Wire rod and bolt for fastening wind tower and manufacturing method thereof

Abstract

A wire rod for fastening a wind tower, according to an embodiment of the present invention, comprises, in weight %: 0.20% to 0.40% of C; 0.10% to 0.40% of Si; 0.30-0.50% of Mn; 1.00% to 1.50% of Cr; 0.01% to 0.10% of Ti; and 0.0005% to 0.0100% of B, the remainder being Fe and other impurities, wherein after hot rolling the microstructure comprises, by area fraction, 30% to 60% pearlite and 40% to 70% ferrite, and satisfies formula (1). Formula (1): 7.50 ≤ 12.1x[C] + 1.5x[Mn] + 3.1x[Cr] ≤ 8.50 (where [C], [Mn] and [Cr] signify the weight % of each element).

Description

Wires, bolts, and manufacturing methods for fastening wind towers

[0001] The present invention relates to wires, bolts, and a method for manufacturing the same for fastening wind towers.

[0002] The existing JS-SCM440 steel used for fastening bolts for wind towers requires spheroidizing heat treatment for cold forging, and the addition of expensive Mo elements reduces its economic feasibility. Meanwhile, the JS-SCr420B steel, which can be cold forged without post-rolling heat treatment for wind tower fastening wire, has low C and Cr contents, which reduces strength after hot-dip galvanizing, necessitating a lower tempering temperature in the QT heat treatment process, and this results in a decrease in low-temperature (-20°C) impact toughness.

[0003] The purpose of the present invention to solve the above-described problem is to provide a wire rod, bolt, and a method for manufacturing the same for fastening a wind power tower, which can simultaneously secure cold forgeability and tensile strength by forming fine carbides during tempering without spheroidizing heat treatment, thereby improving the cold forgeability of a hot-rolled material and suppressing the decrease in strength after hot-dip galvanizing.

[0004] The problems to be solved by the present invention are not limited to the problems mentioned above, and other problems not mentioned will be clearly understood by those skilled in the art from the description below.

[0005] A wire for fastening a wind tower according to one embodiment of the present invention comprises, in weight %, C: 0.20% to 0.40%, Si: 0.10% to 0.40%, Mn: 0.30% to 0.50%, Cr: 1.00% to 1.50%, Ti: 0.01% to 0.10%, and B: 0.0005% to 0.0100%, with the remainder being Fe and other impurities, and a microstructure after hot rolling comprises, in area fraction, pearlite 30% to 60%, ferrite 40% to 70%, and satisfies the following equation (1):

[0006] Equation (1): 7.50 ≤ 12.1x[C] + 1.5x[Mn] + 3.1x[Cr] ≤ 8.50

[0007] (Here, [C], [Mn], and [Cr] represent the weight percent of each element)

[0008] In addition, the wire for fastening a wind tower according to one embodiment of the present invention may have an average austenite grain size of 30 ㎛ or less.

[0009] In addition, the tensile strength of the wire for fastening a wind tower according to one embodiment of the present invention may be 470 MPa to 680 MPa.

[0010] A method for manufacturing a wire for fastening a wind tower according to one embodiment of the present invention comprises the steps of heating a billet, which contains, in wt%, C: 0.20% to 0.40%, Si: 0.10% to 0.40%, Mn: 0.30% to 0.50%, Cr: 1.00% to 1.50%, Ti: 0.01% to 0.10%, and B: 0.0005% to 0.0100%, the remainder being Fe and other impurities, to 950°C to 1200°C; hot-rolling the billet at 850°C to 1000°C; and cooling the billet to room temperature at a cooling rate of 0.1°C / s to 2.0°C / s after coiling:

[0011] Equation (1): 7.50 ≤ 12.1x[C] + 1.5x[Mn] + 3.1x[Cr] ≤ 8.50

[0012] (Here, [C], [Mn], and [Cr] represent the weight percent of each element)

[0013] In addition, in the method for manufacturing a wire for fastening a wind tower according to one embodiment of the present invention, the microstructure of the wire after the hot rolling may include 30% to 60% of pearlite and 40% to 70% of ferrite in terms of area fraction.

[0014] In addition, in the method for manufacturing a wire for fastening a wind tower according to one embodiment of the present invention, the average austenite grain size can be controlled to 30 ㎛ or less in the cooling step.

[0015] In addition, in the method for manufacturing a wire for fastening a wind tower according to one embodiment of the present invention, the spheroidizing heat treatment after the rolling may be omitted.

