The present disclosure relates to a wire rod for fastening a wind tower, a bolt, and a manufacturing method therefor.

Existing JS-SCM440 steel materials used for fastening bolts of wind towers require spheroidizing heat treatment for cold forging, and economic feasibility decreases due to the addition of expensive Mo elements. Meanwhile, JS-SCr420B steel materials, which allow cold forging without heat treatment after rolling of a wire rod for fastening a wind tower, have a problem in that strength decreases after hot-dip galvanizing due to low C and Cr contents, requiring a lowering of a tempering temperature in a QT heat treatment process, which results in a decrease in low-temperature (-20°C) impact toughness.

An objective of the present disclosure for solving the above-mentioned problems is to provide a wire rod for fastening a wind tower, a bolt, and a manufacturing method therefor, which can simultaneously secure cold forgeability and tensile strength by forming fine carbides during tempering without spheroidizing heat treatment to improve cold forgeability of a hot-rolled material and suppressing a decrease in strength after hot-dip galvanizing.

The problems to be solved by the present disclosure 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 following description.

A wire rod for fastening a wind tower according to an embodiment of the present disclosure includes, in percent by weight (wt%), 0.20 to 0.40% of carbon (C), 0.10 to 0.40% of silicon (Si), 0.30 to 0.50% of manganese (Mn), 1.00 to 1.50% of chromium (Cr), 0.01 to 0.10% of titanium (Ti), 0.0005 to 0.0100% of boron (B), and the remainder of iron (Fe) and other inevitable impurities, wherein a microstructure after hot rolling comprises, by area fraction, 30 to 60% of pearlite and 40 to 70% of ferrite, and satisfies Formula (1) below:
$$7.50\le 12.1\mathrm{x}\left[\mathrm{C}\right]+1.5\mathrm{x}\left[\mathrm{Mn}\right]+3.1\mathrm{x}\left[\mathrm{Cr}\right]\le 8.50$$
(wherein [C], [Mn], and [Cr] represent percent by weight (wt%) of each element).

In addition, in the wire rod for fastening a wind tower according to an embodiment of the present disclosure, an average austenite grain size of the wire rod may be 30 µm or less.

In addition, in the wire rod for fastening a wind tower according to an embodiment of the present disclosure, a tensile strength of the wire rod may be 470 MPa to 680 MPa.

A method for manufacturing a wire rod for fastening a wind tower according to an embodiment of the present disclosure includes: heating a billet comprising, in percent by weight (wt%), 0.20 to 0.40% of carbon (C), 0.10 to 0.40% of silicon (Si), 0.30 to 0.50% of manganese (Mn), 1.00 to 1.50% of chromium (Cr), 0.01 to 0.10% of titanium (Ti), 0.0005 to 0.0100% of boron (B), and the remainder of iron (Fe) and other inevitable impurities, and satisfying Formula (1) below, to 950°C to 1200°C; hot rolling at 850°C to 1000°C; and cooling to room temperature at a cooling rate of 0.1°C/s to 2.0°C/s after coiling:
$$7.50\le 12.1\mathrm{x}\left[\mathrm{C}\right]+1.5\mathrm{x}\left[\mathrm{Mn}\right]+3.1\mathrm{x}\left[\mathrm{Cr}\right]\le 8.50$$
(wherein [C], [Mn], and [Cr] represent percent by weight (wt%) of each element).

In addition, in the method for manufacturing a wire rod for fastening a wind tower according to an embodiment of the present disclosure, a microstructure of the wire rod after the hot rolling may comprise, by area fraction, 30 to 60% of pearlite and 40 to 70% of ferrite.

In addition, in the method for manufacturing a wire rod for fastening a wind tower according to an embodiment of the present disclosure, an average austenite grain size may be controlled to 30 µm or less in the cooling.

In addition, in the method for manufacturing a wire rod for fastening a wind tower according to an embodiment of the present disclosure, the method may be performed without spheroidizing heat treatment after the rolling.

