High strength bolt

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 a high strength bolt. More specifically, the present invention relates to a high strength bolt excellent in quenching crack resistance and fatigue characteristics.BACKGROUND ART

[0002] A high strength bolt having a tensile strength of 1200 MPa or more is required as a fastening member for an automobile.

[0003] For example, JP 6988922 B2 (US 11708622 A1) discloses a carbon steel bolt having a composition containing 0.50 mass% or more and 0.65 mass% or less of carbon (C), 1.5 mass% or more and 2.5 mass% or less of silicon (Si), 1.0 mass% or more and 2.0 mass% or less of chromium (Cr), 0.2 mass% or more and 1.0 mass% or less of manganese (Mn), and 1.5 mass% or more and 5.0 mass% or less of molybdenum (Mo), in which a total content of phosphorus (P) and sulfur (S) as impurities is 0.03 mass% or less, and a balance is iron (Fe), the carbon steel bolt including, on a surface, an iron-based oxide film with a film thickness of 5 µm or more and 20 µm or less composed only of Fe 3 O 4 and Fe 2 SiO 4 . As described in the above document, the bolt having the above configuration is excellent in delayed fracture resistance and has a stable fastening axial force.SUMMARY OF INVENTION Technical Problem

[0004] However, according to the study of the present inventors, it has been found that, according to the technique described in the above document, there is a case where thermal cracking occurs due to heat treatment at the time of producing a bolt, or a bolt having sufficient fatigue characteristics cannot be obtained.

[0005] Therefore, an object of the present invention is to provide a high strength bolt excellent in quenching crack resistance and fatigue characteristics.Solution to Problem

[0006] The present inventors have conducted intensive studies to solve the above problems. As a result, the present inventors have found that the above problems can be solved by controlling the composition of steel constituting the bolt and the area ratio of ferrite than can be present on the surface layer of the bolt within specific ranges, and have completed the present invention.

[0007] That is, an embodiment of the present invention relates to a high strength bolt having a tempered martensite structure. The high strength bolt has a composition containing: 0.36 mass% or more and 0.45 mass% or less of carbon (C); 1.75 mass% or more and 2.00 mass% or less of silicon (Si); 0.90 mass% or more and 1.30 mass% or less of chromium (Cr); 0.15 mass% or more and 0.50 mass% or less of manganese (Mn); and 1.50 mass% or more and 2.00 mass% or less of molybdenum (Mo), in which a total of a content of phosphorus (P) and a content of sulfur (S) as impurities is 0.015 mass% or less, and a balance is iron (Fe) and unavoidable impurities, wherein a ferrite area ratio in a range from 100 µm in a depth direction from a threaded portion surface is 3.00% or less.DESCRIPTION OF EMBODIMENTS

[0008] An embodiment of the present invention is a high strength bolt having a tempered martensite structure, having a composition containing: 0.36 mass% or more and 0.45 mass% or less of carbon (C); 1.75 mass% or more and 2.00 mass% or less of silicon (Si); 0.90 mass% or more and 1.30 mass% or less of chromium (Cr); 0.15 mass% or more and 0.50 mass% or less of manganese (Mn); and 1.50 mass% or more and 2.00 mass% or less of molybdenum (Mo), in which a total of a content of phosphorus (P) and a content of sulfur (S) as impurities is 0.015 mass% or less, and a balance is iron (Fe) and unavoidable impurities, wherein a ferrite area ratio in a range from 100 µm in a depth direction from a threaded portion surface is 3.00% or less. According to the present embodiment, a high strength bolt excellent in quenching crack resistance and fatigue characteristics is provided.

[0009] The high strength bolt of the present embodiment is characterized by having a composition containing: 0.36 mass% or more and 0.45 mass% or less of carbon (C); 1.75 mass% or more and 2.00 mass% or less of silicon (Si); 0.90 mass% or more and 1.30 mass% or less of chromium (Cr); 0.15 mass% or more and 0.50 mass% or less of manganese (Mn); and 1.50 mass% or more and 2.00 mass% or less of molybdenum (Mo), in which a total of a content of phosphorus (P) and a content of sulfur (S) as impurities is 0.015 mass% or less, and a balance is iron (Fe) and unavoidable impurities.

