High-strength bolt

Abstract

The purpose of the present invention is to provide a high-strength bolt having excellent firing crack resistance and fatigue characteristics. According to the present invention, provided is a high-strength bolt having a tempered martensite structure, the high-strength bolt having a composition containing 0.36 mass% to 0.45 mass% (inclusive) of carbon (C), 1.75 mass% to 2.00 mass% (inclusive) of silicon (Si), 0.90 mass% to 1.30 mass% (inclusive) of chromium (Cr), 0.15 mass% to 0.50 mass% (inclusive) of manganese (Mn), and 1.50 mass% to 2.00 mass% (inclusive) of molybdenum (Mo), the total of the content of phosphorus (P) and the content of sulfur (S) as impurities is 0.015 mass% or less, the remainder being iron (Fe) and unavoidable impurities, and the ferrite area ratio in a range of 100 [mu] m in the depth direction from the surface of the screw section being 3.00% or less.

Description

Technical Field

[0001] The present invention relates to a high-strength bolt, and more particularly to a high-strength bolt having excellent cracking resistance and fatigue properties. Background Art

[0002] As fastening components used in automobiles, high-strength bolts having a tensile strength of 1200 MPa or more are required.

[0003] For example, Patent No. 6988922 (U.S. Patent No. 11708622) discloses a carbon steel bolt characterized by having a composition containing 0.50% to 0.65% carbon (C), 1.5% to 2.5% silicon (Si), 1.0% to 2.0% chromium (Cr), 0.2% to 1.0% manganese (Mn), and 1.5% to 5.0% molybdenum (Mo). The combined content of phosphorus (P) and sulfur (S) as impurities is 0.03% to 0.03% by mass, with the remainder being iron (Fe). The surface of the carbon steel bolt is provided with an iron-based oxide film composed solely of Fe₃O₄ and Fe₂SiO₄, with a thickness of 5 μm to 20 μm. As shown in the aforementioned document, a bolt having such a structure exhibits excellent delayed fracture resistance and stable tightening axial force.

[0004] Problems to be solved by the invention

[0005] However, the present inventors' research has revealed that the technology described in the above-mentioned document may cause seizures due to heat treatment during bolt production, or may fail to obtain a bolt having sufficient fatigue properties. Summary of the Invention

[0006] Therefore, an object of the present invention is to provide a high-strength bolt having excellent seizure resistance and fatigue properties.

[0007] Technical solutions to solve problems

[0008] The present inventors conducted intensive research to solve the above-mentioned problems and found that the above-mentioned problems can be solved by controlling the composition of the steel constituting the bolt and the area ratio of ferrite that can be present in the surface layer of the bolt within specific ranges, thereby completing the present invention.

[0009] Specifically, one aspect of the present invention relates to a high-strength bolt having a tempered martensite structure. This high-strength bolt is characterized by having a composition containing 0.36% to 0.45% carbon (C), 1.75% to 2.00% silicon (Si), 0.90% to 1.30% chromium (Cr), 0.15% to 0.50% manganese (Mn), and 1.50% to 2.00% molybdenum (Mo). The combined content of phosphorus (P) and sulfur (S) as impurities is 0.015% to 0.015% by mass, with the remainder being iron (Fe) and unavoidable impurities. Furthermore, the high-strength bolt has a ferrite area fraction of 3.00% or less in the range from the thread surface to 100 μm in the depth direction. DETAILED DESCRIPTION

[0010] One aspect of the present invention is a high-strength bolt having a tempered martensite structure. The high-strength bolt has a composition comprising 0.36% to 0.45% carbon (C), 1.75% to 2.00% silicon (Si), 0.90% to 1.30% chromium (Cr), 0.15% to 0.50% manganese (Mn), and 1.50% to 2.00% molybdenum (Mo). The combined content of phosphorus (P) and sulfur (S) as impurities is 0.015% to 0.015% by mass, with the remainder being iron (Fe) and unavoidable impurities. Furthermore, the ferrite area fraction from the thread surface to a depth of 100 μm is 3.00% or less. This aspect provides a high-strength bolt with excellent seizure resistance and fatigue properties.