A bolt for fastening a wind tower according to an embodiment of the present disclosure includes, in percent by weight (wt%), 0.20 to 0.40% of carbon (C), 0.10 to 0.40% of silicon (Si), 0.30 to 0.50% of manganese (Mn), 1.00 to 1.50% of chromium (Cr), 0.01 to 0.10% of titanium (Ti), 0.0005 to 0.0100% of boron (B), and the remainder of iron (Fe) and other inevitable impurities, satisfies Formula (1) below, and has a microstructure after quenching heat treatment and hot-dip galvanizing comprising, by area fraction, 90 to 99% of tempered martensite, 1 to 10% of bainite, and 1% or less of residual austenite:
$$7.50\le 12.1\mathrm{x}\left[\mathrm{C}\right]+1.5\mathrm{x}\left[\mathrm{Mn}\right]+3.1\mathrm{x}\left[\mathrm{Cr}\right]\le 8.50$$
(wherein [C], [Mn], and [Cr] represent percent by weight (wt%) of each element).

In addition, in the bolt for fastening a wind tower according to an embodiment of the present disclosure, a diameter of a bolt body part is 15 mm to 40 mm, and a tensile strength may be 1000 MPa or more.

In addition, in the bolt for fastening a wind tower according to an embodiment of the present disclosure, low-temperature (-20°C) impact toughness may be 50 J or more.

A method for manufacturing a bolt for fastening a wind tower according to an embodiment of the present disclosure includes: preparing the wire rod; drawing the wire rod at a drawing reduction ratio of 20% or less; quenching in oil at 20°C to 80°C after heating at 850°C to 950°C; 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 a microstructure comprises, by area fraction, 90 to 99% of tempered martensite, 1 to 10% of bainite, and 1% or less of residual austenite.

The wire rod for fastening a wind tower according to an embodiment of the present disclosure forms fine carbides during tempering without spheroidizing heat treatment, thereby improving cold forgeability of a hot-rolled material and suppressing a decrease in strength after hot-dip galvanizing, thus simultaneously securing cold forgeability and tensile strength.

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

Hereinafter, preferred embodiments of the present disclosure will be described. However, embodiments of the present disclosure may be modified into various other forms, and the technical spirit of the present disclosure is not limited to the embodiments described below. In addition, the embodiments of the present disclosure are provided to more completely explain the present disclosure to those skilled in the art.

Terms used in this application are merely used to describe specific examples. Therefore, for example, singular expressions include plural expressions unless the context clearly implies otherwise. In addition, it should be noted that terms such as "comprise" or "include" 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 intended to preliminarily exclude the presence of other features, steps, functions, components, or combinations thereof.

Meanwhile, unless defined otherwise, all terms used herein should be construed as having the same meaning as commonly understood by those skilled in the art to which the present disclosure pertains. Therefore, unless explicitly defined in this specification, specific terms should not be interpreted in an excessively ideal or formal sense. For example, singular expressions in this specification include plural expressions unless the context clearly dictates otherwise.

In addition, "about", "substantially", and the like in this specification are used in the sense of the numerical value or close to the numerical value when inherent manufacturing and material tolerances are presented in the stated meaning, and are used to prevent unconscionable infringers from unfairly using the disclosure in which precise or absolute numerical values are mentioned to aid the understanding of the present disclosure.

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

First, the wire rod of the present disclosure will be described.

The wire rod for fastening a wind tower according to an embodiment of the present disclosure includes, in percent by weight (wt%), 0.20 to 0.40% of carbon (C), 0.10 to 0.40% of silicon (Si), 0.30 to 0.50% of manganese (Mn), 1.00 to 1.50% of chromium (Cr), 0.01 to 0.10% of titanium (Ti), 0.0005 to 0.0100% of boron (B), and the remainder of iron (Fe) and other inevitable impurities, wherein a microstructure after hot rolling comprises, by area fraction, 30 to 60% of pearlite and 40 to 70% of ferrite, and satisfies Formula (1) below:
$$7.50\le 12.1\mathrm{x}\left[\mathrm{C}\right]+1.5\mathrm{x}\left[\mathrm{Mn}\right]+3.1\mathrm{x}\left[\mathrm{Cr}\right]\le 8.50$$
(wherein [C], [Mn], and [Cr] represent percent by weight (wt%) of each element).