[0016] A bolt for fastening a wind tower according to one embodiment of the present invention contains, in wt%, C: 0.20% to 0.40%, Si: 0.10% to 0.40%, Mn: 0.30% to 0.50%, Cr: 1.00% to 1.50%, Ti: 0.01% to 0.10%, and B: 0.0005% to 0.0100%, with the remainder being Fe and other impurities, and satisfies the following formula (1), and after quenching heat treatment and hot-dip galvanizing, the microstructure includes tempered martensite of 90% to 99% in area fraction, bainite of 1% to 10%, and retained austenite of 1% or less:

[0017] Equation (1): 7.50 ≤ 12.1x[C] + 1.5x[Mn] + 3.1x[Cr] ≤ 8.50

[0018] (Here, [C], [Mn], and [Cr] represent the weight percent of each element)

[0019] In addition, a bolt for fastening a wind tower according to one embodiment of the present invention may have a bolt body with a diameter of 15 mm to 40 mm and a tensile strength of 1000 MPa or more.

[0020] In addition, a bolt for fastening a wind tower according to one embodiment of the present invention may have a low-temperature (-20°C) impact toughness of 50 J or more.

[0021] A method for manufacturing a bolt for fastening a wind tower according to one embodiment of the present invention comprises the steps of: preparing the wire; drawing the wire at a drawing reduction rate of 20% or less; heating at 850°C to 950°C and then quenching in oil at 20°C to 80°C; performing a tempering heat treatment at 450°C to 550°C for 3000 seconds to 10000 seconds; and hot-dip galvanizing at a temperature of 500°C to 550°C, wherein the microstructure includes tempered martensite of 90% to 99% in area fraction, bainite of 1% to 10%, and retained austenite of 1% or less.

[0022] According to one embodiment of the present invention, a wire for fastening a wind tower can simultaneously secure cold forgeability and tensile strength by forming fine carbides during tempering without spheroidizing heat treatment, thereby improving the cold forgeability of a hot-rolled material and suppressing a decrease in strength after hot-dip galvanizing.

[0023] A bolt for fastening a wind tower according to one embodiment of the present invention may have a body diameter of 15 mm to 40 mm or more, a tensile strength of 1000 MPa or more, and a low-temperature (-20°C) impact toughness of 50 J or more.

[0024] Preferred embodiments of the present invention are described below. However, the embodiments of the present invention may be modified in various ways, 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 fully explain the present invention to those of ordinary skill in the art.

[0025] The terminology used in this application is solely for the purpose of describing specific examples. Therefore, for example, singular expressions include plural expressions unless the context clearly dictates otherwise. Additionally, it should be noted that terms such as "comprise" or "have" 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 presence of other features, steps, functions, components, or combinations thereof.

[0026] Meanwhile, unless otherwise defined, all terms used herein should be considered to have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Therefore, unless explicitly defined herein, specific terms should not be interpreted in an overly idealistic or formal sense. For example, singular expressions herein include plural expressions unless the context clearly indicates otherwise.

[0027] In addition, the terms "about", "substantially", etc. in this specification are used in the sense of or close to the numerical value when manufacturing and material tolerances inherent to the meanings mentioned are presented, and are used to prevent unscrupulous infringers from unfairly using the disclosure contents in which exact or absolute numerical values ​​are mentioned to aid in the understanding of the present invention.

[0028] The basic principle of the present invention is to limit the C content to minimize the increase in tensile strength after hot rolling, to limit the Mn content to reduce the solid solution strengthening effect of hot rolled material, and to minimize the Si content to secure cold forgeability. In addition, B is added to secure hardenability during quenching heat treatment, and Cr is added to suppress the decrease in strength after hot-dip galvanizing at high temperatures of 500°C or higher, thereby contributing to improving the softening resistance of tempering.

[0029] First, the wire of the present invention will be described.

[0030] A wire for fastening a wind tower according to one embodiment of the present invention comprises, in wt%, C: 0.20% to 0.40%, Si: 0.10% to 0.40%, Mn: 0.30% to 0.50%, Cr: 1.00% to 1.50%, Ti: 0.01% to 0.10%, and B: 0.0005% to 0.0100%, with the remainder being Fe and other impurities, and a microstructure after hot rolling comprises, in area fraction, pearlite 30% to 60%, ferrite 40% to 70%, and satisfies the following equation (1):

[0031] Equation (1): 7.50 ≤ 12.1x[C] + 1.5x[Mn] + 3.1x[Cr] ≤ 8.50

[0032] (Here, [C], [Mn], and [Cr] represent the weight percent of each element)

[0033] Below, the reasons for limiting the composition range of each alloy element are explained. Unless otherwise specified, the unit is weight percent.

