, resulting in defects such as surface blemishes. If the scale thickness is too thick, scale loss increases, reducing productivity. Scale thickness varies with the coiling temperature. If the coiling temperature is below 750°C, the scale thickness is small, and if it exceeds 950°C, the scale thickness becomes excessively thick. Therefore, it is recommended to control the temperature between 750 and 950°C.

[0055] The minimum temperature (T) at which the above coiled wire undergoes pearlite transformation low ) and maximum temperature (T high ) control step

[0056] The above coiled wire has the lowest temperature T at which pearlite is transformed according to the carbon equivalent (Ceq). low is T low Controlled to (℃)≥702.3-76.0x[Ceq], and the highest temperature T during pearlite transformation high is T high(℃)≥715.3-44.9x[Ceq] can be controlled. Here, [Ceq]=[C]+[Mn] / 6+[Si] / 24, and [C], [Mn], and [Si] represent the weight% of each element.

[0057] Figure 1 shows the temperature changes measured in high-carbon steel wire during conventional stelmor cooling. After coiling, the temperature of the wire continuously decreases during air cooling, and then increases as the transformation from austenite to pearlite begins. This occurs due to the release of energy accompanying the transformation, as the transformation from austenite to pearlite is an exothermic reaction. The mechanical properties of high-carbon wire with pearlite structure are determined by the actual transformation temperature, but the actual pearlite transformation initiation temperature cannot be controlled during the wire production process. After the initiation of pearlite transformation, the temperature of the wire decreases and then increases with a slight delay, and the minimum temperature (T) at this time is low ) to T low By controlling to (℃)≥702.3-76.0x[Ceq], the overall transformation can be prevented from occurring at too low a temperature.

[0058] In addition, the pearlite transformation completion temperature must be controlled, but like the transformation initiation temperature, the exact transformation completion temperature cannot be controlled during the process. However, when the temperature of the wire increases due to the transformation heat generation and the heat energy supply due to the transformation heat generation and the heat energy release due to the external cooling reach an equilibrium, the temperature of the wire decreases again to the maximum temperature (T). high ) to T high (℃)≥715.3-44.9x[Ceq] can be controlled, and the transformation completion temperature can also be controlled through this.

[0059] In addition, in order to increase the amount of fresh reduction, the tensile strength must be lowered to lower the work hardening rate. However, while increasing the transformation temperature can lower the tensile strength, proeutectoid cementite may be generated during the continuous cooling process. In addition, since the risk of proeutectoid cementite generation increases at the same temperature as the carbon equivalent increases, the generation of proeutectoid cementite can be suppressed by controlling the minimum and maximum temperatures. If the temperature is lower than the above-mentioned range, the tensile strength increases, reducing the total amount of fresh reduction that can be granted.

[0060] Therefore, in order to maximize the average lamella spacing in continuous cooling while suppressing the occurrence of the proeutectoid cementite phase, the lowest temperature T during pearlite transformation is low is T low Control at (℃)≥702.3-76.0x[Ceq], and T high It is desirable to control it to (℃)≥715.3-44.9x[Ceq].

[0061] In addition, the method for manufacturing a wire rod according to one embodiment of the present invention may have a pearlite transformation time of 12 to 24 seconds. If cooling is controlled so that the pearlite transformation time is less than 12 seconds, the overall microstructure becomes finer, which increases the tensile strength and thus the work hardening rate. Furthermore, if the transformation time exceeds 24 seconds, the process time becomes longer, which reduces productivity. Therefore, it is preferable to control the transformation time to 24 seconds or less.

[0062] In addition, in the method for manufacturing a wire according to one embodiment of the present invention, the average lamellar spacing during the pearlite transformation may be 0.230 to 0.350 μm.

[0063] The mechanical properties of pearlite are governed by the average lamellar spacing, which in turn is influenced by the temperature at which pearlite transformation occurs. Lowering the transformation temperature leads to a finer average lamellar spacing and increased tensile strength, while increasing the average lamellar spacing leads to decreased tensile strength. Accordingly, by controlling the maximum and minimum temperatures, the average lamellar spacing can be controlled to a range of 0.230 to 0.350 μm.

[0064] In addition, in a method for manufacturing a wire according to one embodiment of the present invention, the wire can be subjected to a constant temperature transformation heat treatment once after fresh processing.

[0065] In addition, in the method for manufacturing a wire according to one embodiment of the present invention, the tensile strength (TS) of the wire during the pearlite transformation can be determined according to the value of the following formula.

