High-strength bolts

Abstract

The purpose of the present invention is to provide a high strength bolt having excellent quenching crack resistance and fatigue characteristics. The present invention is capable of providing a high strength bolt which has a tempered martensite structure, has a composition containing 0.36-0.45 mass% of carbon (C), 1.75-2.00 mass% of silicon (Si), 0.90-1.30 mass% of chromium (Cr), 0.15-0.50 mass% of manganese (Mn) and 1.50-2.00 mass% of molybdenum (Mo) and containing a total of 0.015 mass% or less of phosphorus (P) and sulfur (S) as impurities, with the remainder comprising iron (Fe) and unavoidable impurities, and in which the areal percentage of ferrite is 3.00% or less in a region up to 100 μm in the depth direction from a surface of a thread part.

Description

【Technical Field】 【0001】 The present invention relates to high-strength bolts. More specifically, it relates to high-strength bolts having excellent crack resistance and fatigue characteristics. 【Background Art】 【0002】 As a fastening member for automobiles, high-strength bolts having a tensile strength of 1200 MPa or more are required. 【0003】 For example, in Japanese Patent No. 6988922 (U.S. Patent No. 11708622), 0.50% by mass or more and 0.65% by mass or less of carbon (C), 1.5% by mass or more and 2.5% by mass or less of silicon (Si), 1.0% by mass or more and 2.0% by mass or less of chromium (Cr), 0.2% by mass or more and 1.0% by mass or less of manganese (Mn), and 1.5% by mass or more and 5.0% by mass or less of molybdenum (Mo) are contained, and the total content of impurities phosphorus (P) and sulfur (S) is 0.03% by mass or less, and the balance is iron (Fe). A carbon steel bolt is disclosed which is provided with an iron-based oxide film having a film thickness of 5 μm or more and 20 μm or less and consisting only of Fe3O4 and Fe2SiO4 on the surface of the carbon steel bolt having the composition. As shown in the above document, the bolt having the above configuration is excellent in stress corrosion cracking resistance and has a stable fastening axial force. 【Summary of the Invention】 【Problems to be Solved by the Invention】 【0004】 However, according to the studies of the present inventors, it has been found that according to the technology described in the above document, cracking may occur during heat treatment during bolt manufacturing, or bolts having sufficient fatigue characteristics may not be obtained. 【0005】 Therefore, an object of the present invention is to provide a high-strength bolt having excellent crack resistance and fatigue characteristics. 【Means for Solving the Problems】 【0006】 The inventors of this invention conducted diligent research to solve the above problems. As a result, they discovered that the above problems could be solved by controlling the composition of the steel constituting the bolt and the area ratio of ferrite that may be present on the surface of the bolt to within specific ranges, and thus completed the present invention. 【0007】 In other words, one embodiment of the present invention relates to a high-strength bolt having a tempered martensitic structure. The high-strength bolt contains 0.36% to 0.45% by mass of carbon (C), 1.75% to 2.00% by mass of silicon (Si), 0.90% to 1.30% by mass of chromium (Cr), 0.15% to 0.50% by mass of manganese (Mn), and 1.50% to 2.00% by mass of molybdenum (Mo), with a total content of phosphorus (P) and sulfur (S) as impurities of 0.015% by mass or less, and the remainder being iron (Fe) and unavoidable impurities, and is characterized in that the ferrite area ratio in the range from the surface of the threaded portion to 100 μm in the depth direction is 3.00% or less. [Modes for carrying out the invention] 【0008】 One embodiment of the present invention is a high-strength bolt having a tempered martensitic structure, containing 0.36% to 0.45% by mass of carbon (C), 1.75% to 2.00% by mass of silicon (Si), 0.90% to 1.30% by mass of chromium (Cr), 0.15% to 0.50% by mass of manganese (Mn), and 1.50% to 2.00% by mass of molybdenum (Mo), wherein the total content of the impurities phosphorus (P) and sulfur (S) is 0.015% by mass or less, with the remainder being iron (Fe) and unavoidable impurities, and the ferrite area ratio in the range from the surface of the threaded portion to 100 μm in the depth direction is 3.00% or less. According to this embodiment, a high-strength bolt with excellent resistance to quench cracking and fatigue properties is provided. 【0009】 The high-strength bolt of this embodiment is characterized by having a composition containing 0.36% to 0.45% by mass of carbon (C), 1.75% to 2.00% by mass of silicon (Si), 0.90% to 1.30% by mass of chromium (Cr), 0.15% to 0.50% by mass of manganese (Mn), and 1.50% to 2.00% by mass of molybdenum (Mo), with a total content of phosphorus (P) and sulfur (S) as impurities of 0.015% by mass or less, and the remainder being iron (Fe) and unavoidable impurities. 