Hereinafter, the reasons for limiting the composition range of each alloying element will be described.

Unless otherwise specified below, the unit is percent by weight (wt%).

The content of carbon (C) may be 0.20 to 0.40%.

C is an element capable of most effectively increasing the strength of a material, and a content of less than 0.20% leads to insufficient hardenability during QT heat treatment. In addition, an excess of C exceeding 0.40% results in an excessive increase in tensile strength during cooling after rolling of the wire rod, which decreases the life of a forging die during cold forging, thereby increasing costs. Preferably, the content of C may be more than 0.20% and 0.39% or less.

The content of silicon (Si) may be 0.10 to 0.40%.

Si is not only useful for deoxidation of steel but also effective in securing strength through solid solution strengthening, but is an element that makes cold forgeability inferior. A Si content of less than 0.10% is insufficient for securing strength through deoxidation and solid solution strengthening of steel, and a Si content exceeding 0.40% may result in inferior cold forgeability due to solid solution strengthening, which is not preferable. Preferably, the content of Si may be 0.10 to 0.35%. More preferably, the content of Si may be 0.15 to 0.35%.

The content of manganese (Mn) may be 0.30 to 0.50%.

Mn is an alloying element advantageous for securing strength by improving hardenability of steel, and plays a role in increasing rollability and reducing brittleness.

Addition of Mn in an amount less than 0.30% makes it difficult to secure sufficient strength, and addition of Mn in an amount exceeding 0.50% tends to generate a hard structure during cooling after hot rolling, and generates a large amount of MnS inclusions, raising a concern that fatigue characteristics may deteriorate. Preferably, the content of Mn may be 0.31 to 0.48%.

The content of chromium (Cr) may be 1.00 to 1.50%.

Cr is an element effective in improving hardenability together with Mn to secure hardness, and may be added in an amount of 1.00% or more. However, an excessive content causes a problem of forming coarse carbides, so the upper limit thereof may be limited to 1.50%. Preferably, the content of Cr may be more than 1.00% and 1.42% or less.

The content of titanium (Ti) may be 0.01 to 0.10%.

Ti combines with nitrogen introduced into steel to form titanium carbonitrides, thereby preventing boron from combining with nitrogen. A titanium content of less than 0.01% is not sufficient to form nitrogen introduced during a steelmaking process into titanium carbonitrides, making it difficult to utilize the effect of boron, and a content exceeding 0.10% forms coarse carbonitrides, causing occurrence of microcracks and making delayed fracture resistance inferior. Preferably, the content of Ti may be more than 0.01% and 0.04% or less.

The content of boron (B) may be 0.0005 to 0.0100%.

B is a hardenability improving element. A B content of less than 0.0005% makes it difficult to expect a hardenability improvement effect, and a content exceeding 0.0100% forms Fe23(CB)6 carbides at grain boundaries, causing brittleness of austenite grain boundaries, which is not preferable. Preferably, the content of B may be more than 0.0005% and 0.0025% or less. More preferably, the content of B may be 0.0015 to 0.0025% or less.

The wire rod for fastening a wind tower according to an embodiment of the present disclosure must satisfy Formula (1) in addition to the above composition, and by satisfying Formula (1) of 7.50 or more and 8.50 or less, Cr-based fine carbides are sufficiently secured to improve cold forgeability according to an optimal tensile strength of the wire rod of 470 MPa to 680 MPa, so that cold forging is possible without spheroidizing heat treatment.