[0010] The content of carbon (C) is 0.36 mass% or more and 0.45 mass% or less. When the content of carbon is less than 0.36 mass%, fatigue characteristics may be deteriorated. Since sufficient tempering hardness cannot be obtained and tempering at a high temperature (preferably 520°C or higher, more preferably 570°C or higher) (hereinafter, also simply referred to as "high-temperature tempering") cannot be performed, delayed fracture resistance may be deteriorated. When the content of carbon is more than 0.45 mass%, the quenching crack resistance may be deteriorated. Since the amount of cementite that accumulates hydrogen is significantly increased, delayed fracture resistance may be deteriorated. From the viewpoint of improving the tensile strength, the content of carbon is preferably 0.38 mass% or more and 0.45 mass% or less, more preferably 0.40 mass% or more and 0.45 mass% or less, and further preferably 0.42 mass% or more and 0.45 mass% or less.

[0011] The content of silicon (Si) is 1.75 mass% or more and 2.00 mass% or less. When the content of silicon is less than 1.75 mass%, sufficient tempering softening resistance cannot be obtained, and high-temperature tempering cannot be performed, so that delayed fracture resistance may be deteriorated. By increasing the content of silicon, the diffusion coefficient of hydrogen in the steel is lowered, and concentration of hydrogen causing delayed fracture can be suppressed. However, when the content of silicon is more than 2.00 mass%, forgeability is significantly deteriorated, so that a predetermined bolt may not be molded.

[0012] The content of chromium (Cr) is 0.90 mass% or more and 1.30 mass% or less. When the content of chromium is less than 0.90 mass%, sufficient tempering softening resistance cannot be obtained, and high-temperature tempering cannot be performed, so that delayed fracture resistance may be deteriorated. When the content of chromium is more than 1.30 mass%, cold forgeability of a steel material may be deteriorated.

[0013] The content of manganese (Mn) is 0.15 mass% or more and 0.50 mass% or less. By containing manganese, hardenability can be improved. When the content of manganese is less than 0.15 mass%, the tensile strength may be deteriorated. When the content of manganese is more than 0.50 mass%, segregation to crystal grain boundaries is promoted, so that grain boundary strength may be reduced, leading to deterioration of delayed fracture resistance.

[0014] The content of molybdenum (Mo) is 1.50 mass% or more and 2.00 mass% or less. By containing molybdenum, hardenability for obtaining a martensite structure can be improved. The softening resistance can be increased during the tempering treatment, and the hardness can be improved. However, when the content of molybdenum exceeds 2.00 mass%, these effects cannot be obtained. When the content of molybdenum is less than 1.50 mass%, the amount of molybdenum-based carbide to be a hydrogen trap site generated is not sufficient, so that hydrogen embrittlement cannot be suppressed, and delayed fracture resistance may be deteriorated.

[0015] The contents of phosphorus (P) and sulfur (S) as impurities are preferably small. Specifically, the total content of phosphorus (P) and sulfur (S) is 0.015 mass% or less. When the total amount of phosphorus (P) and sulfur (S) exceeds 0.015 mass%, grain boundary segregation is promoted, the grain boundary bonding force decreases, and the grain boundary strength decreases, so that delayed fracture resistance may be deteriorated.

[0016] In the present specification, as the composition of the high strength bolt, a value obtained by measuring the composition of the steel at a position of a shaft portion center line of a shaft of the bolt is adopted as described in Examples below. Note that, in order to set the composition of the high strength bolt within the above range, the composition of a steel material, which is a raw material of the bolt, may be controlled to be a value within the above range.

[0017] The high strength bolt of the present embodiment is also characterized in that the ferrite area ratio in a range from 100 µm in a depth direction from a threaded portion surface is 3.00% or less. With such a configuration, it is possible to obtain a high strength bolt excellent in quenching crack resistance and fatigue characteristics (particularly, fatigue characteristics). The ferrite area ratio is more preferably 1.00% or less, more preferably 0.70% or less, and further preferably 0.03% or less, and most preferably 0.00%. In the present specification, as the ferrite area ratio, a value measured by a method described in Examples described later is adopted. Note that, in order to set the ferrite area ratio within the above range, a difference (content of carbon in composition - CP) between the content [unit: mass%] of carbon in the composition of the high strength bolt and the carbon potential (CP) [unit: mass%] of the quenching atmosphere may be controlled to be small. Specifically, the difference (content of carbon in composition - CP) is preferably 0.20 mass% or less, more preferably 0.18 mass% or less, and further preferably 0.16 mass% or less (lower limit value: 0 mass%).