【0010】 The carbon (C) content is between 0.36% by mass and 0.45% by mass. If the carbon content is less than 0.36% by mass, the fatigue properties may deteriorate. Also, sufficient temper hardness cannot be obtained, and tempering at high temperatures (preferably 520°C or higher, more preferably 570°C or higher) (hereinafter also simply referred to as "high-temperature tempering") may not be possible, which may reduce the resistance to delayed fracture. If the carbon content exceeds 0.45% by mass, the resistance to quench cracking may deteriorate. Also, the amount of cementite that accumulates hydrogen increases significantly, which may reduce the resistance to delayed fracture. From the viewpoint of improving tensile strength, the carbon content is preferably between 0.38% by mass and 0.45% by mass, more preferably between 0.40% by mass and 0.45% by mass, and even more preferably between 0.42% by mass and 0.45% by mass. 【0011】 The silicon (Si) content is between 1.75% by mass and 2.00% by mass. If the silicon content is less than 1.75% by mass, sufficient tempering softening resistance cannot be obtained, and high-temperature tempering cannot be performed, which may reduce the delayed fracture resistance. Increasing the silicon content lowers the hydrogen diffusion coefficient in the steel, which can suppress the concentration of hydrogen that causes delayed fracture. However, if the silicon content exceeds 2.00% by mass, the forgeability deteriorates significantly, and it may not be possible to form the specified bolts. 【0012】 The chromium (Cr) content is between 0.90% by mass and 1.30% by mass. If the chromium content is less than 0.90% by mass, sufficient tempering softening resistance cannot be obtained, and high-temperature tempering cannot be performed, which may reduce the delayed fracture resistance. If the chromium content exceeds 1.30% by mass, the cold forgeability of the steel may decrease. 【0013】 The manganese (Mn) content is between 0.15% by mass and 0.50% by mass. The inclusion of manganese may improve hardenability. If the manganese content is less than 0.15% by mass, the tensile strength may decrease. If the manganese content exceeds 0.50% by mass, segregation at the grain boundaries may be promoted, which may reduce the grain boundary strength and decrease the resistance to delayed fracture. 【0014】 The molybdenum (Mo) content is between 1.50% by mass and 2.00% by mass. The inclusion of molybdenum improves hardenability, enabling the formation of a martensitic structure. It also increases softening resistance during tempering, thereby improving hardness. However, these effects are lost if the molybdenum content exceeds 2.00% by mass. If the molybdenum content is less than 1.50% by mass, the amount of molybdenum-based carbides that act as hydrogen trapping sites will not be sufficient, potentially leading to insufficient suppression of hydrogen embrittlement and reduced resistance to delayed fracture. 【0015】 It is preferable that the content of phosphorus (P) and sulfur (S), which are impurities, be low. Specifically, the total content of phosphorus (P) and sulfur (S) should be 0.015% by mass or less. If the total amount of phosphorus (P) and sulfur (S) exceeds 0.015% by mass, grain boundary segregation will be promoted, the grain boundary bonding force will decrease, and the grain boundary strength will decrease, which may reduce the delayed fracture resistance. 【0016】 In this specification, the composition of a high-strength bolt shall be determined by measuring the composition of the steel at the center line of the bolt's shaft, as described in the examples below. In order to keep the composition of the high-strength bolt within the above range, the composition of the steel material used as the bolt's raw material should be controlled to fall within that range. 【0017】 The high-strength bolt of this embodiment is also characterized in that the ferrite area ratio in the range from the surface of the threaded portion to 100 μm in the depth direction is 3.00% or less. By adopting such a configuration, a high-strength bolt with excellent resistance to quench cracking and fatigue properties (especially fatigue properties) can be obtained. The ferrite area ratio is more preferably 1.00% or less, more preferably 0.70% or less, even more preferably 0.03% or less, and most preferably 0.00%. In this specification, the ferrite area ratio shall be the value measured by the method described in the examples below. In order to keep the ferrite area ratio within the above range, the difference between the carbon content in the composition of the high-strength bolt [unit: mass%] and the carbon potential (CP) of the quenching atmosphere [unit: mass%] (carbon content in composition - CP) should be controlled to be small. Specifically, the difference (carbon content in the composition - CP) is preferably 0.20% by mass or less, more preferably 0.18% by mass or less, and even more preferably 0.16% by mass or less (lower limit: 0% by mass). 