[0034] The C (carbon) content can be 0.20% to 0.40%.

[0035] C is the element that most effectively increases the strength of materials. However, if the content is less than 0.20%, hardenability is not sufficiently secured during QT heat treatment. Furthermore, if the content exceeds 0.40%, the tensile strength increases excessively during cooling after rolling of the wire rod, which can reduce the life of the forging die during cold forging and increase the cost. Preferably, the C content is greater than 0.20% and less than 0.39%.

[0036] The content of Si (silicon) can be 0.10% to 0.40%.

[0037] Silicon (Si) is not only useful for deoxidation of steel but also effective in securing strength through solid solution strengthening, but it is an element that deteriorates cold forgeability. When Si content is less than 0.10%, securing strength through deoxidation and solid solution strengthening of steel is insufficient, and when it exceeds 0.40%, cold forgeability due to solid solution strengthening may be deteriorated, so it is not desirable. Preferably, the Si content may be 0.10% to 0.35%. More preferably, the Si content may be 0.15% to 0.35%.

[0038] The content of Mn (manganese) can be 0.30% to 0.50%.

[0039] Manganese (Mn) is an alloying element that enhances the hardenability of steel, thereby securing strength. It also plays a role in increasing rollability and reducing brittleness. If Mn is added at less than 0.30%, it is difficult to secure sufficient strength. If it exceeds 0.50%, hard structures are likely to develop during cooling after hot rolling, and a large amount of MnS inclusions may be generated, potentially reducing fatigue properties. Preferably, the Mn content is 0.31% to 0.48%.

[0040] The content of Cr (chromium) can be 1.00% to 1.50%.

[0041] Cr, together with Mn, is an element that is effective in improving hardenability and securing hardness. It can be added in amounts of 1.00% or more. However, if the content is excessive, there is a problem of coarse carbide being formed, so the upper limit may be limited to 1.50%. Preferably, the content of Cr may be greater than 1.00% and less than 1.42%.

[0042] The content of Ti (titanium) can be 0.01% to 0.10%.

[0043] Ti combines with nitrogen introduced into the steel to form titanium carbonitride, thereby preventing boron from combining with nitrogen. When the titanium content is less than 0.01%, it is difficult to utilize the effect of boron because the nitrogen introduced during the steelmaking process is not sufficient to form titanium carbonitride. When it exceeds 0.10%, coarse carbonitride is formed, causing microcracks to occur and lowering the resistance to delayed fracture. Preferably, the Ti content may be more than 0.01% and less than 0.04%.

[0044] The content of B (boron) can be 0.0005% to 0.0100%.

[0045] B is an element that improves hardenability. If the content of B is less than 0.0005%, it is difficult to expect the effect of improving hardenability, and if it exceeds 0.0100%, Fe is present at the grain boundaries. 23 (CB)6 carbide is formed, which is not desirable because it causes embrittlement of austenite grain boundaries. Preferably, the B content may be greater than 0.0005% and less than or equal to 0.0025%. More preferably, the B content may be from 0.0015% to 0.0025%.

[0046] In addition to the above composition, the wire for fastening a wind tower according to one embodiment of the present invention must satisfy the above formula (1), and by satisfying formula (1) of 7.50 or more and 8.50 or less, sufficient fine carbides of Cr-type are secured, so that the cold forging property of the wire is improved according to the optimal tensile strength of 470 MPa to 680 MPa, and thus cold forging is possible without spheroidizing heat treatment.

[0047] In addition, by satisfying the above formula (1), when formula (1) is satisfied, the Cr-based carbide ratio of 90% to 95% is satisfied in the final bolt. When the Cr-based carbide ratio exceeds 95%, the impact toughness is lowered to 50J or less, and when the Cr-based carbide ratio is less than 90%, the tensile strength is inferior to less than 1000MPa. That is, when the above formula (1) is satisfied and the Cr-based carbide ratio is 90% to 95%, the tensile strength is secured, and the final product, the bolt, can secure a strength of 1000MPa or more and a low-temperature (-20℃) impact toughness of 50J or more.