[0011] The high-strength bolt of the present invention is characterized in that it has a composition containing 0.36 mass% to 0.45 mass% carbon (C), 1.75 mass% to 2.00 mass% silicon (Si), 0.90 mass% to 1.30 mass% chromium (Cr), 0.15 mass% to 0.50 mass% manganese (Mn), and 1.50 mass% to 2.00 mass% molybdenum (Mo), with the total content of phosphorus (P) and sulfur (S) as impurities being 0.015 mass% or less, and the remainder being iron (Fe) and unavoidable impurities.

[0012] The carbon (C) content is not less than 0.36% by mass and not more than 0.45% by mass. If the carbon content is less than 0.36% by mass, fatigue properties may be reduced. In addition, sufficient tempering hardness cannot be obtained, and tempering at high temperature (preferably not less than 520°C, more preferably not less than 570°C) (hereinafter also referred to as "high temperature tempering") cannot be implemented, so delayed fracture resistance may be reduced. If the carbon content exceeds 0.45% by mass, burnout resistance may be reduced. In addition, the amount of cementite that accumulates hydrogen increases significantly, so delayed fracture resistance may be reduced. From the viewpoint of improving tensile strength, the carbon content is preferably not less than 0.38% by mass and not more than 0.45% by mass, more preferably not less than 0.40% by mass and not more than 0.45% by mass, and further preferably not less than 0.42% by mass and not more than 0.45% by mass.

[0013] The silicon (Si) content is between 1.75% and 2.00% by mass. If the silicon content is less than 1.75% by mass, sufficient resistance to temper softening cannot be achieved, making high-temperature tempering impossible. Consequently, delayed fracture resistance may be reduced. Increasing the silicon content reduces the diffusion coefficient of hydrogen in the steel, suppressing hydrogen concentration that can cause delayed fracture. However, if the silicon content exceeds 2.00% by mass, forgeability significantly deteriorates, making it impossible to form the desired bolt.

[0014] The chromium (Cr) content is 0.90% to 1.30% by mass. If the chromium content is less than 0.90% by mass, sufficient resistance to temper softening is not achieved, and high-temperature tempering cannot be performed, which may reduce delayed fracture resistance. If the chromium content exceeds 1.30% by mass, the cold forgeability of the steel may be reduced.

[0015] The manganese (Mn) content is 0.15% to 0.50% by mass. The inclusion of manganese improves hardenability. If the manganese content is less than 0.15% by mass, tensile strength may decrease. If the manganese content exceeds 0.50% by mass, segregation at grain boundaries may be promoted, resulting in reduced grain boundary strength and decreased delayed fracture resistance.

[0016] The molybdenum (Mo) content is 1.50% to 2.00% by mass. The inclusion of Mo improves hardenability, resulting in a martensitic structure. Furthermore, during tempering, it increases softening resistance and improves hardness. However, if the Mo content exceeds 2.00% by mass, these effects are not achieved. If the Mo content is less than 1.50% by mass, the formation of molybdenum-based carbides, which serve as hydrogen traps, is insufficient, resulting in an inability to suppress hydrogen embrittlement and a potential reduction in delayed fracture resistance.

[0017] The content of phosphorus (P) and sulfur (S) as impurities is preferably low. Specifically, the total content of phosphorus (P) and sulfur (S) is 0.015% by mass or less. If the total content of phosphorus (P) and sulfur (S) exceeds 0.015% by mass, grain boundary segregation is promoted, the grain boundary bonding strength decreases, and the grain boundary strength decreases, which may reduce the delayed fracture resistance.

[0018] In this specification, the composition of a high-strength bolt is determined by measuring the steel composition at the centerline of the bolt's shaft, as described in the Examples below. Furthermore, to ensure that the composition of the high-strength bolt falls within the aforementioned range, the composition of the steel material used as the bolt's raw material must be controlled within the aforementioned range.