【0018】 The tensile strength of the high-strength bolt in this embodiment is preferable as high as possible. Specifically, the tensile strength is preferably 1500 MPa or higher, more preferably 1550 MPa or higher, even more preferably 1600 MPa or higher, and particularly preferably 1650 MPa or higher. There is no particular upper limit to the tensile strength, but it is usually 1750 MPa or lower. In this specification, the tensile strength will be the value measured by the method described in the examples below. In order to keep the tensile strength within the above range, the carbon content in the composition of the high-strength bolt can be controlled to be high. 【0019】 In this embodiment of high-strength bolts, it is preferable that the difference (H1-H2) between the Vickers hardness (H1) at a depth of 0.5 mm from the shaft surface and the Vickers hardness (H2) at a depth of 0.05 mm from the shaft surface is between 0 HV and 50 HV. By adopting such a configuration, it is possible to improve delayed fracture resistance while maintaining excellent fatigue properties. The difference (H1-H2) is more preferably between 9 HV and 43 HV. In this specification, the Vickers hardness (H2) and (H1) shall be the values ​​measured by the method described in the examples below. In order to keep the above difference (H1-H2) within the above range, the difference between the carbon content in the composition of the high-strength bolt [unit: mass%] and the carbon potential (CP) of the quenching atmosphere [unit: mass%] (carbon content in composition - CP) should be controlled within a specific range. Specifically, the difference (carbon content in the composition - CP) is preferably controlled to be greater than 0 mass% and 0.20 mass% or less, more preferably between 0.01 mass% and 0.18 mass%, and even more preferably between 0.03 mass% and 0.16 mass%, thereby bringing the above difference (H1-H2) within the above range. 【0020】 In this embodiment of high-strength bolts, it is preferable that the percentage of carbon concentration (C2) at a position 0.05 mm from the shaft surface in the depth direction relative to the carbon concentration (C1) at a position 0.5 mm from the shaft surface in the depth direction is between 60% and 100%. By adopting such a configuration, fatigue characteristics can be improved. From the viewpoint of improving delayed fracture resistance, it is more preferable that the percentage be less than 100%, and even more preferable that be 90% or less. In this specification, the carbon concentrations (C1) and (C2) shall be the values ​​measured by the method described in the examples below. In order to keep the above percentages within the above range, the difference between the carbon content in the composition of the high-strength bolt [unit: mass%] and the carbon potential (CP) of the quenching atmosphere [unit: mass%] (carbon content in composition - CP) should be controlled to be small. Specifically, the difference (carbon content in the composition - CP) is preferably 0.20% by mass or less, more preferably 0.18% by mass or less, and even more preferably 0.16% by mass or less (lower limit: 0% by mass). 【0021】 As a method for manufacturing high-strength bolts in this embodiment, for example, a high-strength bolt can be obtained by first cold forging a high-strength bolt steel having a predetermined composition, then performing heat treatment by quenching at 900°C or higher, tempering at 520°C or higher (preferably 570°C or higher), and finally performing thread rolling. The order of the above heat treatment (quenching and tempering) and thread rolling may be reversed. Since this heat treatment involves quenching and tempering from the austenite single-phase region, the high-strength bolt will naturally have a structure mainly composed of tempered martensite (specifically, a structure in which the area ratio of martensite is 85% or more, as determined by the image analysis method described in the examples). 【0022】 In the high-strength bolt of this embodiment, in order to keep the ferrite area ratio below 3.00%, the difference between the carbon content in the composition of the high-strength bolt [unit: mass%] and the carbon potential (CP) of the quenching atmosphere [unit: mass%] (carbon content in composition - CP) can be controlled to be small. The preferred numerical range for the difference (carbon content in composition - CP) is as described above. In this case, the value of the carbon potential (CP) of the quenching atmosphere is preferably 0.25 mass% or more and 0.35 mass% or less, and more preferably 0.28 mass% or more and 0.35 mass% or less. In the manufacture of the high-strength bolt of this embodiment, any known heat treatment furnace, such as a batch-type heat treatment furnace or a continuous-type heat treatment furnace, can be used without particular restriction, as long as it is capable of setting the above temperature and CP value. Generally, the setting limit for the CP value tends to be lower in continuous heat treatment furnaces compared to batch heat treatment furnaces. However, since the carbon content in the composition of this high-strength bolt is between 0.36% and 0.45% by mass, it is possible to achieve the desired ferrite area ratio even when using a continuous heat treatment furnace. In other words, since this high-strength bolt can be manufactured using a continuous heat treatment furnace, mass production is possible. Therefore, this embodiment allows for the provision of low-cost high-strength bolts. [Examples] 【0023】 The present invention will be described in more detail below with reference to examples. However, the technical scope of the present invention is not limited to the following examples. Unless otherwise specified, operations and measurements of physical properties were performed under conditions of room temperature of 20-25°C and relative humidity of 40-50%RH. 