[0048] In addition, the wire for fastening a wind tower according to one embodiment of the present invention may have an average austenite grain size of 30 µm or less. Preferably, the average austenite grain size may be 15 µm to 30 µm or less.

[0049] In addition, the wire for fastening a wind tower according to one embodiment of the present invention may have a tensile strength of 470 MPa to 680 MPa.

[0050] If the tensile strength of the wire rod is less than 470 MPa, the bolt after QT heat treatment may have inferior strength and impact toughness, with a tensile strength of less than 1000 MPa and a low-temperature (-20°C) impact toughness of less than 50 J. Furthermore, if the tensile strength of the wire rod exceeds 680 MPa, the cold forgeability of the wire rod may be poor, requiring spheroidizing heat treatment to address this. The tensile strength may preferably be 500 to 650 MPa.

[0051] The following describes the manufacturing method of wire for fastening wind towers.

[0052] A method for manufacturing a wire for fastening a wind tower according to one embodiment of the present invention comprises the steps of heating a billet, which contains, in wt%, C: 0.20% to 0.40%, Si: 0.10% to 0.40%, Mn: 0.30% to 0.50%, Cr: 1.00% to 1.50%, Ti: 0.01% to 0.10%, and B: 0.0005% to 0.0100%, with the remainder being Fe and other impurities, to 950°C to 1200°C; hot-rolling the billet at 850°C to 1000°C; and cooling the billet to room temperature at a cooling rate of 0.1°C / s to 2.0°C / s after coiling.

[0053] Equation (1): 7.50 ≤ 12.1x[C] + 1.5x[Mn] + 3.1x[Cr] ≤ 8.50

[0054] (Here, [C], [Mn], and [Cr] represent the weight percent of each element)

[0055] In addition, in a method for manufacturing a wire for fastening a wind tower according to another embodiment of the present invention, the microstructure after hot rolling may include an area fraction of 30% to 60% pearlite and 40% to 70% ferrite.

[0056] In addition, in the method for manufacturing a wire rod according to another embodiment of the present invention, the average austenite grain size can be controlled to 30 ㎛ or less in the cooling step. When the cooling rate after coiling satisfies 0.1℃ / s to 2.0℃ / s or less, the average austenite grain size is controlled to 30 ㎛ or less, thereby preventing cracks from occurring inside the bolt during cold forging without spheroidizing heat treatment since the subsequent wire rod tensile strength is in the range of 470 MPa to 680 MPa.

[0057] If the post-coiling cooling rate exceeds 2.0°C / s, the tensile strength of the wire exceeds 680 MPa, which can lead to cracks within the bolt during cold forging. Therefore, prior spheroidization heat treatment is necessary to prevent this. If the cooling rate is less than 0.1°C / s, the cooling rate becomes excessively slow, which can lead to decreased productivity due to the delayed cooling time.

[0058] According to one embodiment of the present invention, a method for manufacturing a wire for fastening a wind turbine tower can omit the spheroidizing heat treatment after the rolling. By satisfying the range of the above formula (1) according to the present invention, the tensile strength of the wire can be controlled to a range of 470 MPa to 680 MPa, and thus, cracking can be prevented during subsequent cold forging without the spheroidizing heat treatment.

[0059] Next, a bolt manufactured using the wire for fastening a wind tower according to the present invention is described.

[0060] A bolt for fastening a wind tower according to one embodiment of the present invention comprises, in wt%, C: 0.20% to 0.40%, Si: 0.10% to 0.40%, Mn: 0.30% to 0.50%, Cr: 1.00% to 1.50%, Ti: 0.01% to 0.10%, and B: 0.0005% to 0.0100%, with the remainder being Fe and other impurities, and satisfies the following formula (1), and after quenching heat treatment and hot-dip galvanizing, the microstructure comprises tempered martensite of 90% to 99% in area fraction, bainite of 1% to 10%, and retained austenite of 1% or less.

[0061] Equation (1): 7.50 ≤ 12.1x[C] + 1.5x[Mn] + 3.1x[Cr] ≤ 8.50

[0062] (Here, [C], [Mn], and [Cr] represent the weight percent of each element)

[0063] In addition, a bolt for fastening a wind tower according to another embodiment of the present invention may have a body diameter of 15 mm to 40 mm and a tensile strength of 1000 MPa or more.

[0064] In addition, a bolt for fastening a wind tower according to another embodiment of the present invention may have a low-temperature (-20°C) impact toughness of 50 J or more.