[0018] The tensile strength of the high strength bolt of the present embodiment is preferably as high as possible. Specifically, the tensile strength is preferably 1500 MPa or more, more preferably 1550 MPa or more, further preferably 1600 MPa or more, and particularly preferably 1650 MPa or more. The upper limit of the tensile strength is not particularly limited, and is usually 1750 MPa or less. In the present specification, as the tensile strength, a value measured by a method described in Examples described later is adopted. Note that in order to set the tensile strength within the above range, the content of carbon in the composition of the high strength bolt may be controlled to be increased.

[0019] In the high strength bolt of the present embodiment, a difference (H 1 - H 2 ) between a Vickers hardness (H 1 ) at a position of 0.5 mm from a shaft portion surface in the depth direction and a Vickers hardness (H 2 ) at a position of 0.05 mm from the shaft portion surface in the depth direction is preferably 0 HV or more and 50 HV or less. With such a configuration, delayed fracture resistance can be improved while maintaining excellent fatigue characteristics. The difference (H 1 - H 2 ) is more preferably 9 HV or more and 43 HV or less. In the present specification, as the Vickers hardnesses (H 2 ) and (H 1 ), a value measured by a method described in Examples described later is adopted. Note that, in order to set the difference (H 1 - H 2 ) within the above range, a difference (content of carbon in composition - CP) between the content [unit: mass%] of carbon in the composition of the high strength bolt and the carbon potential (CP) [unit: mass%] of the quenching atmosphere may be controlled within a specific range. Specifically, the difference (H 1 - H 2 ) can be set within the above range by controlling the difference (content of carbon in composition - CP) within a range of preferably more than 0 mass% and 0.20 mass% or less, more preferably 0.01 mass% or more and 0.18 mass% or less, and further preferably 0.03 mass% or more and 0.16 mass% or less.

[0020] In the high strength bolt of the present embodiment, a percentage of a carbon concentration (C 2 ) at a position of 0.05 mm from a shaft portion surface in the depth direction to a carbon concentration (C 1 ) at a position of 0.5 mm from the shaft portion surface in the depth direction is preferably 60% or more and 100% or less. With such a configuration, fatigue characteristics can be improved. From the viewpoint of improving delayed fracture resistance, the percentage is more preferably less than 100% and further preferably 90% or less. In the present specification, as the carbon concentrations (C 1 ) and (C 2 ), a value measured by a method described in Examples described later is adopted. Note that, in order to set the percentage within the above range, a difference (content of carbon in composition - CP) between the content [unit: mass%] of carbon in the composition of the high strength bolt and the carbon potential (CP) [unit: mass%] of the quenching atmosphere may be controlled to be small. Specifically, the difference (content of carbon in composition - CP) is preferably 0.20 mass% or less, more preferably 0.18 mass% or less, and further preferably 0.16 mass% or less (lower limit value: 0 mass%).

[0021] As a method for producing a high strength bolt of the present embodiment, for example, a steel for a high strength bolt having a predetermined composition is first subjected to cold forging, then quenched at 900°C or higher, subjected to heat treatment of tempering at 520°C or higher (preferably 570°C or higher), and further subjected to screw rolling, whereby a high strength bolt can be obtained. The heat treatment (quenching and tempering) and the screw rolling may be performed in a different order. In the present heat treatment, quenching and tempering are performed from the austenite single phase region, and thus the high strength bolt naturally has a structure mainly composed of tempered martensite (specifically, a structure in which the area ratio of martensite by the image analysis method described in Examples is 85% or more).