In addition, as Formula (1) is satisfied, a Cr-based carbide ratio of 90 to 95% is satisfied in the final bolt. A Cr-based carbide ratio exceeding 95% results in a decrease in impact toughness to 50 J or less, and a Cr-based carbide ratio of less than 90% results in inferior tensile strength of less than 1000 MPa. That is, satisfaction of Formula (1) and a Cr-based carbide ratio of 90 to 95% ensure tensile strength, and the bolt, which is the final product, can secure a strength of 1000 MPa or more and low-temperature (-20°C) impact toughness of 50 J or more.

In addition, in the wire rod for fastening a wind tower according to an embodiment of the present disclosure, an average austenite grain size may be 30 µm or less. Preferably, the average austenite grain size may be 15 µm to 30 µm or less.

In addition, in the wire rod for fastening a wind tower according to an embodiment of the present disclosure, a tensile strength may be 470 MPa to 680 MPa.

A tensile strength of the wire rod of less than 470 MPa may result in inferior strength and impact toughness of the bolt after QT heat treatment with a tensile strength of less than 1000 MPa and low-temperature (-20°C) impact toughness of less than 50 J. In addition, a tensile strength of the wire rod exceeding 680 MPa results in inferior cold forgeability of the wire rod, requiring spheroidizing heat treatment to resolve this. Preferably, the tensile strength may be 500 MPa to 650 MPa.

Next, a method for manufacturing a wire rod for fastening a wind tower will be described.

A method for manufacturing a wire rod for fastening a wind tower according to an embodiment of the present disclosure includes: heating a billet comprising, in percent by weight (wt%), 0.20 to 0.40% of carbon (C), 0.10 to 0.40% of silicon (Si), 0.30 to 0.50% of manganese (Mn), 1.00 to 1.50% of chromium (Cr), 0.01 to 0.10% of titanium (Ti), 0.0005 to 0.0100% of boron (B), and the remainder of iron (Fe) and other inevitable impurities, and satisfying Formula (1) below, to 950°C to 1200°C; hot rolling at 850°C to 1000°C; and cooling to room temperature at a cooling rate of 0.1°C/s to 2.0°C/s after coiling.$$7.50\le 12.1\mathrm{x}\left[\mathrm{C}\right]+1.5\mathrm{x}\left[\mathrm{Mn}\right]+3.1\mathrm{x}\left[\mathrm{Cr}\right]\le 8.50$$
(wherein [C], [Mn], and [Cr] represent percent by weight (wt%) of each element).

In addition, in a method for manufacturing a wire rod for fastening a wind tower according to another embodiment of the present disclosure, a microstructure after hot rolling may comprise, by area fraction, 30 to 60% of pearlite and 40 to 70% of ferrite.

In addition, in a method for manufacturing a wire rod according to another embodiment of the present disclosure, an average austenite grain size may be controlled to 30 µm or less in the cooling. Satisfaction of a cooling rate after coiling of 0.1°C/s to 2.0°C/s or less controls the average austenite grain size to be 30 µm or less, so that the tensile strength of the wire rod is subsequently in the range of 470 MPa to 680 MPa, thereby preventing cracks from occurring inside the bolt during cold forging without spheroidizing heat treatment.

A cooling rate after coiling exceeding 2.0°C/s results in the tensile strength of the wire rod exceeding 680 MPa, causing cracks inside the bolt during cold forging, so spheroidizing heat treatment is required in advance to prevent this. A cooling rate of less than 0.1°C/s results in an excessively slow cooling rate, and productivity may decrease due to delay in cooling time.

In the method for manufacturing a wire rod for fastening a wind tower according to an embodiment of the present disclosure, spheroidizing heat treatment after the rolling may be omitted. Satisfaction of the range of Formula (1) according to the present disclosure enables control of the tensile strength of the wire rod in the range of 470 MPa to 680 MPa, thereby preventing crack occurrence during subsequent cold forging even without spheroidizing heat treatment.

Next, a bolt manufactured using the wire rod for fastening a wind tower according to the present disclosure will be described.