[0019] The high-strength bolt of this aspect is also characterized by a ferrite area fraction of 3.00% or less over a depth of 100 μm from the threaded portion surface. This structure enables a high-strength bolt with excellent seizure resistance and fatigue properties (particularly fatigue properties). The ferrite area fraction 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 fraction is the value measured using the method described in the Examples below. To maintain the ferrite area fraction within the above range, the difference between the carbon content (in mass %) in the high-strength bolt composition and the carbon potential (CP) (in mass %) of the quenching atmosphere (carbon content in the composition - CP) can be minimized. Specifically, the difference (carbon content in the composition - CP) is preferably 0.20% or less, more preferably 0.18% or less, and even more preferably 0.16% or less (lower limit: 0% by mass).

[0020] The tensile strength of the high-strength bolts of this aspect is preferably 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. The upper limit of the tensile strength is not particularly limited, but is typically 1750 MPa or lower. In this specification, the tensile strength is the value measured using the method described in the Examples below. Furthermore, to achieve the tensile strength within the above range, the carbon content in the high-strength bolt composition can be increased.

[0021] For the high-strength bolt of this aspect, the difference (H1-H2) between the Vickers hardness (H1) at a point 0.5 mm from the shaft surface in the depth direction and the Vickers hardness (H2) at a point 0.05 mm from the shaft surface in the depth direction is preferably 0 HV or higher and 50 HV or lower. This structure maintains excellent fatigue properties while improving delayed fracture resistance. The difference (H1-H2) is more preferably 9 HV or higher and 43 HV or lower. In this specification, the Vickers hardness (H2) and (H1) are values measured using the methods described in the Examples below. In order to keep the difference (H1-H2) within the above range, it is sufficient to control the difference between the carbon content (unit: mass %) in the high-strength bolt composition and the carbon potential (CP) (unit: mass %) of the quenching atmosphere (carbon content in composition - CP) within a specific range. Specifically, the difference (carbon content in the composition - CP) is preferably controlled within the range of greater than 0 mass % and 0.20 mass % or less, more preferably 0.01 mass % or more and 0.18 mass % or less, and even more preferably 0.03 mass % or more and 0.16 mass % or less, so that the difference (H1 - H2) can be within the above range.

[0022] In the high-strength bolt of this aspect, the percentage of the 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 preferably 60% or more and 100% or less. This structure can improve fatigue properties. From the perspective of improving delayed fracture resistance, this percentage is more preferably less than 100%, and even more preferably less than 90%. In this specification, the carbon concentrations (C1) and (C2) are values measured using the methods described in the Examples below. To maintain these percentages within the above ranges, the difference between the carbon content (unit: mass %) in the high-strength bolt composition and the carbon potential (CP) (unit: mass %) of the quenching atmosphere (carbon content in the composition - CP) can be minimized. Specifically, the difference (carbon content in the composition - CP) is preferably 0.20% or less, more preferably 0.18% or less, and even more preferably 0.16% or less (lower limit: 0% by mass).

[0023] The method for manufacturing high-strength bolts according to the present invention involves, for example, first cold forging high-strength bolt steel having a predetermined composition, followed by heat treatment of quenching at a temperature of 900°C or higher and tempering at a temperature of 520°C or higher (preferably 570°C or higher), and then thread rolling. This produces a high-strength bolt. The order of these heat treatments (quenching and tempering) and thread rolling can also be varied. Since quenching and tempering are performed in the austenite single-phase region during this heat treatment, the high-strength bolt naturally has a structure primarily composed of tempered martensite (specifically, a structure in which the area ratio of martensite, as calculated using the image analysis method described in the Examples, is 85% or higher).

[0024] In the high-strength bolt of this aspect, to achieve the aforementioned ferrite area ratio of 3.00% or less, the difference between the carbon content (unit: mass %) in the high-strength bolt composition and the carbon potential (CP) of the quenching atmosphere (unit: mass %) (carbon content in composition - CP) can be minimized. The preferred numerical range for this difference (carbon content in composition - CP) is as described above. In this case, the carbon potential (CP) of the quenching atmosphere is preferably 0.25% to 0.35% by mass, and more preferably 0.28% to 0.35% by mass. In the production of the high-strength bolt of this aspect, any known heat treatment furnace, such as a batch heat treatment furnace or a continuous heat treatment furnace, can be used without particular limitation, as long as it can be set to the aforementioned temperature and CP value. Typically, the set limit for the CP value in a continuous heat treatment furnace tends to be lower than in a batch heat treatment furnace. However, the carbon content in the high-strength bolt of the present invention is 0.36% to 0.45% by mass, so even using a continuous heat treatment furnace, the desired ferrite area ratio can be achieved. In other words, because the high-strength bolt of the present invention can be manufactured in a continuous heat treatment furnace, it can be mass-produced. Therefore, according to the present invention, a low-cost high-strength bolt can be provided.

