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(11) | EP 4 800 142 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
| published in accordance with Art. 153(4) EPC |
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| (54) | WIRE ROD, BOLT AND MANUFACTURING METHOD THEREOF |
| (57) A wire rod according to an embodiment of the present disclosure includes, in percent
by weight (wt%), 0.20 to 0.35% of carbon (C), 0.05 to 0.50% of silicon (Si), 0.30
to 0.60% of manganese (Mn), 1.00 to 1.60% of chromium (Cr), 0.02 to 0.10% of aluminum
(Al), 0.02 to 0.05% of titanium (Ti), and 0.0005 to 0.0100% of boron (B), the remainder
of iron (Fe) and inevitable impurities, satisfies Formula (1) below, and includes
30% to 60% of pearlite and 40% to 70% of ferrite in an area fraction as a microstructure
after hot rolling. (Where, [C], [Mn], and [Cr] refer to the weight percent of each element). |
[Technical Field]
[0001] The present disclosure relates to a wire rod, a bolt, and a method for manufacturing the same.
[Background Art]
[0002] Existing JS-SCM440 steel, which is a raw material for bolts used for fastening wind towers, requires spheroidizing heat treatment for cold forging, and economic feasibility is reduced because a high-priced Mo element is added. Meanwhile, JS-SCr420B steel, which enables cold forging without heat treatment after rolling of a wire rod, has 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 reduces low-temperature (-40°C) impact toughness.
[Disclosure]
[Technical Problem]
[0003] An objective of the present disclosure for solving the above-mentioned problems is to improve cold forgeability of a hot-rolled material by forming fine carbides during tempering even without spheroidizing heat treatment.
[0004] In addition, the present disclosure is to provide a wire rod, a bolt, and a method for manufacturing the same capable of simultaneously securing cold forgeability and Vickers hardness.
[0005] 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.
[Technical Solution]
[0006] A wire rod according to an embodiment of the present disclosure includes, in percent by weight (wt%), 0.20 to 0.35% of carbon (C), 0.05 to 0.50% of silicon (Si), 0.30 to 0.60% of manganese (Mn), 1.00 to 1.60% of chromium (Cr), 0.02 to 0.10% of aluminum (Al), 0.02 to 0.05% of titanium (Ti), and 0.0005 to 0.0100% of boron (B), the remainder of iron (Fe) and inevitable impurities, satisfies Formula (1) below, and includes 30% to 60% of pearlite and 40% to 70% of ferrite in an area fraction as a microstructure after hot rolling.
[0008] Also, the wire rod according to an embodiment of the present disclosure may satisfy Formula (2) below.
[0010] Also, in the wire rod according to an embodiment of the present disclosure, an average austenite grain size of the wire rod may be 25 µm or less.
[0011] Also, in the wire rod according to an embodiment of the present disclosure, a Vickers hardness of the wire rod may be 180 HV or less.
[0012] A method for manufacturing a wire rod according to an embodiment of the present disclosure includes: heating a billet including, in percent by weight (wt%), 0.20 to 0.35% of carbon (C), 0.05 to 0.50% of silicon (Si), 0.30 to 0.60% of manganese (Mn), 1.00 to 1.60% of chromium (Cr), 0.02 to 0.10% of aluminum (Al), 0.02 to 0.05% of titanium (Ti), and 0.0005 to 0.0100% of boron (B), the remainder of iron (Fe) and inevitable impurities, and satisfying Formula (1) below to 950°C to 1200°C; hot-rolling at 850°C to 950°C; coiling at 750°C to 850°C; and cooling to room temperature at a cooling rate of 0.1°C/s to 0.5°C/s.
[0014] Also, in the method for manufacturing the wire rod according to another embodiment of the present disclosure, a microstructure of the wire rod after the hot-rolling may include 30% to 60% of pearlite and 40% to 70% of ferrite in an area fraction.
[0015] Also, in the method for manufacturing the wire rod according to another embodiment of the present disclosure, an average austenite grain size may be controlled to 25 µm or less in the cooling.