[0022] In the high strength bolt of the present embodiment, in order to set the above-described ferrite area ratio to 3.00% or less, the difference (content of carbon in composition - CP) between the content [unit: mass%] of carbon in the composition of the high strength bolt and the carbon potential (CP) [unit: mass%] of the quenching atmosphere can be controlled to be small. The preferable numerical range of the difference (content of carbon in composition - CP) is as described above. At this time, the value of carbon potential (CP) in 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 production of the high strength bolt of the present embodiment, a known heat treatment furnace such as a batch type heat treatment furnace or a continuous heat treatment furnace can be used without particular limitation as long as it is a heat treatment furnace that can be set to the above temperature and CP value. In general, the setting limit of the CP value in a continuous heat treatment furnace tends to be lower than that in a batch type heat treatment furnace, but in the high strength bolt of the present embodiment, the content of carbon in the composition is 0.36 mass% or more and 0.45 mass% or less, and thus a desired ferrite area ratio can be achieved even when the continuous heat treatment furnace is used. In other words, since the high strength bolt of the present embodiment can be produced by a continuous heat treatment furnace, mass production is possible. Therefore, according to the present embodiment, a low-cost high strength bolt can be provided.Examples

[0023] Hereinafter, the present invention will be described in more detail with reference to Examples. However, the technical scope of the present invention is not limited only to the following Examples. Note that, unless otherwise specified, operations and measurements of physical properties and the like are performed under the conditions of room temperature of 20 to 25°C / relative humidity of 40 to 50% RH.<Production of high strength bolt>[Example 1]

[0024] A steel for a high strength bolt having a composition containing C: 0.36 mass%, Si: 1.81 mass%, Cr: 1.00 mass%, Mn: 0.19 mass%, Mo: 1.51 mass%, and a total amount of S and P: 0.012 mass% with a balance being Fe was subjected to cold forging, and then screw rolling. Thereafter, a heat treatment of quenching at 930°C for 30 minutes and tempering at 520°C for 100 minutes was performed in an atmosphere having a carbon potential (CP) of 0.30 mass% to obtain a high strength bolt (M11×1.0, neck length: 26 mm).[Examples 2 to 5 and 7 to 9 and Comparative Examples 1 to 3]

[0025] A high strength bolt (M11×1.0, neck length: 26 mm) of each of Examples and Comparative Examples was obtained by the same method as in Example 1 described above except that the composition of the steel for a high strength bolt, the timing of performing screw rolling, and the heat treatment conditions were changed as shown in Tables 1 and 2 below.[Example 6]

[0026] A steel for a high strength bolt having a composition containing C: 0.42 mass%, Si: 1.79 mass%, Cr: 1.01 mass%, Mn: 0.41 mass%, Mo: 1.51 mass%, and a total amount of S and P: 0.008 mass% with a balance being Fe was subjected to cold forging. Then, a heat treatment of quenching at 930°C for 30 minutes and tempering at 575°C for 100 minutes was performed in an atmosphere having a carbon potential (CP) of 0.35 mass%. Thereafter, screw rolling was performed to obtain a high strength bolt (M11×1.0, neck length: 26 mm).<Measurement of physical properties>[Composition]

[0027] The compositions of the high strength bolts produced in Examples and Comparative Examples described above were measured by the following method. First, the steel at a position of a shaft portion center line of a shaft of the bolt was cut to prepare a measurement sample. Measurement samples for C and S were prepared in the form of chips of 1 g or more. A measurement sample for other elements was prepared into a rod shape having φ 5 and a length of 10 mm or more. C and S were measured in accordance with JIS G 1211-3:2018 Part 3: Infrared absorption method after combustion. Other elements were measured by wet chemical analysis. As a result, it was confirmed that the composition of the high strength bolt was the same as the composition of the steel for a high strength bolt used for producing the bolt (results of measuring a measurement sample of each element prepared from molten steel by the following method in accordance with JIS G 0321:2017 Product analysis and its tolerance for wrought steel). The composition of the steel for a high strength bolt was measured by the following method. Measurement samples for C and S were prepared in the form of chips of 1 g or more. A measurement sample for other elements was prepared into a block shape having φ 30 to 35 mm and a thickness of 10 mm or more. C and S were measured in accordance with JIS G 1211-3:2018 Part 3: Infrared absorption method after combustion. The other elements were measured in accordance with JIS G 1256:1997 Iron and steel-Method for X-ray fluorescence spectrometric analysis.[Ferrite area ratio]