[0025] Example

[0026] The present invention is described in more detail below using 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 at room temperature of 20-25°C and relative humidity of 40-50%.

[0027] Production of high-strength bolts

[0028] [Example 1]

[0029] High-strength bolt steel with a composition consisting of 0.36 mass% C, 1.81 mass% Si, 1.00 mass% Cr, 0.19 mass% Mn, 1.51 mass% Mo, a combined S and P content of 0.012 mass%, and the balance Fe was cold forged and then thread rolled. This was then heat treated in an atmosphere with a carbon potential (CP) of 0.30 mass% by quenching at 930°C for 30 minutes and tempering at 520°C for 100 minutes to produce high-strength bolts (M11×1.0, shank length 26 mm).

[0030] [Examples 2 to 5, 7 to 9 and Comparative Examples 1 to 3]

[0031] High-strength bolts (M11×1.0, shank length 26 mm) according to each example and comparative example were obtained by the same method as in Example 1, except that the composition of the high-strength bolt steel, the timing of thread rolling, and the heat treatment conditions were changed to those described in Tables 1 and 2 below.

[0032] [Example 6]

[0033] High-strength bolt steel containing 0.42 mass% C, 1.79 mass% Si, 1.01 mass% Cr, 0.41 mass% Mn, 1.51 mass% Mo, and a combined S and P content of 0.008 mass%, with the balance consisting of Fe, was cold forged. Subsequently, the steel was heat treated in an atmosphere with a carbon potential (CP) of 0.35 mass%, undergoing quenching at 930°C for 30 minutes and tempering at 575°C for 100 minutes. Thread rolling was then performed to produce high-strength bolts (M11×1.0, shank length 26 mm).

[0034] <Measurement of physical properties>

[0035] [composition]

[0036] The composition of the high-strength bolts produced in the above-mentioned Examples and Comparative Examples was measured using the following method. First, the steel was cut at the centerline of the bolt shaft to prepare a sample for measurement. Samples for C and S measurements were prepared into powder weighing at least 1g. Samples for other elements were prepared into rods with a diameter of 5 mm and a length of at least 10mm. 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. The results confirmed that the composition of the high-strength bolts was identical to that of the high-strength bolt steel used in bolt production (measured using the following method on samples prepared from molten steel according to JIS G0321:2017, Methods of Analysis of Steel Products and Permissible Variations). The composition of the high-strength bolt steel was measured using the following method. Samples for C and S measurements were prepared into powder weighing at least 1g. Samples for the measurement of other elements were prepared into blocks with a diameter of 30-35 mm and a thickness of at least 10 mm. 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 - Fluorescent X-ray Analysis Method.

[0037] [Ferrite area ratio]

[0038] The ferrite area ratio of the high-strength bolts produced in the aforementioned Examples and Comparative Examples was measured using the following method. First, the boundary between the bolt's shaft and thread (the initial groove bottom portion, as viewed from the shaft) was cut into a circular shape (perpendicular to the shaft's centerline) to prepare a measurement sample. The sample was mirror-polished, subjected to Nettal etching, and imaged using an optical microscope. The image brightness threshold was set to allow for discrimination between ferrite and martensite structures, and binarization was performed to measure the area of the ferrite structure. The percentage of the ferrite structure within this range relative to the total area of the area from the surface to a depth of 100 μm was calculated. Furthermore, the images confirmed that the high-strength bolts produced in the aforementioned Examples and Comparative Examples contained a tempered martensite structure with an area ratio of 85% or more.