[0065] A method for manufacturing a bolt for fastening a wind tower according to one embodiment of the present invention comprises the steps of: providing a wire manufactured by the method for manufacturing a wire for fastening a wind tower according to the present invention; drawing the wire at a drawing reduction rate of 20% or less; heating the wire at 850°C to 950°C and then quenching it in oil at 20°C to 80°C; performing a tempering heat treatment at 450°C to 550°C for 3000 seconds to 10000 seconds; and hot-dip galvanizing the wire at a temperature of 500°C to 550°C, wherein the microstructure includes tempered martensite of 90% to 99% in area fraction, bainite of 1% to 10%, and retained austenite of 1% or less.

[0066] When trying to control a bolt with a diameter of 15 mm to 40 mm in the body with a tensile strength of 1000 MPa or more using general steel, tempering must be performed at a low temperature of less than 450°C, which makes it difficult to secure sufficient fine carbides, resulting in a problem in that the low-temperature (-20°C) impact toughness becomes less than 50 J.

[0067] Specifically, a wire having the above-described alloy composition, microstructure, and formula (1) and having an average austenite grain size of 30 ㎛ or less is drawn at a drawing reduction rate of 20% or less, heated to a temperature between 850°C and 950°C, quenched in oil at 20°C to 80°C, and subjected to tempering heat treatment at 450°C to 550°C for 3,000 seconds to 10,000 seconds, and then a bolt for fastening a wind tower can secure a microstructure of 90% to 99% tempered martensite, 1% to 10% bainite, and 1% or less of retained austenite.

[0068] That is, cold forging is possible without spheroidizing heat treatment of the wire rod, and by subsequent tempering at high temperatures, the thin film-shaped carbides that are mainly formed at the grain boundaries of old austenite are suppressed and spheroidized, and the fine carbides are dispersed and distributed inside and outside the grain boundaries, so that the final bolt with a diameter of 15 mm to 40 mm in the body portion that has undergone hot-dip galvanizing at 500°C to 550°C can secure high strength and high impact toughness of 1000 MPa or more in tensile strength and 50 J or more in low-temperature (-20°C) impact toughness.

[0069] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and further illustrate the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0070] {Example}

[0071] Billets having various alloy compositions as shown in Table 1 below were heated to 950°C to 1200°C, then hot-rolled and coiled at 850°C to 1000°C. The cooling rate after coiling was set to 0.1°C / s to 2.0°C / s.

[0072] Alloy composition (wt%) CSiMnCrTiB Inventive material 10.250.210.421.350.030.0020 Inventive material 20.380.150.321.070.030.0018 Inventive material 30.210.320.481.420.040.0022 Inventive material 40.390.180.411.020.030.0018 Inventive material 50.220.150.311.420.030.0019 Comparative material 10.180.220.321.170.030.0021 Comparative material 20.420.190.311.050.030.0017 Comparative material 30.320.140.211.250.030.0019Comparative material 40.350.250.320.890.040.0021Comparative material 50.520.220.331.270.030.0018Comparative material 60.210.210.421.020.030.0020Comparative material 70.260.210.351.040.030.0020Comparative material 80.280.210.391.020.030.0020Comparative material 90.350.210.371.240.030.0020Comparative material 100.320.210.341.480.030.0020Comparative material 110.370.210.451.450.030.0020

[0073] Equation (1): 7.50 ≤ 12.1x[C] + 1.5x[Mn] + 3.1x[Cr] ≤ 8.50 Table 2 below shows the microstructure and properties of the wire rod. In the case of cold forging formability, if the tensile strength of the wire rod exceeds 680 MPa, the case in which the cold forging formability is poor is marked with X, and if the tensile strength of 470 MPa to 680 MPa is good, the cold forging formability is marked with O. In addition, in the case of the wire rod and bolt, each specimen was processed in accordance with the ASTM E8 standard and then subjected to a tensile test.