[0066] TS(MPa)≤69.1+1161.0 x [Ceq]

[0067] Here, [Ceq]=[C]+[Mn] / 6+[Si] / 24, and [C], [Mn], and [Si] represent the weight% of each element.

[0068] Hereinafter, the present invention will be described in more detail through examples. However, the description of these examples is merely intended to illustrate the implementation of the present invention and is not intended to limit the present invention. This is because the scope of the present invention is determined by the matters set forth in the claims and matters reasonably inferred therefrom.

[0069] {Example}

[0070] In the present invention, a steel having a composition as shown in Table 1 below was produced in a converter, and then cast into a bloom, and then either rolled into a billet or directly cast into a continuous casting billet to produce a billet with a cross-sectional area of ​​160x160㎟. Afterwards, the billet was maintained at a temperature of around 1100℃ for 90 minutes in a heating furnace, and rolled to a wire diameter of 5.5㎜ through wire rod rolling under typical conditions. The coiling temperature was controlled variously from 750 to 950℃, and the minimum temperature was set to T by adjusting the airflow in the stelmor cooling according to the carbon equivalent (Ceq). low Controlled to (℃)≥702.3-76.0x[Ceq], and the highest temperature is T high (℃)≥715.3-44.9x[Ceq] was controlled. In addition, the pearlite transformation time was controlled to within 12 to 24 seconds.

[0071] The alloy composition and carbon equivalent [Ceq] of the steel used in the present invention are indicated in Table 1 below.

[0072] Classification Alloy composition (wt.%) Carbon equivalent [Ceq] CSiMn Experimental example 10.710.100.400.78 Experimental example 20.730.150.410.80 Experimental example 30.720.20.430.80 Experimental example 40.700.210.460.79 Experimental example 50.690.230.490.78 Experimental example 60.680.250.480.77 Experimental example 70.800.130.440.88 Experimental example 80.810.160.450.89 Experimental example 90.830.200.420.91 Experimental example 100.820.230.400.90 Experimental example 110.810.240.410.89 Experimental example 120.830.250.420.91 Experimental example 130.850.220.450.93 Experimental example 140.860.250.440.94 Experimental example 150.860.240.420.94 Experimental example 160.870.210.410.95 Experimental example 170.880.240.400.96 Experimental example 180.850.160.430.93 Comparative material 10.730.20.450.81 Comparative material 20.700.20.450.78 Comparative material 30.730.20.550.83 Comparative material 40.810.20.450.89Comparative material 50.830.20.450.91Comparative material 60.840.20.490.93Comparative material 70.850.20.490.94Comparative material 80.860.20.550.96Comparative material 90.860.20.450.94Comparative material 100.870.20.450.95

[0073] [Ceq]=[C]+[Mn] / 6+[Si] / 24 Tables 2 and 3 below show the minimum temperature, maximum temperature, transformation time, average Raella spacing, and tensile strength during cooling of the wire rod, along with those of the comparative material.

[0074] Classification Coiling temperature (℃) Minimum temperature (℃) Maximum temperature (℃) Transformation time (sec.) Average lamella spacing (㎛) Tensile strength (MPa) Experimental example 1780.0645.6686.4140.292914.7 Experimental example 2810.0648.0686.4160.291934.7 Experimental example 3840.0650.4686.4170.303903.3 Experimental example 4870.0655.2688.8180.318854.3 Experimental example 5900.0657.6688.8190.323835.3 Experimental example 6930.0660.0686.2200.333806.4 Experimental example 7780.0645.6681.6160.2581056.2Experimental example 8810.0645.2679.2170.2621058.4Experimental example 9840.0643.2679.2190.2711061.9Experimental example 10870.0655.2684.0190.2881022.9Experimental example 11900.0657.6684.0200.295999.8Experimental example 12930.0657.6681.6210.2881031.3Experimental example 13780.0640.8679.2170.2401128.3Experimental example Experimental Example 14810.0643.2679.2180.2471127 Experimental Example 15840.0643.2681.6190.2611106 Experimental Example 16870.0648.0681.6200.2691102 Experimental Example 17900.0655.2679.2210.2711108 Experimental Example 18930.0652.8681.6210.2791066

[0075] Classification Coiling temperature (℃) Minimum temperature (℃) Maximum temperature (℃) Transformation time (sec.) Average lamella spacing (㎛) Tensile strength (MPa) Comparative material 1910.0631.2674.470.2171042.7 Comparative material 2910.0636.0674.470.2251010.5 Comparative material 3910.0628.8669.680.2061049.6 Comparative material 4910.0631.2667.280.1971136.3 Comparative material 5910.0628.8667.280.1921153.5 Comparative material 6910.0624.0664.880.1841168.6 Comparative material 7910.0626.4662.480.1831169.0Comparative material 8910.0612.2657.680.1601211.1Comparative material 9890.0624.0662.480.1741200.3Comparative material 10830.0621.6660.680.1751200.1