[0016] Also, in the method for manufacturing the wire rod according to another embodiment of the present disclosure, the method is performed without a spheroidizing heat treatment after the hot rolling.
[0017] A bolt according to an embodiment of the present disclosure includes, in percent by weight (wt%), 0.20 to 0.35% of carbon (C), 0.05 to 0.50% of silicon (Si), 0.30 to 0.60% of manganese (Mn), 1.00 to 1.60% of chromium (Cr), 0.02 to 0.10% of aluminum (Al), 0.02 to 0.05% of titanium (Ti), and 0.0005 to 0.0100% of boron (B), the remainder of iron (Fe) and inevitable impurities, satisfies Formula (1) below, and includes 90% to 99% of tempered martensite, 1% to 10% of bainite, and 1% or less of residual austenite in an area fraction as a microstructure after quenching heat treatment and hot-dip galvanizing.
[0019] Also, the bolt according to another embodiment of the present disclosure may satisfy Formula (2) below.
[0021] Also, in the bolt according to another embodiment of the present disclosure, a diameter of a bolt body portion may be 15 mm to 40 mm, and a Vickers hardness may be 320 HV or more.
[0022] Also, the bolt according to another embodiment of the present disclosure may have a low-temperature (-40°C) impact toughness of 50 J or more.
[0023] A method for manufacturing a bolt according to an embodiment of the present disclosure includes: preparing the wire rod according to the present disclosure; drawing the wire rod at a drawing reduction ratio of 20% or less; heating at 850°C to 950°C and then quenching at a temperature of 20°C to 80°C; performing tempering heat treatment at 450°C to 550°C for 3000 seconds to 10000 seconds; and performing hot-dip galvanizing at a temperature of 500°C to 550°C, wherein a microstructure includes 90% to 99% of tempered martensite, 1% to 10% of bainite, and 1% or less of residual austenite in an area fraction.
[Advantageous Effects]
[0024] The wire rod according to an embodiment of the present disclosure secures a Vickers hardness of 180 HV or less, thereby improving cold forgeability, so that cold forging is possible without spheroidizing heat treatment.
[0025] In addition, the bolt according to an embodiment of the present disclosure may have a Vickers hardness of 320 HV or more and an impact toughness at -40°C of 50 J or more.
[Mode for Invention]
[0026] Hereinafter, preferred embodiments of the present disclosure will be described. However, the embodiments of the present disclosure may be modified in 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 fully describe the present disclosure to those skilled in the art.
[0027] Terms used in this application are only used to describe specific examples. Therefore, for example, singular expressions include plural expressions unless the context clearly indicates that they must be singular. In addition, it should be noted that terms such as "include" or "have" used in this application are used to clearly indicate the existence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to preliminarily exclude the existence of other features, steps, functions, components, or combinations thereof.
[0028] Meanwhile, unless otherwise defined, all terms used herein should be viewed as having the same meaning as commonly understood by those skilled in the art to which the present disclosure belongs. Therefore, unless clearly defined herein, specific terms should not be interpreted in an excessively ideal or formal sense. For example, singular expressions herein include plural expressions unless the context clearly indicates an exception.
[0029] In addition, "about", "substantially", etc. in the present specification are used in the sense of at or close to the numerical value when manufacturing and material tolerances inherent in the stated meaning are suggested, 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.
[0030] The basic principle of the present disclosure is designed to limit the C content to minimize an increase in Vickers hardness after hot rolling, limit the Mn content to reduce a solid solution strengthening effect of the hot-rolled material, and 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 a decrease in strength after hot-dip galvanizing at a high temperature of 500°C or higher, thereby contributing to improving softening resistance of tempering.
[0032] The wire rod according to an embodiment of the present disclosure includes, in percent by weight (wt%), 0.20 to 0.35% of carbon (C), 0.05 to 0.50% of silicon (Si), 0.30 to 0.60% of manganese (Mn), 1.00 to 1.60% of chromium (Cr), 0.02 to 0.10% of aluminum (Al), 0.02 to 0.05% of titanium (Ti), and 0.0005 to 0.0100% of boron (B), the remainder of iron (Fe) and inevitable impurities, satisfies Formula (1) below, and includes 30% to 60% of pearlite and 40% to 70% of ferrite in an area fraction as a microstructure after hot rolling.