[0074] Equation (1) Tensile strength of wire rod (MPa) Cold forging composition Inventive material 17.84570O Inventive material 28.40625O Inventive material 37.66535O Inventive material 48.50630O Inventive material 57.53530O Comparative material 16.29522O Comparative material 28.80721X Comparative material 38.06612O Comparative material 47.47621O Comparative material 510.72746X Comparative material 66.33465O Comparative material 76.90500O Comparative material 87.14512O Comparative material 98.63682X Comparative material 108.97688X Comparative material 119.65710X

[0075] Table 3 below shows the values ​​of the austenite grain size measured according to the cooling rate during the cooling stage of the wire rod, and the occurrence of microcracks in the drawn and final bolts thereafter. If cracks occurred, they were marked as O, and if no cracks occurred, they were marked as X. The average austenite grain size of the wire rod was measured at three random locations according to the ASTM E 112 standard and calculated as the average value. In addition, the presence or absence of cracks was confirmed by a delayed fracture simulation method in which the presence or absence of microcracks was observed in the threads, which are stress concentration areas, before and after immersing in a 5% hydrochloric acid + 95% distilled water solution for 10 minutes after fastening to the target steel plate after the final product heat treatment.

[0076] Cooling rate (℃ / s)Average austenite grain size (㎛)Presence of microcracksInventive material 10.418XInventive material 20.622XInventive material 30.316XInventive material 41.226XInventive material 51.628XComparative material 10.014XComparative material 20.036XComparative material 30.057XComparative material 40.079XComparative material 50.0811XComparative material 62.233OComparative material 72.536OComparative material 82.840OComparative material 93.242OComparative material 104.146OComparative material 114.552O

[0077] Referring to Table 3 above, it can be confirmed that when the cooling rate exceeds 2.0℃ / s in the cooling step, the average austenite grain size exceeds 30㎛, causing microcracks to occur in the final bolt. Next, the hot-rolled wire was drawn with a drawing reduction rate of 20% or less, heated at 940℃ for 3600 seconds, and then quenched in oil at 70℃. After that, a tempering heat treatment was performed at 450℃ to 550℃ for 3,000 seconds to 10,000 seconds, and then hot-dip galvanized at 500℃ to 550℃ to manufacture bolts. The impact toughness of the bolts was measured by processing V-notch specimens according to ASTM E23 standard and conducting a Charpy impact test at low temperature (-20℃), and the results are shown in Table 4 below.

[0078] Additionally, the Cr carbide ratio of the bolts for fastening wind towers was measured using an image analyzer according to the ASTM E 552 standard method.

[0079] Bolt Classification Tensile Strength (MPa) Low Temperature (-20℃) Impact Toughness (J) Cr Carbide Ratio (%) Formula (1) Inventive Material 17.8410508293.3 Inventive Material 28.4011217294.2 Inventive Material 37.66104210292.5 Inventive Material 48.5011356894.8 Inventive Material 57.53103112590.2 Comparative Material 16.2989515682.5 Comparative Material 28.8011554595.3 Comparative Material 38.069856993.6 Comparative Material 47.479737588.2 Comparative Material 510.7211894298.5 Comparative Material 66.339507884.2 Comparative material 76.909756586.8 Comparative material 87.149907887.5 Comparative material 98.6311804895.1 Comparative material 108.9712104596.5 Comparative material 119.6512303597.2

[0080] Inventive Examples 1 to 5 have a tensile strength of 1000 MPa or more and a low-temperature (-20°C) impact toughness of 50 J or more by satisfying the composition, austenite average grain size, and formula (1) within the scope of the present invention, whereas Comparative Examples 1 to 11, which do not satisfy the composition, austenite average grain size, or formula (1), have a tensile strength of less than 1000 MPa or a low-temperature (-20°C) impact toughness of less than 50 J. In the case of Comparative Material 1, the C content was less than 0.20%, and the value of formula (1) was less than 7.50, so the tensile strength of the final product, the bolt, was inferior to less than 1000 MPa.

[0081] For Comparative Materials 2 and 5, the C content exceeded 0.40%, and the value of Equation (1) exceeded 8.50, so that the tensile strength of the wire rod exceeded 680 MPa. Accordingly, it can be confirmed that the cold forging formability is poor and spheroidizing heat treatment is required.

[0082] In the case of comparative material 3, the range of equation (1) according to the present invention was satisfied, but since the Mn content was less than 0.30%, the tensile strength of the bolt was inferior at less than 1000 MPa.

[0083] In the case of comparative material 4, the Cr content was less than 1.00% and did not satisfy the range of formula (1) of the present invention, so the tensile strength of the bolt was inferior at less than 1000 MPa.

[0084] Furthermore, for comparative materials 6 to 11, the physical properties were compared by changing the C, Mn, and Cr contents of formula (1) of the present invention in the alloy composition of inventive material 1. Comparative materials 6 to 8 satisfied the range of the alloy composition according to the present invention, but the value of formula (1) was less than 7.50, so Cr-based carbides were not sufficiently secured, resulting in inferior tensile strength of the final bolts.