【0024】 <Manufacturing of high-strength bolts> [Example 1] For a high-strength bolt steel with a composition of C: 0.36% by mass, Si: 1.81% by mass, Cr: 1.00% by mass, Mn: 0.19% by mass, Mo: 1.51% by mass, total amount of S and P: 0.012% by mass, and the balance being Fe, cold forging was performed, and then thread rolling was carried out. Thereafter, heat treatment was performed by quenching at 930°C for 30 minutes and tempering at 520°C for 100 minutes in an atmosphere where the carbon potential (CP) was 0.30% by mass, and a high-strength bolt (M11×1.0, under-head length 26 mm) was obtained. 【0025】 [Examples 2 to 5, 7 to 9 and Comparative Examples 1 to 3] Except that the composition of the high-strength bolt steel, the timing of thread rolling, and the heat treatment conditions were changed as described in Table 1 and Table 2 below, high-strength bolts (M11×1.0, under-head length 26 mm) of each example and comparative example were obtained by the same method as in Example 1 described above. 【0026】 [Example 6] For a high-strength bolt steel with a composition containing C: 0.42% by mass, Si: 1.79% by mass, Cr: 1.01% by mass, Mn: 0.41% by mass, Mo: 1.51% by mass, total amount of S and P: 0.008% by mass, and the balance being Fe, cold forging was performed. Then, heat treatment was performed by quenching at 930°C for 30 minutes and tempering at 575°C for 1 hundred minutes in an atmosphere where the carbon potential (CP) was 0 point 35% by mass. Thereafter, thread rolling was carried out, and a high-strength bolt (M11×1.0, under-head length 26 mm) was obtained. 【0027】 [Measurement of Physical Properties] [Composition] The composition of the high-strength bolts produced in the above examples and comparative examples was measured using the following methods. First, a sample for measurement was prepared by cutting the steel at the position of the center line of the bolt shaft. Samples for C and S were prepared in the form of chips weighing 1 g or more. Samples for other elements were prepared in the form of rods with a diameter of φ5 and a length of 10 mm or more. C and S were measured according to JIS G1211-3:2018 Part 3: Combustion - Infrared Absorption Method. Other elements were measured by wet chemical analysis. As a result, it was confirmed that the composition of the high-strength bolts was the same as the composition of the high-strength bolt steel used in the manufacture of the bolts (results of measuring each elemental sample prepared from molten steel using the following method, according to JIS G0321:2017 Product Analysis Method and Permissible Variation Values ​​for Steel Materials). The composition of the high-strength bolt steel was measured using the following method. Samples for C and S were prepared in the form of chips weighing 1 g or more. Samples for the measurement of other elements were prepared in block form with a diameter of φ30-35 mm and a thickness of 10 mm or more. C and S were measured according to JIS G1211-3:2018 Part 3: Combustion - Infrared absorption method. Other elements were measured according to JIS G1256:1997 Iron and steel - X-ray fluorescence analysis method. 【0028】 [Ferrite area ratio] The ferrite area ratio of the high-strength bolts fabricated in the above examples and comparative examples was measured using the following method. First, a sample for measurement was prepared by cutting a cross-section (perpendicular to the centerline of the shaft) of the boundary between the shaft and threaded portion of the bolt (the bottom of the first threaded root when viewed from the shaft side). The sample was mirror-polished, etched with nital, and an image was taken using an optical microscope. The image was binarized by setting a brightness threshold so that the ferrite and martensite structures could be distinguished, and the area of ​​the ferrite structure was measured. Then, the ratio of the area of ​​the ferrite structure contained in the range up to 100 μm in depth from the surface to the total area was calculated as a percentage. Furthermore, from the above images, it was confirmed that the high-strength bolts fabricated in the above examples and comparative examples had a tempered martensite structure with an area ratio of 85% or more. 【0029】 [Tensile strength] The tensile strength of the high-strength bolts produced in the above examples and comparative examples was measured in accordance with JIS B1051:2014 Mechanical properties of fasteners made of carbon steel and alloy steel - Bolts, machine screws and stud bolts with specified strength grades - Coarse threads and fine threads. 