[0034] Hereinafter, reasons for limiting the component ranges of each alloying element are described. Hereinafter, the unit is wt% unless otherwise stated.
[0036] C is an element capable of most effectively increasing the strength of a material, and securing hardenability during QT heat treatment is not sufficient at a content of less than 0.20%. In addition, exceeding 0.35% excessively increases tensile strength during cooling after rolling of the wire rod, and may reduce the lifespan of a forging die during cold forging, thereby increasing costs. Preferably, the content of C may be 0.21 to 0.34%.
[0038] Si is not only useful for deoxidation of steel, but also effective in securing strength through solid solution strengthening, but is an element that deteriorates cold forgeability. A content of Si of less than 0.05% is insufficient for securing strength through deoxidation and solid solution strengthening of steel, and a content exceeding 0.50% is not preferable because cold forgeability due to solid solution strengthening may be inferior. Preferably, the content of Si may be 0.06 to 0.29%. More preferably, the content of Si may be 0.16 to 0.22%.
[0040] Mn is an alloy element advantageous for securing strength by improving hardenability of steel, and plays a role in increasing rollability and reducing brittleness. When Mn is added in an amount of less than 0.30%, it is difficult to secure sufficient strength, and when added in an amount exceeding 0.60%, hard structures are likely to occur during cooling after hot rolling, and a large amount of MnS inclusions are generated, which may reduce fatigue properties. Preferably, the content of Mn may be 0.31 to 0.48%.
[0042] Cr is an element that is 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 may cause the formation of coarse carbides, so the upper limit may be limited to 1.60%. Preferably, the content of Cr may be more than 1.10% and 1.49% or less.
[0044] Al is widely used as a deoxidizer in a steelmaking process, and is effective in refining austenite grains by AlN formed by reacting with N. When added in an amount of less than 0.02%, the number of nitrogen compounds is not sufficient, so a grain refinement effect is reduced, and when added in an amount exceeding 0.10%, generation of non-metallic inclusions such as alumina is excessive, thereby intensifying the occurrence of defects in steel, so it is necessary to limit the content. Preferably, the content may be more than 0.02% and 0.04% or less.
[0046] Ti combines with nitrogen introduced into steel to form titanium carbonitrides, preventing boron from combining with nitrogen. When the content of titanium is less than 0.02%, it is difficult to utilize the effect of boron because it is not sufficient to form nitrogen introduced during the steelmaking process into titanium carbonitrides, and when the content exceeds 0.05%, coarse carbonitrides are formed, causing microcracks and deteriorating delayed fracture resistance. Preferably, the content of Ti may be more than 0.02% and 0.04% or less.
[0048] B is a hardenability improving element. When the content of B is less than 0.0005%, it is difficult to expect a hardenability improvement effect, and when the content exceeds 0.0100%, Fe23(CB)6 carbides are formed 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 0.0090% or less. More preferably, the content of B may be 0.0018% to 0.0022%.
[0049] The wire rod according to an embodiment of the present disclosure should satisfy Formula (1) in addition to the above composition, and satisfying 5.90 or more and 6.30 or less of Formula (1) sufficiently secures Cr-based fine carbides, so that the wire rod secures a Vickers hardness of 180 HV or less while improving cold forgeability, thereby enabling cold forging without spheroidizing heat treatment. In addition, satisfying Formula (1) satisfies a Cr-based carbide ratio of 90% to 95% in the final bolt. That is, when Formula (1) is satisfied and the Cr-based carbide ratio is 90% to 95%, a Vickers hardness of 320 HV or more may be secured.
[0050] Also, the wire rod according to an embodiment of the present disclosure may satisfy Formula (2) below.