[0076] Table 2 above shows the coiling temperature, minimum temperature, maximum temperature, transformation time, average lamella spacing, and tensile strength of experimental examples and comparative materials according to the control of the cooling rate after rolling of the wire rod. The lamella spacing stipulated in the present invention can be defined as “(lamella cementite thickness + lamella ferrite thickness)”, and can be specifically measured by the following method. After polishing the C-section of the steel wire or wire rod, it is etched to reveal pearlite. Then, using a scanning electron microscope (SEM), 10 fields of view are taken at 1 / 2 points in the surface direction from the center of the C-section to obtain a tissue photograph of the sample. At this time, the magnification is 1,000 times. Among the tissue photographs of the sample, 5 points are selected from the range in the field of view where the lamella directions are aligned, where 5 lamella spacings can be measured. For the five selected points, draw a straight line perpendicular to the lamella and calculate the length of the 5-interval lamella. Select the two points with the smallest 5-interval lamella lengths measured at the five selected points, and divide the measured 5-interval lengths by 5 to calculate the lamella spacing. In this way, two lamella spacings can be calculated from one tissue photograph, and 20 lamella spacings can be calculated from a total of 10 photographs, and then the average lamella spacing can be measured by taking the arithmetic mean.

[0077] In addition, the tensile strength of the wire was measured by cutting the wire to a length of 40 cm and mounting it on a standard tensile testing device. Since the wire cannot be processed into a separate gauge section in its original state, the wire was gripped 5 cm from each end in its original state, and a 30 cm specimen was pulled between the upper and lower grips under displacement control at a speed of 10 mm / min. for measurement.

[0078] Referring to Tables 2 and 3 above, it can be confirmed that the tensile strength of the experimental examples is controlled lower than that of the comparative material at the same carbon equivalent, depending on the differences in minimum temperature, maximum temperature, and transformation time between the experimental examples and the comparative material. To compare this more clearly, Fig. 2 shows the difference in minimum temperature according to carbon equivalent between the experimental examples and the comparative material, Fig. 3 shows the maximum temperature, and Fig. 4 shows the transformation time.

[0079] In particular, for Comparative Materials 1 to 10, the transformation time is less than 12 seconds, which means that the transformation proceeds at a temperature close to the nose of the pearlite phase transformation, and since the lamella spacing becomes fine, the tensile strength increases and the work hardening rate also appears high.

[0080] Specifically, referring to Fig. 2, the allowable range of the minimum temperature decreases as the carbon equivalent increases. This requires a faster cooling rate to suppress the formation of proeutectoid cementite due to the increase in carbon equivalent, and accordingly, the airflow must be increased. This phenomenon occurs because the cooling rate is large until the temperature of the wire rises again after the initiation of transformation. It can be seen that the minimum temperature of the experimental example is controlled to be significantly higher than that of the comparative materials at the same carbon equivalent level, and the reference temperature (T) indicated by the dotted line low It can be confirmed that it was controlled to (℃)≥702.3-76.0x[Ceq] or higher.

[0081] In addition, as shown in Fig. 3, the maximum temperature according to the carbon equivalent of the experimental example and the comparative material is shown. As in Fig. 2, as the carbon equivalent increases, more cooling is required to suppress the formation of proeutectoid cementite. Accordingly, the maximum temperature decreases. However, in the case of the experimental example, the maximum temperature was controlled (T) significantly higher at the same carbon equivalent level compared to the comparative material. high It can be confirmed that (℃)≥715.3-44.9xCeq).

[0082] Figure 4 shows the transformation times for the experimental and comparative materials. While the experimental materials are controlled to be within 12 seconds ≤ transformation time ≤ 24 seconds, the comparative materials are shown to be completely transformed in less than 12 seconds. The rapid completion of transformation in less than 12 seconds indicates that the transformation energy of the austenite-to-pearlite transformation, which is characterized by an exothermic reaction, was rapidly dissipated, indicating that cooling occurred rapidly due to the airflow during the transformation. In this case, the overall interlamellar spacing becomes finer and the tensile strength increases, making it difficult to secure a high draw reduction.