[0028] For the high strength bolts produced in Examples and Comparative Examples described above, the ferrite area ratio was measured by the following method. First, a boundary between the shaft portion and the threaded portion of the bolt (the valley bottom portion of the first threaded valley when viewed from the shaft portion side) was cut into round slices (perpendicular to the shaft portion center line) to prepare a measurement sample. The sample was mirror-polished and subjected to nital corrosion, and an image was taken with an optical microscope. In order to be able to distinguish the ferrite structure and the martensite structure, a threshold value of brightness of the image was set and binarization processing was performed, and the area of the ferrite structure was measured. The ratio of the area of the ferrite structure included in the range up to 100 µm in the depth direction from the surface to the total area was calculated as a percentage. Note that, from the above images, it was also confirmed that the high strength bolts produced in Examples and Comparative Examples described above had a tempered martensite structure of 85% or more in area ratio.[Tensile strength]

[0029] The tensile strength of each of the high strength bolts produced in Examples and Comparative Examples described above was measured in accordance with JIS B 1051:2014 Mechanical properties of fasteners made of carbon steel and alloy steel-Bolts, screws and studs with specified property classes-Coarse thread and fine pitch thread.[Vickers hardness]

[0030] The Vickers hardness of each of the high strength bolts produced in Examples and Comparative Examples described above was measured in accordance with the measurement method by 6.2 Hardness Test in JIS G 0558:2020 Steels-Determination of depth of decarburization. Note that the measurement was performed at a position at the center of the shaft portion (1 / 2 of the shaft portion length) of the bolt. The Vickers hardness (H 1 ) at a position of 0.5 mm from the surface in the depth direction and the Vickers hardness (H 2 ) at a position of 0.05 mm from the surface in the depth direction were measured, and the difference (H 1 - H 2 ) was determined.[Carbon concentration]

[0031] The carbon concentration of each of the high strength bolts produced in Examples and Comparative Examples described above was measured in accordance with JIS G 1211-3:2018 Part 3: Infrared absorption method after combustion. Note that the measurement was performed at a position at the center of the shaft portion (1 / 2 of the shaft portion length) of the bolt. The carbon concentration (C 1 ) at a position of 0.5 mm from the surface in the depth direction and the carbon concentration (C 2 ) at a position of 0.05 mm from the surface in the depth direction were measured, and the ratio of C 2 to C 1 was calculated as a percentage.<Evaluation>[Quenching crack resistance]

[0032] For the high strength bolts produced in Examples and Comparative Examples described above, the presence or absence of thermal cracking was confirmed by magnetic powder inspection. The results are shown in Table 3 below. In Table 3 below, the case without thermal cracking is described as "○", and the case with thermal cracking is described as "×".[Fatigue characteristics]

[0033] For the high strength bolts produced in Examples and Comparative Examples described above, the fatigue strength (MPa) was measured on the basis of JIS B 1081:1997 Threaded fasteners-Axial load fatigue testing-Test methods and evaluation of results. A fatigue test was performed by applying a cyclic tensile load of 2×10 6< times at a maximum stress of 1572 MPa in an air atmosphere at room temperature (25°C). After the fatigue test, the fatigue strength (MPa) was measured by a staircase method. The results are shown in Table 3 below. In Table 3 below, a sample having a ratio of the fatigue strength (MPa) to a required fatigue strength (MPa) of 1.1 or more is described as "○", and a sample having a ratio of less than 1.1 is described as "×".[Delayed fracture resistance]

[0034] The high strength bolts produced in Examples and Comparative Examples described above were immersed in a 15% aqueous hydrochloric acid solution at room temperature (25°C) for 4 minutes. With this cycle as one cycle, the presence or absence of breakage of the bolt was confirmed after 14 cycles of repetition. The results are shown in Table 3 below. In Table 3 below, the case without breakage is described as "○", and the case with breakage is described as "△".[Table 1]