【0030】 [Vickers hardness] The Vickers hardness of the high-strength bolts fabricated in the above examples and comparative examples was measured in accordance with JIS G0558:2020 Method for Measuring the Depth of the Decarburized Layer of Steel, 6.2 Measurement Method by Hardness Test. The measurement was performed at the center of the bolt shaft (1 / 2 of the shaft length). The Vickers hardness at a position 0.5 mm from the surface in the depth direction (H1) and the Vickers hardness at a position 0.05 mm from the surface in the depth direction (H2) were measured, and the difference (H1-H2) was calculated. 【0031】 [Carbon concentration] The carbon concentration of the high-strength bolts prepared in the above examples and comparative examples was measured according to JIS G1211-3:2018 Part 3: Combustion - Infrared Absorption Method. The measurement was performed at the center of the bolt shaft (half the length of the shaft). The carbon concentration at a depth of 0.5 mm from the surface (C1) and the carbon concentration at a depth of 0.05 mm from the surface (C2) were measured, and the ratio of C2 to C1 was calculated as a percentage. 【0032】 <Rating> [Burning crack resistance] The high-strength bolts fabricated in the above examples and comparative examples were checked for the presence or absence of quench cracks using magnetic particle testing. The results are shown in Table 3 below. In Table 3, "○" indicates no quench cracks, and "×" indicates quench cracks. 【0033】 [Fatigue characteristics] The fatigue strength (MPa) of the high-strength bolts fabricated in the above examples and comparative examples was measured based on JIS B1081:1997 Screw parts - Tensile fatigue test - Test method and evaluation of results. The fatigue test was conducted at room temperature (25°C) in an atmospheric environment, with a maximum stress of 1572 MPa for 2 × 10⁻¹⁶ bolts. 6The fatigue test was conducted by applying repeated tensile loads. After the fatigue test, the fatigue strength (MPa) was measured using the staircase method. The results are shown in Table 3 below. In Table 3 below, "○" indicates that the ratio of fatigue strength (MPa) to required fatigue strength (MPa) was 1.1 or higher, and "×" indicates that it was less than 1.1. 【0034】 [Delayed fracture resistance] The high-strength bolts prepared in the above examples and comparative examples were immersed in a 15% hydrochloric acid aqueous solution at room temperature (25°C) for 4 minutes. This cycle was considered one cycle, and the presence or absence of bolt damage was checked after 14 cycles. The results are shown in Table 3 below. In Table 3 below, "○" indicates no damage, and "△" indicates damage. 【0035】 [Table 1] 【0036】 [Table 2] 【0037】 [Table 3] 【0038】 The results shown in Table 3 demonstrate that the present invention provides high-strength bolts with excellent resistance to quench cracking and fatigue properties. 【0039】 Examples 1 to 8 show that the difference (H1-H2) is 0HV or greater, indicating that they have excellent resistance to quench cracking and fatigue properties, as well as excellent resistance to delayed fracture. 【0040】 This application is based on Japanese Patent Application No. 2023-003637, filed on 13 January 2023, the disclosures of which are cited in their entirety by reference.

Claims

[Claim 1] A high-strength bolt having a tempered martensitic structure, 0.36% by mass or more and 0.45% by mass or less of carbon (C), 1.75% by mass or more and 2.00% by mass or less of silicon (Si), 0.90% by mass or more and 1.30% by mass or less of chromium (Cr), Manganese (Mn) of 0.15% by mass or more and 0.50% by mass or less, It contains 1.50% by mass or more and 2.00% by mass or less of molybdenum (Mo), The total content of phosphorus (P) and sulfur (S), which are impurities, is 0.015% by mass or less. The composition consists of iron (Fe) and unavoidable impurities. A high-strength bolt in which the ferrite area ratio in the range from the surface of the threaded portion to a depth of 100 μm is 3.00% or less. [Claim 2] A high-strength bolt according to claim 1, wherein the tensile strength is 1500 MPa or more. [Claim 3] Vickers hardness (H) at a position 0.5 mm in depth from the surface of the shaft １ ) and the Vickers hardness (H) at a position 0.05 mm in depth from the surface of the shaft. ２ ) difference (H １ -H ２ A high-strength bolt according to claim 1 or 2, wherein the strength is 0 HV or more and 50 HV or less. [Claim 4] Carbon concentration (C) at a position 0.5 mm in depth from the surface of the shaft １ The carbon concentration (C) at a position 0.05 mm in depth from the surface of the shaft relative to the carbon concentration (C) ２ A high-strength bolt according to claim 1 or 2, wherein the percentage of ) is 60% or more and 100% or less. [Claim 5] Carbon concentration (C) at a position 0.5 mm in depth from the surface of the shaft １ The carbon concentration (C) at a position 0.05 mm in depth from the surface of the shaft relative to the carbon concentration (C) ２ The high-strength bolt according to claim 3, wherein the percentage of ) is 60% or more and 100% or less.

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