[0083] Figure 5 shows the tensile strength of the experimental examples and comparative materials. It can be seen that the tensile strength of experimental examples 1 to 18 according to the present invention is maintained significantly lower than that of comparative materials having the same carbon equivalent. When a lower tensile strength is secured at a certain carbon equivalent based on the indicated dotted line (TS = 69.1 + 1161.0 x [Ceq]), a high elongation reduction rate can be secured.

[0084] The experimental examples and comparative materials above were measured by controlling the strain per pass to 0.2 using a draw bench. The limiting freshness reduction was determined by whether or not delamination occurred through a torsion test. If no delamination occurred in three tests, it was determined as passing, and if delamination occurred even once, it was determined as failing. Table 4 below shows the change in wire diameter and the applied strain according to the drawing process, and shows the limiting strain for delamination in the torsion test after drawing. If no delamination occurred, it was marked as X, and the materials in which delamination occurred according to the accumulated strain were described.

[0085] Fresh pass diameter (mm) Cumulative deformation Delamination occurrence Material 05.500X14.980.2X24.500.4X34.070.6X43.690.8X53.341.0X63.021.2X72.731.4X82.471.6X92.241.8X102.022.0X111.832.2X121.662.4Comparative material 5 to 10131.502.6Comparative material 4141.362.8Comparative material 1 to 3151.233.0Experimental examples 13 to 17161.113.2Experimental examples 1 to 12, 18

[0086] Table 4 above shows experimental examples or comparative materials in which delamination occurred according to the drawing pass schedule and strain applied amount using a draw bench. Referring to this, it can be confirmed that all of Experimental Examples 1 to 18, which satisfy both the alloy composition and manufacturing conditions according to the present invention, have a total applied strain exceeding 2.88. Through this, it can be seen that, based on the initial wire diameter of 5.50 mm, drawing is possible without isothermal transformation heat treatment up to the final wire diameter of 1.24 mm. On the other hand, it can be seen that delamination occurred in the comparative material from the total applied strain level of 2.4, which means that drawing is possible without delamination up to the wire diameter of 1.67 mm. Therefore, it can be confirmed that, when the wire diameter is less than 1.67 mm, the conventional isothermal transformation heat treatment must be performed twice.

Claims

1. Containing, by weight%, C: 0.60 to 1.00%, Si: 0.10 to 0.40%, and Mn: 0.20 to 0.60%, the remainder being Fe and other impurities, A wire having a microstructure comprising, in area fraction, 98% or more of pearlite and 2% or less of proeutectoid cementite, and an average lamellar spacing of 0.230 to 0.350 ㎛.

2. In claim 1, The tensile strength (TS) of the above wire is controlled by the value of the following formula. TS(MPa)≤69.1+1161.0 x [Ceq] (Here, [Ceq]=[C]+[Mn] / 6+[Si] / 24, and [C], [Mn], and [Si] represent the weight% of each element.) 3. In claim 1, The wire having a diameter of 1.30 to 5.50 mm is a wire in which delamination does not occur in a range of total strain exceeding 0 and less than 3.

4. A step of heating a billet containing C: 0.60 to 1.00% by weight, Si: 0.10 to 0.40%, and Mn: 0.20 to 0.60%, the remainder being Fe and other impurities, at 1000 to 1250°C; A step of hot rolling the above billet at a temperature of 1000 to 1100℃ to prepare a wire rod; A step of winding the above wire at a temperature range of 750 to 950°C; and The above coiled wire has the lowest temperature (T) at which pearlite is transformed according to the carbon equivalent (Ceq). low ) is T low Controlled to (℃)≥702.3-76.0x[Ceq], and the highest temperature (T high ) is T high A method for manufacturing a wire rod, comprising a step of controlling the temperature to (℃)≥715.3-44.9x[Ceq]. (Here, [Ceq]=[C]+[Mn] / 6+[Si] / 24, and [C], [Mn], and [Si] represent the weight% of each element.) 5. In claim 4, A method for manufacturing a wire rod, wherein the above pearlite transformation time is controlled to 12 to 24 seconds.

6. In claim 4, A method for manufacturing a wire rod, wherein the average lamellar spacing during the above pearlite transformation is 0.230 to 0.350 ㎛.

7. In claim 4, A method for manufacturing a wire rod, wherein the above wire rod is subjected to a constant temperature transformation heat treatment once after fresh processing.

8. In claim 4, The tensile strength (TS) of the wire during the above pearlite transformation is controlled according to the value of the following formula. TS(MPa)≤69.1+1161.0 x [Ceq] (Here, [Ceq]=[C]+[Mn] / 6+[Si] / 24, and [C], [Mn], and [Si] represent the weight% of each element.)

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