A bolt for fastening a wind tower according to an embodiment of the present disclosure includes, in percent by weight (wt%), 0.20 to 0.40% of carbon (C), 0.10 to 0.40% of silicon (Si), 0.30 to 0.50% of manganese (Mn), 1.00 to 1.50% of chromium (Cr), 0.01 to 0.10% of titanium (Ti), 0.0005 to 0.0100% of boron (B), and the remainder of iron (Fe) and other inevitable impurities, satisfies Formula (1) below, and has a microstructure after quenching heat treatment and hot-dip galvanizing comprising, by area fraction, 90 to 99% of tempered martensite, 1 to 10% of bainite, and 1% or less of residual austenite.
$$7.50\le 12.1\mathrm{x}\left[\mathrm{C}\right]+1.5\mathrm{x}\left[\mathrm{Mn}\right]+3.1\mathrm{x}\left[\mathrm{Cr}\right]\le 8.50$$
(wherein [C], [Mn], and [Cr] represent percent by weight (wt%) of each element).

In addition, in a bolt for fastening a wind tower according to another embodiment of the present disclosure, a diameter of a bolt body part for fastening a wind tower is 15 mm to 40 mm, and a tensile strength may be 1000 MPa or more.

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

A method for manufacturing a bolt for fastening a wind tower according to an embodiment of the present disclosure includes: preparing a wire rod manufactured by the method for manufacturing a wire rod for fastening a wind tower according to the present disclosure; drawing the wire rod at a drawing reduction ratio of 20% or less; quenching in oil at 20°C to 80°C after heating at 850°C to 950°C; 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 a microstructure comprises, by area fraction, 90 to 99% of tempered martensite, 1 to 10% of bainite, and 1% or less of residual austenite.

Controlling a bolt having a body part diameter of 15 mm to 40 mm to a tensile strength of 1000 MPa or more with general steel materials requires tempering at a low temperature of less than 450°C, which causes a problem that low-temperature (-20°C) impact toughness becomes less than 50 J because fine carbides cannot be sufficiently secured.

Specifically, after drawing a wire rod satisfying the above-described alloy composition, microstructure, and Formula (1), and having an average austenite grain size of 30 µm or less at a drawing reduction ratio of 20% or less, heating to a temperature between 850°C and 950°C, quenching in oil at 20°C to 80°C, and tempering heat treatment at 450°C to 550°C for 3,000 seconds to 10,000 seconds, the bolt for fastening a wind tower can secure a microstructure of 90 to 99% of tempered martensite, 1 to 10% of bainite, and 1% or less of residual austenite.

That is, cold forging is possible without spheroidizing heat treatment of the wire rod, and subsequent tempering at high temperature suppresses thin film-shaped carbides mainly generated at prior austenite grain boundaries and allows spherical and fine carbides to be dispersedly distributed inside and outside the grain boundaries, so that the final bolt having a body part diameter of 15 mm to 40 mm, which has undergone hot-dip galvanizing at 500°C to 550°C, can secure high strength and high impact toughness of a tensile strength of 1000 MPa or more and low-temperature (-20°C) impact toughness of 50 J or more.

Hereinafter, the present disclosure will be described in more detail through examples. However, it should be noted that the following examples are only for illustrating the present disclosure in more detail and are not intended to limit the scope of rights of the present disclosure.

This is because the scope of rights of the present disclosure is determined by the matters described in the claims and matters reasonably inferred therefrom.