[0035] Table 1CSiCrMnMoS + PFeExample 10.361.811.000.191.510.012BalanceExample 20.381.820.990.191.510.012BalanceExample 30.401.801.120.201.510.011BalanceExample 40.421.791.010.411.510.008BalanceExample 50.421.791.010.411.510.008BalanceExample 60.421.791.010.411.510.008BalanceExample 70.441.991.200.311.610.015BalanceExample 80.451.791.300.411.510.008BalanceExample 90.451.791.300.411.510.008BalanceComparative Example 10.381.820.990.191.510.012BalanceComparative Example 20.381.820.990.191.510.012BalanceComparative Example 30.501.981.000.302.000.009Balance(unit: mass%) [Table 2]

[0036] Table 2CPQuenchingTemperingRolling(mass%)Temperature (°C)Time (min)Temperature (°C)Time (min)Example 10.3093030520100Before heat treatmentExample 20.3393030560100Before heat treatmentExample 30.2893030560100Before heat treatmentExample 40.2893030575100Before heat treatmentExample 50.3093030575100Before heat treatmentExample 60.3593030575100After heat treatmentExample 70.2893030580100Before heat treatmentExample 80.3093030585100Before heat treatmentExample 90.3593030585100Before heat treatmentComparative Example 10.2893030575100Before heat treatmentComparative Example 20.2593030575100Before heat treatmentComparative Example 30.2593030595100Before heat treatment [Table 3]

[0037] Table 3Physical propertiesEvaluationFerrite area ratio (%)Tensile strength (MPa)H 1 -H 2 (HV)C 1 (mass%)C 2 (mass%)C 2 / C 1 percentage (%)Quenching crack resistanceFatigue characteristicsDelayed fracture resistanceExample 13.001589380.360.3083○○○Example 20.031645230.380.3387○○○Example 30.70166890.400.2870○○○Example 40.031650430.420.2867○○○Example 50.001640280.420.3071○○○Example 60.001665150.420.3583○○○Example 70.021659100.440.2864○○○Example 80.001704210.450.3067○○○Example 90.001723-70.450.3578○○△Comparative Example 16.201615550.380.2874○×○Comparative Example 211.001613700.380.2566○×○Comparative Example 30.001693270.500.2550×○○

[0038] From the results shown in Table 3, it is found that according to the present invention, it is possible to provide a high strength bolt excellent in quenching crack resistance and fatigue characteristics.

[0039] In Examples 1 to 8, since the difference (H 1 - H 2 ) is 0 HV or more, it can be seen that in addition to excellent quenching crack resistance and fatigue characteristics, delayed fracture resistance is also excellent.

[0040] The present application is based on Japanese Patent Application No. 2023-003637 filed on January 13, 2023, the disclosure content of which is incorporated herein by reference in its entirety.

Claims

1. A high strength bolt having a tempered martensite structure, comprising a composition containing: 0.36 mass% or more and 0.45 mass% or less of carbon (C); 1.75 mass% or more and 2.00 mass% or less of silicon (Si); 0.90 mass% or more and 1.30 mass% or less of chromium (Cr); 0.15 mass% or more and 0.50 mass% or less of manganese (Mn); and 1.50 mass% or more and 2.00 mass% or less of molybdenum (Mo), in which a total of a content of phosphorus (P) and a content of sulfur (S) as impurities is 0.015 mass% or less, and a balance is iron (Fe) and unavoidable impurities, wherein a ferrite area ratio in a range from 100 µm in a depth direction from a threaded portion surface is 3.00% or less.

2. The high strength bolt according to claim 1, wherein a tensile strength is 1500 MPa or more.

3. The high strength bolt according to claim 1 or 2, wherein a difference (H1 - H2) between a Vickers hardness (H1) at a position of 0.5 mm from a shaft portion surface in the depth direction and a Vickers hardness (H2) at a position of 0.05 mm from the shaft portion surface in the depth direction is 0 HV or more and 50 HV or less.

4. The high strength bolt according to claim 1 or 2, wherein a percentage of a carbon concentration (C2) at a position of 0.05 mm from a shaft portion surface in the depth direction to a carbon concentration (C1) at a position of 0.5 mm from the shaft portion surface in the depth direction is 60% or more and 100% or less.

5. The high strength bolt according to claim 3, wherein a percentage of a carbon concentration (C2) at a position of 0.05 mm from a shaft portion surface in the depth direction to a carbon concentration (C1) at a position of 0.5 mm from the shaft portion surface in the depth direction is 60% or more and 100% or less.

Images