[0052] Formula (2) relates to Ti and B, and Ti is also used as an element for solid-solving nitrogen to secure an amount of free boron in boron steel. Therefore, Formula (2) should satisfy 0.99 to 1.36 in order to fix nitrogen for securing the amount of free boron (free B) for grain boundary strengthening or hardenability improvement through boron, and to improve hardenability and impact toughness of steel through grain refinement by precipitation as fine TiN. When Formula (2) is less than 0.99, the contents of Ti and B become insufficient, and when it exceeds 1.36, coarse carbonitrides are formed. Therefore, failing to satisfy the range of Formula (2) prevents the impact toughness at -40°C of the final bolt from satisfying 50 J or more.
[0053] Also, the wire rod according to an embodiment of the present disclosure may have an average austenite grain size of 25 µm or less. Preferably, it may be 1 µm to 24 µm.
[0054] Also, the wire rod according to an embodiment of the present disclosure may have a Vickers hardness of 180 HV or less. When the Vickers hardness of the wire rod exceeds 180 HV, the cold forgeability of the wire rod is inferior, requiring spheroidizing heat treatment to resolve this. Preferably, the Vickers hardness may be 150 HV to 180 HV.
[0056] A method for manufacturing a wire rod according to an embodiment of the present disclosure includes: heating a billet including, in percent by weight (wt%), 0.20 to 0.35% of carbon (C), 0.05 to 0.50% of silicon (Si), 0.30 to 0.60% of manganese (Mn), 1.00 to 1.60% of chromium (Cr), 0.02 to 0.10% of aluminum (Al), 0.02 to 0.05% of titanium (Ti), and 0.0005 to 0.0100% of boron (B), the remainder of iron (Fe) and inevitable impurities, and satisfying Formula (1) below to 950°C to 1200°C; hot-rolling at 850°C to 950°C; coiling at 750°C to 850°C; and cooling to room temperature at a cooling rate of 0.1°C/s to 0.5°C/s.
[0058] Also, in the method for manufacturing the wire rod according to another embodiment of the present disclosure, a microstructure after hot rolling may include 30% to 60% of pearlite and 40% to 70% of ferrite in an area fraction.
[0059] Also, in the method for manufacturing the wire rod according to another embodiment of the present disclosure, an average austenite grain size may be controlled to 25 µm or less in the cooling. Satisfying the cooling rate of 0.1°C/s to 0.5°C/s after coiling controls the average austenite grain size to be 25 µm or less, so that the Vickers hardness of the wire rod is subsequently in the range of 180 HV or less, thereby preventing cracks from occurring inside the final bolt after cold forging without spheroidizing heat treatment.
[0060] When the cooling rate after coiling exceeds 0.5°C/s, the Vickers hardness of the wire rod exceeds 180 HV, causing microcracks inside the final bolt after cold forging, so spheroidizing heat treatment is required in advance to prevent this. When the cooling rate is less than 0.1°C/s, the cooling rate becomes too slow, which may reduce productivity due to the delay in cooling time.
[0061] Also, in the method for manufacturing the wire rod according to another embodiment of the present disclosure, spheroidizing heat treatment may be omitted after the rolling. Satisfying the range of Formula (1) according to the present disclosure enables controlling the Vickers hardness of the wire rod to a range of 180 HV or less through an appropriate ratio of Cr-based carbides, thereby preventing crack occurrence during subsequent cold forging even without spheroidizing heat treatment.
[0062] Next, a bolt manufactured using the wire rod according to the present disclosure is described.
[0063] A bolt according to an embodiment of the present disclosure includes, in percent by weight (wt%), 0.20 to 0.35% of carbon (C), 0.05 to 0.50% of silicon (Si), 0.30 to 0.60% of manganese (Mn), 1.00 to 1.60% of chromium (Cr), 0.02 to 0.10% of aluminum (Al), 0.02 to 0.05% of titanium (Ti), and 0.0005 to 0.0100% of boron (B), the remainder of iron (Fe) and inevitable impurities, satisfies Formula (1) below, and includes 90% to 99% of tempered martensite, 1% to 10% of bainite, and 1% or less of residual austenite in an area fraction as a microstructure after quenching heat treatment and hot-dip galvanizing.