Billets having various alloy compositions as shown in Table 1 below were heated to 950°C to 1200°C, and 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. Alloy Composition (wt%) C Si Mn Cr Ti B Inventive Material 1 0.25 0.21 0.42 1.35 0.03 0.0020 Inventive Material 2 0.38 0.15 0.32 1.07 0.03 0.0018 Inventive Material 3 0.21 0.32 0.48 1.42 0.04 0.0022 Inventive Material 4 0.39 0.18 0.41 1.02 0.03 0.0018 Inventive Material 5 0.22 0.15 0.31 1.42 0.03 0.0019 Comparative Material 1 0.18 0.22 0.32 1.17 0.03 0.0021 Comparative Material 2 0.42 0.19 0.31 1.05 0.03 0.0017 Comparative Material 3 0.32 0.14 0.21 1.25 0.03 0.0019 Comparative Material 4 0.35 0.25 0.32 0.89 0.04 0.0021 Comparative Material 5 0.52 0.22 0.33 1.27 0.03 0.0018 Comparative Material 6 0.21 0.21 0.42 1.02 0.03 0.0020 Comparative Material 7 0.26 0.21 0.35 1.04 0.03 0.0020 Comparative Material 8 0.28 0.21 0.39 1.02 0.03 0.0020 Comparative Material 9 0.35 0.21 0.37 1.24 0.03 0.0020 Comparative Material 10 0.32 0.21 0.34 1.48 0.03 0.0020 Comparative 0.37 0.21 0.45 1.45 0.03 0.0020 Material 11 $$7.50\le 12.1\mathrm{x}\left[\mathrm{C}\right]+1.5\mathrm{x}\left[\mathrm{Mn}\right]+3.1\mathrm{x}\left[\mathrm{Cr}\right]\le 8.50$$

Table 2 below shows microstructures and physical properties of the wire rods. Inferior cold forgeability (tensile strength > 680 MPa) is indicated by X, and good cold forgeability (tensile strength 470 to 680 MPa) is indicated by O. In addition, in the case of wire rods and bolts, each specimen was processed according to ASTM E8 standard, and then a tensile test was performed. Formula (1) Wire Rod Tensile Strength (MPa) Cold Forgeability Inventive Material 1 7.84 570 O Inventive Material 2 8.40 625 O Inventive Material 3 7.66 535 O Inventive Material 4 8.50 630 O Inventive Material 5 7.53 530 O Comparative Material 1 6.29 522 O Comparative Material 2 8.80 721 X Comparative Material 3 8.06 612 O Comparative Material 4 7.47 621 O Comparative Material 5 10.72 746 X Comparative Material 6 6.33 465 O Comparative Material 7 6.90 500 O Comparative Material 8 7.14 512 O Comparative Material 9 8.63 682 X Comparative Material 10 8.97 688 X Comparative Material 11 9.65 710 X

Table 3 below shows measured values of austenite grain sizes according to the cooling rate in the cooling step of the wire rod, and shows the presence or absence of microcrack occurrence in subsequent drawing and final bolts. The occurrence of cracks is indicated by O, and non-occurrence is indicated by X. The average austenite grain size of the wire rod was calculated as an average value by measuring at arbitrary three locations according to ASTM E 112 standard. In addition, checking for cracks was performed by a delayed fracture simulation method of observing the presence or absence of microcracks in a thread, which is a stress concentration part, before/after immersing in a 5% hydrochloric acid + 95% distilled water solution for 10 minutes after fastening to a target steel plate after heat treatment of the final product. Cooling Rate (°C /s) Average Austenite Grain Size (µm) Microcrack Presence Inventive Material 1 0.4 18 X Inventive Material 2 0.6 22 X Inventive Material 3 0.3 16 X Inventive Material 4 1.2 26 X Inventive Material 5 1.6 28 X Comparative Material 1 0.01 4 X Comparative Material 2 0.03 \6 X Comparative Material 3 0.05 7 X Comparative 0.07 9 X Material 4 Comparative Material 5 0.08 11 X Comparative Material 6 2.2 33 O Comparative Material 7 2.5 36 O Comparative Material 8 2.8 40 O Comparative Material 9 3.2 42 O Comparative Material 10 4.1 46 O Comparative Material 11 4.5 52 O

Referring to Table 3, it can be confirmed that a cooling rate in the cooling step exceeding 2.0°C/s causes the average austenite grain size to exceed 30 µm, causing microcracks in the final bolt. Next, the hot-rolled wire rod was drawn at a drawing reduction ratio of 20% or less, heated at 940°C for 3600 seconds, and then quenched by immersion in oil at 70°C. Thereafter, tempering heat treatment of holding at 450°C to 550°C for 3,000 seconds to 10,000 seconds was performed, and then hot-dip galvanizing at 500°C to 550°C was performed to manufacture a bolt. The impact toughness of the bolt was measured by processing a V-notch specimen of ASTM E23 standard and performing a Charpy impact test at low temperature (-20°C), and is shown in Table 4 below.