[0039] [Tensile strength]

[0040] The tensile strength of the high-strength bolts produced in the above-mentioned Examples and Comparative Examples was measured in accordance with JIS B1051:2014 Mechanical properties of carbon steel and alloy steel fasteners - Strength zones for bolts, screws and studs - Coarse and fine threads.

[0041] [Vickers hardness]

[0042] The Vickers hardness of the high-strength bolts produced in the examples and comparative examples described above was measured in accordance with JIS G0558:2020, "Methods for determination of decarburized layer depth in steel," 6.2, "Methods for determination of decarburized layer depth in steel." Measurements were performed at the center of the bolt's shaft (half the length). The Vickers hardness (H1) at a depth of 0.5 mm from the surface and the Vickers hardness (H2) at a depth of 0.05 mm from the surface were measured, and the difference (H1 - H2) was calculated.

[0043] [Carbon concentration]

[0044] The carbon concentration of the high-strength bolts produced in the examples and comparative examples was measured according to JIS G1211-3:2018, Part 3: Combustion - Infrared Absorption Method. Measurements were performed at the center of the bolt shaft (half the length). The carbon concentration (C1) at a depth of 0.5 mm from the surface and the carbon concentration (C2) at a depth of 0.05 mm from the surface were measured, and the ratio of C2 to C1 was calculated as a percentage.

[0045] <Evaluation>

[0046] [Crack resistance]

[0047] The high-strength bolts produced in the above-mentioned Examples and Comparative Examples were examined by magnetic particle inspection to determine whether they had seizures. The results are shown in Table 3. In Table 3, no seizures are indicated by "○" and seizures are indicated by "×."

[0048] [Fatigue properties]

[0049] The fatigue strength (MPa) of the high-strength bolts produced in the above-mentioned Examples and Comparative Examples was measured in accordance with JIS B1081:1997, Threaded Parts - Tensile Fatigue Tests - Test Methods and Evaluation of Results. The fatigue tests were conducted at room temperature (25°C) in an atmospheric atmosphere, applying a tensile load of 2 × 10⁶ cycles at a maximum stress of 1572 MPa. After the fatigue tests, the fatigue strength (MPa) was measured using a step method. The results are shown in Table 3 below. In Table 3, a ratio of fatigue strength (MPa) to required fatigue strength (MPa) of 1.1 or greater is indicated by "○," while a ratio of less than 1.1 is indicated by "×."

[0050] [Delayed destruction resistance]

[0051] The high-strength bolts produced in the 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, and the bolts were inspected for damage after 14 cycles. The results are shown in Table 3. In Table 3, no damage is indicated by "○" and damage is indicated by "△."

[0052] [Table 1]

[0053]

[0054] [Table 2]

[0055]

[0056] [Table 3]

[0057]

[0058] From the results shown in Table 3, it can be seen that according to the present invention, a high-strength bolt having excellent seizure resistance and fatigue properties can be provided.

[0059] It is also understood that in Examples 1 to 8, since the difference (H1−H2) is 0 HV or more, in addition to being excellent in seizure resistance and fatigue properties, the delayed fracture resistance is also excellent.

[0060] This application is based on Japanese Patent Application No. 2023-003637 filed on January 13, 2023, the disclosure of which is incorporated herein by reference in its entirety.

Claims

1. A high-strength bolt having a tempered martensite structure, wherein: The high-strength bolt has the following composition: Contains: 0.36 mass % or more and 0.45 mass % or less 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), The total content of phosphorus (P) and sulfur (S) as impurities is 0.015 mass % or less, and the remainder is iron (Fe) and unavoidable impurities. The ferrite area ratio in the range from the thread portion surface to 100 μm in the depth direction is 3.00% or less.

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

3. The high-strength bolt according to claim 1 or 2, wherein: The difference (H1-H2) between the Vickers hardness (H1) at a position 0.5 mm from the shaft surface in the depth direction and the Vickers hardness (H2) at a position 0.05 mm from the shaft 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: The percentage of the 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 60% or more and 100% or less.

5. The high-strength bolt according to claim 3, wherein: The percentage of the 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 60% or more and 100% or less.

Images