[0065] Also, the bolt according to another embodiment of the present disclosure may satisfy Formula (2) below.
[0067] Also, in the bolt according to another embodiment of the present disclosure, a diameter of a bolt body portion may be 15 mm to 40 mm, and a Vickers hardness may be 320 HV or more. Preferably, it may be 320 HV to 400 HV.
[0068] Also, the bolt according to another embodiment of the present disclosure may have a low-temperature (-40°C) impact toughness of 50 J or more.
[0069] A method for manufacturing a bolt according to an embodiment of the present disclosure includes: preparing the wire rod according to the present disclosure; drawing the wire rod at a drawing reduction ratio of 20% or less; heating at 850°C to 950°C and then quenching in oil at 20°C to 80°C; performing tempering heat treatment at 450°C to 550°C for 3000 seconds to 10000 seconds; and performing hot-dip galvanizing at a temperature of 500°C to 550°C, wherein a microstructure includes 90% to 99% of tempered martensite, 1% to 10% of bainite, and 1% or less of residual austenite in an area fraction.
[0070] When controlling a bolt having a body portion diameter of 15 mm to 40 mm to a Vickers hardness of 320 HV or more with general steel, a temperature of 450°C to 550°C is preferable for tempering heat treatment.
[0071] Specifically, after drawing a wire rod satisfying the above-described alloy composition, microstructure, and Formula (1) and having an average austenite grain size of 25 µm or less at a drawing reduction ratio of 20% or less, heating at a temperature between 850°C and 950°C, quenching in oil at 20°C to 80°C, and performing tempering heat treatment at 450°C to 550°C for 3,000 seconds to 10,000 seconds, the bolt may secure a microstructure of 90% to 99% of tempered martensite, 1% to 10% of bainite, and 1% or less of residual austenite.
[0072] That is, cold forging is possible without spheroidizing heat treatment of the wire rod, and subsequent tempering at a high temperature suppresses thin film-type carbides mainly generated at prior austenite grain boundaries and allows spheroidized and fine carbides to be dispersed and distributed inside and outside the grain boundaries, so that the final bolt having a body portion diameter of 15 mm to 40 mm, which has undergone hot-dip galvanizing at 500°C to 550°C, may secure high strength and high impact toughness of a Vickers hardness of 320 HV or more and a low-temperature (-40°C) impact toughness of 50 J or more.
[0073] 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 the present disclosure. This is because the scope of the rights of the present disclosure is determined by matters described in the scope of claims and matters reasonably inferred therefrom.
{Example}
[0074] Billets having alloy compositions as shown in Table 1 below were heated to 950°C to 1200°C, then hot-rolled at 850°C to 950°C, and coiled at 800°C. The cooling rate after coiling was 0.1°C/s to 0.5°C/s.
[Table 1]
| Alloy composition (wt%) | |||||||
| C | Si | Mn | Cr | Ti | B | Al | |
| Inventive Steel 1 | 0.24 | 0.22 | 0.38 | 1.42 | 0.03 | 0.0021 | 0.03 |
| Inventive Steel 2 | 0.33 | 0.16 | 0.35 | 1.10 | 0.03 | 0.0021 | 0.03 |
| Inventive Steel 3 | 0.22 | 0.22 | 0.49 | 1.36 | 0.04 | 0.0018 | 0.03 |
| Inventive Steel 4 | 0.34 | 0.19 | 0.30 | 1.12 | 0.03 | 0.0022 | 0.03 |
| Inventive Steel 5 | 0.21 | 0.17 | 0.31 | 1.49 | 0.03 | 0.0019 | 0.03 |
| Comparative Steel 1 | 0.04 | 0.03 | |||||
| 0.19 | 0.22 | 0.32 | 1.21 | 0.0017 | |||
| Comparative Steel 2 | 0.03 | ||||||
| 0.41 | 0.19 | 0.33 | 1.11 | 0.03 | 0.0018 | ||