In addition, the Cr-based carbide ratio of the bolt for fastening a wind tower was measured by ASTM E 552 standard method using an image analyzer. Bolt Category Formula (1) Tensile Strength (MPa) Low-Temperature (-20°C) Impact Toughness (J) Cr-based Carbide Ratio (%) Inventive Material 1 7.84 1050 82 93.3 Inventive Material 2 8.40 1121 72 94.2 Inventive Material 3 7.66 1042 102 92.5 Inventive Material 4 8.50 1135 68 94.8 Inventive Material 5 7.53 1031 125 90.2 Comparative Material 1 6.29 895 156 82.5 Comparative Material 2 8.80 1155 45 95.3 Comparative Material 3 8.06 985 69 93.6 Comparative Material 4 7.47 973 75 88.2 Comparative Material 5 10.72 1189 42 98.5 Comparative Material 6 6.33 950 78 84.2 Comparative Material 7 6.90 975 65 86.8 Comparative Material 8 7.14 990 78 87.5 Comparative Material 9 8.63 1180 48 95.1 Comparative Material 10 8.97 1210 45 96.5 Comparative Material 11 9.65 1230 35 97.2

It can be confirmed that Inventive Examples 1 to 5 satisfied all the ranges of the present disclosure for composition, average austenite grain size, and Formula (1), thereby having a tensile strength of 1000 MPa or more and low-temperature (-20°C) impact toughness of 50 J or more, whereas Comparative Examples 1 to 11, which did not satisfy the composition, average austenite grain size, or Formula (1), had a tensile strength of less than 1000 MPa or low-temperature (-20°C) impact toughness of less than 50 J. In the case of Comparative Material 1, a C content of less than 0.20% and a Formula (1) value of less than 7.50 resulted in inferior tensile strength of the final product bolt at less than 1000 MPa.

In the case of Comparative Materials 2 and 5, a C content exceeding 0.40% and a Formula (1) value exceeding 8.50 resulted in the tensile strength of the wire rod exceeding 680 MPa. Accordingly, it can be confirmed that cold forgeability was inferior, requiring spheroidizing heat treatment.

In the case of Comparative Material 3, satisfaction of the range of Formula (1) according to the present disclosure but a Mn content of less than 0.30% resulted in inferior tensile strength of the bolt at less than 1000 MPa.

In the case of Comparative Material 4, a Cr content of less than 1.00% and failure to satisfy the range of Formula (1) of the present disclosure resulted in inferior tensile strength of the bolt at less than 1000 MPa.

Furthermore, in the case of Comparative Materials 6 to 11, physical properties were compared by varying the contents of C, Mn, and Cr of Formula (1) of the present disclosure in the alloy composition of Inventive Material 1. Comparative Materials 6 to 8 satisfied the range of the alloy composition according to the present disclosure, but a Formula (1) value of less than 7.50 failed to sufficiently secure Cr-based carbides, so the tensile strength of the final bolt was inferior.

In addition, Comparative Materials 9 to 11 satisfied the range of the alloy composition according to the present disclosure, but as a Formula (1) value exceeded 8.50, the tensile strength of the wire rod exceeded 680 MPa, resulting in inferior cold forgeability, and accordingly, the low-temperature (-20°C) impact toughness of the bolt was also inferior at 50 J or less.

Although exemplary embodiments of the present disclosure have been described above, the present disclosure is not limited thereto, and it will be understood by those skilled in the art that various changes and modifications can be made within the scope without departing from the concept and scope of the claims described below.