[0085] In addition, Comparative Materials 9 to 11 satisfied the range of alloy composition according to the present invention, but since the value of Formula (1) exceeded 8.50, the tensile strength of the wire rod exceeded 680 MPa, resulting in poor cold forging properties, and the low-temperature (-20°C) impact toughness of the bolt was also poor at 50 J or less.

[0086] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and those skilled in the art will understand that various changes and modifications are possible within the scope and spirit of the claims set forth below.

Claims

1. Containing, by weight%, C: 0.20% to 0.40%, Si: 0.10% to 0.40%, Mn: 0.30% to 0.50%, Cr: 1.00% to 1.50%, Ti: 0.01% to 0.10%, and B: 0.0005% to 0.0100%, the remainder being Fe and other impurities, After hot rolling, the microstructure contains, in area fraction, 30 to 60% pearlite and 40 to 70% ferrite. A wire for fastening a wind power tower, satisfying the following equation (1). Equation (1): 7.50 ≤ 12.1x[C] + 1.5x[Mn] + 3.1x[Cr] ≤ 8.50 (Here, [C], [Mn], and [Cr] represent the weight% of each element) 2. In claim 1, A wire for fastening a wind tower, wherein the average austenite grain size of the above wire is 30㎛ or less.

3. In claim 1, A wire for fastening a wind tower, the tensile strength of the wire being 470 MPa to 680 MPa.

4. Containing, by weight%, C: 0.20% to 0.40%, Si: 0.10% to 0.40%, Mn: 0.30% to 0.50%, Cr: 1.00% to 1.50%, Ti: 0.01% to 0.10%, and B: 0.0005% to 0.0100%, the remainder being Fe and other impurities, A step of heating a billet satisfying the following equation (1) to 950°C to 1200°C; A step of hot rolling at 850℃ to 1000℃; and A method for manufacturing a wire for fastening a wind tower, comprising a step of cooling to room temperature at a cooling rate of 0.1°C / s to 2.0°C / s after winding. Equation (1): 7.50 ≤ 12.1x[C] + 1.5x[Mn] + 3.1x[Cr] ≤ 8.50 (Here, [C], [Mn], and [Cr] represent the weight% of each element) 5. In claim 4, A method for manufacturing a wire for fastening a wind tower, wherein the microstructure of the wire after the hot rolling includes 30 to 60% of pearlite and 40 to 70% of ferrite in terms of area fraction.

6. In claim 4, A method for manufacturing a wire for fastening a wind tower, wherein the average austenite grain size is controlled to 30 ㎛ or less in the above cooling step.

7. In claim 4, A method for manufacturing a wire for fastening a wind tower, wherein the spheroidizing heat treatment after the above rolling can be omitted.

8. Containing, by weight%, C: 0.20% to 0.40%, Si: 0.10% to 0.40%, Mn: 0.30% to 0.50%, Cr: 1.00% to 1.50%, Ti: 0.01% to 0.10%, and B: 0.0005% to 0.0100%, the remainder being Fe and other impurities, and satisfying the following formula (1): A bolt for fastening a wind tower, the microstructure of which after quenching heat treatment and hot-dip galvanizing comprises an area fraction of 90 to 99% tempered martensite, 1 to 10% bainite, and 1% or less of retained austenite. Equation (1): 7.50 ≤ 12.1x[C] + 1.5x[Mn] + 3.1x[Cr] ≤ 8.50 (Here, [C], [Mn], and [Cr] represent the weight% of each element) 9. In claim 8, A bolt for fastening a wind tower, having a body diameter of 15 mm to 40 mm and a tensile strength of 1000 MPa or more.

10. In claim 8, Bolts for fastening wind towers with a low temperature (-20℃) impact toughness of 50J or more.

11. A step of preparing the precursors of claims 1 to 3; A step of freshening the above raw material with a freshness reduction rate of 20% or less; A step of heating at 850°C to 950°C and then quenching in oil at 20°C to 80°C; A step of tempering heat treatment at 450℃ to 550℃ for 3000 seconds to 10000 seconds; and Comprising a step of hot-dip galvanizing at a temperature of 500°C to 550°C, A method for manufacturing a bolt for fastening a wind tower, wherein the microstructure comprises tempered martensite of 90% to 99% in area fraction, bainite of 1% to 10%, and retained austenite of 1% or less.