| Comparative Steel 3 | 0.03 | ||||||
| 0.32 | 0.14 | 0.21 | 1.25 | 0.03 | 0.0016 | ||
| Comparative Steel 4 | 0.03 | ||||||
| 0.34 | 0.25 | 0.35 | 0.91 | 0.04 | 0.0030 | ||
| Comparative Steel 5 | 0.03 | ||||||
| 0.51 | 0.22 | 0.41 | 1.21 | 0.03 | 0.0018 | ||
| Comparative Steel 6 | 0.03 | ||||||
| 0.22 | 0.22 | 0.37 | 1.42 | 0.03 | 0.0006 | ||
| Comparative Steel 7 | 0.03 | ||||||
| 0.25 | 0.22 | 0.35 | 1.42 | 0.04 | 0.0035 | ||
| Comparative Steel 8 | 0.03 | ||||||
| 0.23 | 0.22 | 0.38 | 1.42 | 0.03 | 0.0052 | ||
[0075] Table 2 below shows microstructures and physical properties of the wire rods, and for cold forgeability, X is indicated when cold forgeability is inferior as the Vickers hardness of the wire rod exceeds 180 HV, and O is indicated when good cold forgeability is shown according to the Vickers hardness of 180 HV or less. In addition, in the case of Vickers hardness of the wire rod and the bolt, each specimen was measured using a Vickers hardness tester.
[Table 2]
| Formula (1) | Formula (2) | Vickers hardness (HV) | Cold forgeability | |
| Inventive Steel 1 | 6.20 | 1.01 | 169 | O |
| Inventive Steel 2 | 6.19 | 1.01 | 177 | O |
| Inventive Steel 3 | 5.98 | 1.22 | 167 | O |
| Inventive Steel 4 | 6.27 | 1.02 | 178 | O |
| Inventive Steel 5 | 6.02 | 0.99 | 166 | O |
| Comparative Steel 1 | 5.14 | 1.21 | 163 | O |
| Comparative Steel 2 | 6.93 | 0.97 | 225 | X |
| Comparative Steel 3 | 6.32 | 0.94 | 186 | X |
| Comparative Steel 4 | 5.80 | 1.37 | 178 | O |
| Comparative Steel 5 | 8.18 | 0.97 | 233 | X |
| Comparative Steel 6 | 6.00 | 0.82 | 165 | O |
| Comparative Steel 7 | 6.26 | 1.43 | 158 | O |
| Comparative Steel 8 | 6.10 | 1.39 | 162 | O |
5.9 ≤ 9.2x[C] + 1.2x(0.9x[Mn] + 2.1x[Cr]) ≤ 6.3 Formula (2): 0.99 ≤ 25x[Ti] + 124x[B]
≤ 1.36
[0076] Table 3 below shows measured values of austenite grain sizes according to cooling rates in the cooling step of the wire rods. In addition, the presence or absence of microcracks in the final bolt by the following manufacturing method is shown. O is indicated when cracks occurred, and X is indicated when cracks did not occur. The average grain size of austenite of the wire rod was measured for random 3 locations according to the ASTM E 112 standard and calculated as an average value.
[0077] In addition, the confirmation of the presence or absence of cracks was proceeded 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.
[Table 3]
| Cooling rate (°C/s) | Average austenite grain size (µm) | Presence of microcracks | |
| Inventive Steel 1 | 0.15 | 5 | X |
| Inventive Steel 2 | 0.24 | 22 | X |
| Inventive Steel 3 | 0.42 | 16 | X |
| Inventive Steel 4 | 0.28 | 20 | X |
| Inventive Steel 5 | 0.45 | 24 | X |
| Comparative Steel 1 | 0.08 | 4 | X |
| Comparative Steel 2 | 0.03 | 6 | X |
| Comparative Steel 3 | 0.05 | 7 | X |
| Comparative Steel 4 | 0.07 | 9 | X |
| Comparative Steel 5 | 0.08 | 11 | X |
| Comparative Steel 6 | 2.5 | 28 | O |
| Comparative Steel 7 | 3.2 | 36 | O |
| Comparative Steel 8 | 4.4 | 40 | O |
[0078] For the manufacture of the bolt, the wire rod was drawn at a drawing reduction ratio of 20% or less, heated at 940°C for 3600 seconds, and then immersed in oil at 70°C and quenched. Thereafter, tempering heat treatment was performed maintaining at 450°C to 550°C for 3000 seconds to 10000 seconds, and then hot-dip galvanizing was performed at 500°C to 550°C 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 a low temperature (-40°C), and is shown in Table 4 below. Also, the Cr-based carbide ratio of the bolt was measured by the ASTM E 552 standard method using an Image analyzer.
[Table 4]
| Bolt | |||||
| Category | Formula (1) | Formula (2) | Vickers hardness (HV) | Low temperature (-40°C) impact toughness (J) | Cr-based carbide ratio (%) |
| Inventive Steel 1 | 6.20 | 1.01 | 332 | 68 | 93.3 |
| Inventive Steel 2 | 6.19 | 1.01 | 346 | 55 | 94.2 |
| Inventive Steel 3 | 5.98 | 1.22 | 322 | 76 | 92.5 |
| Inventive Steel 4 | 6.27 | 1.02 | 352 | 57 | 94.8 |
| Inventive Steel 5 | 6.02 | 0.99 | 325 | 82 | 90.2 |
| Comparative Steel 1 | 5.14 | 1.21 | 280 | 75 | 82.5 |
| Comparative Steel 2 | 6.93 | 0.97 | 361 | 25 | 95.3 |
| Comparative Steel 3 | 6.32 | 0.94 | 311 | 45 | 96.6 |
| Comparative Steel 4 | 5.80 | 1.37 | 302 | 43 | 88.2 |
| Comparative Steel 5 | 8.18 | 0.97 | 372 | 22 | 98.5 |
| Comparative Steel 6 | 6.00 | 0.82 | 324 | 32 | 94.5 |
| Comparative Steel 7 | 6.26 | 1.43 | 354 | 18 | 92.8 |
| Comparative Steel 8 | 6.10 | 1.39 | 362 | 15 | 93.6 |
[0079] Inventive Steels 1 to 5 satisfied all the ranges of the alloy composition, average austenite grain size, Formula (1), and Formula (2) of the wire rod according to the present disclosure, so that the final bolts could have a Vickers hardness of 320 HV or more and a low-temperature (-40°C) impact toughness value of 50 J or more. On the other hand, it can be confirmed that Comparative Examples 1 to 5, which do not satisfy the composition, average austenite grain size, Formula (1) or Formula (2), have a Vickers hardness of less than 320 HV or a low-temperature (-40°C) impact toughness of less than 50 J.
[0080] Furthermore, in the case of Comparative Steels 6 to 8, the ranges of the alloy composition and Formula (1) according to the present disclosure were satisfied, but Formula (2) was not satisfied. Specifically, in the case of Comparative Steel 6, the value of Formula (2) was less than 0.99, so a sufficient content of free boron was not secured, resulting in inferior impact toughness, and in the case of Comparative Steels 7 and 8, the value of Formula (2) exceeded 1.36, resulting in the generation of coarse carbonitrides. Accordingly, it was confirmed that Comparative Steels 6 to 8 were inferior as the impact toughness at -40°C was less than 50 J.
[0081] In the above, exemplary embodiments of the present disclosure have been described, but the present disclosure is not limited thereto, and those skilled in the art will understand that various changes and modifications are possible without departing from the concept and scope of the claims described below.
1. A wire rod comprising, in percent by weight (wt%), 0.20 to 0.35% of carbon (C), 0.05
to 0.50% of silicon (Si), 0.30 to 0.60% of manganese (Mn), 1.00 to 1.60% of chromium
(Cr), 0.02 to 0.10% of aluminum (Al), 0.02 to 0.05% of titanium (Ti), and 0.0005 to
0.0100% of boron (B), the remainder of iron (Fe) and inevitable impurities, wherein
Formula (1) below is satisfied, and
a microstructure after hot rolling comprises 30% to 60% of pearlite and 40% to 70% of ferrite in an area fraction.
(Where, [C], [Mn], and [Cr] refer to the weight percent of each element).
a microstructure after hot rolling comprises 30% to 60% of pearlite and 40% to 70% of ferrite in an area fraction.
(Where, [C], [Mn], and [Cr] refer to the weight percent of each element).
2. The wire rod of claim 1, wherein Formula (2) below is satisfied.
(Where, [Ti] and [B] refer to the weight percent of each element).
(Where, [Ti] and [B] refer to the weight percent of each element).
3. The wire rod of claim 1, wherein an average austenite grain size of the wire rod is
25 µm or less.
4. The wire rod of claim 1, wherein a Vickers hardness of the wire rod is 180 HV or less.
5. A method for manufacturing a wire rod, comprising:
heating a billet comprising, in percent by weight (wt%), 0.20 to 0.35% of carbon (C), 0.05 to 0.50% of silicon (Si), 0.30 to 0.60% of manganese (Mn), 1.00 to 1.60% of chromium (Cr), 0.02 to 0.10% of aluminum (Al), 0.02 to 0.05% of titanium (Ti), and 0.0005 to 0.0100% of boron (B), the remainder of iron (Fe) and inevitable impurities, and satisfying Formula (1) below to 950°C to 1200°C;
hot-rolling at 850°C to 950°C;
coiling at 750°C to 850°C; and
cooling to room temperature at a cooling rate of 0.1°C/s to 0.5°C/s.
(Where, [C], [Mn], and [Cr] refer to the weight percent of each element).
6. The method of claim 5, wherein a microstructure of the wire rod after the hot-rolling
comprises 30% to 60% of pearlite and 40% to 70% of ferrite in an area fraction.
7. The method of claim 5, wherein an average austenite grain size is controlled to 25
µm or less in the cooling.
8. The method of claim 5, wherein the method is performed without a spheroidizing heat
treatment after the hot rolling.
9. A bolt comprising, in percent by weight (wt%), 0.20 to 0.35% of carbon (C), 0.05 to
0.50% of silicon (Si), 0.30 to 0.60% of manganese (Mn), 1.00 to 1.60% of chromium
(Cr), 0.02 to 0.10% of aluminum (Al), 0.02 to 0.05% of titanium (Ti), and 0.0005 to
0.0100% of boron (B), the remainder of iron (Fe) and inevitable impurities, wherein
Formula (1) below is satisfied, and
a microstructure after quenching heat treatment and hot-dip galvanizing comprises 90% to 99% of tempered martensite, 1% to 10% of bainite, and 1% or less of residual austenite in an area fraction.
(Where, [C], [Mn], and [Cr] refer to the weight percent of each element).
a microstructure after quenching heat treatment and hot-dip galvanizing comprises 90% to 99% of tempered martensite, 1% to 10% of bainite, and 1% or less of residual austenite in an area fraction.
(Where, [C], [Mn], and [Cr] refer to the weight percent of each element).
10. The bolt of claim 9, wherein Formula (2) below is satisfied.
(Where, [Ti] and [B] refer to the weight percent of each element).
(Where, [Ti] and [B] refer to the weight percent of each element).
11. The bolt of claim 9, wherein a diameter of a bolt body portion is 15 to 40 mm, and
a Vickers hardness is 320 HV or more.
12. The bolt of claim 9, wherein a low-temperature (-40°C) impact toughness is 50 J or
more.
13. A method for manufacturing a bolt, comprising:
preparing the wire rod of any one of claims 1 to 4;
drawing the wire rod at a drawing reduction ratio of 20% or less;
heating at 850°C to 950°C and then quenching at a temperature of 20°C to 80°C;
performing tempering heat treatment at 450°C to 550°C for 3000 seconds to 10000 seconds; and
performing hot-dip galvanizing at a temperature of 500°C to 550°C,
wherein a microstructure comprises 90% to 99% of tempered martensite, 1% to 10% of bainite, and 1% or less of residual austenite in an area fraction.