High Strength Bolts

Abstract

To provide a high-strength bold that is superior in corrosion resistance and fatigue strength.SOLUTION: A high-strength bolt having a tensile strength of 1,500 MPa or more is provided which is formed by using steel for bolt which includes 0.50 to 0.65 mass% of C, 1.5 to 2.5 mass% of Si, 1.0 to 1.6 mass% of Cr, 0.4 mass% or less of Mn, and 1.5 to 2.2 mass% of Mo, wherein the bolt has a surface roughness of equal to or less than 0.48 μm, and also has on its surface a surface-treated film selected from a chemical conversion film, a plating film and a coating film.SELECTED DRAWING: None

Description

[Technical field]

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

[0002] Generally, alloy steels (SCM435, SCM440, etc.) are used for bolts, but when they are used at high strength, heat treatment such as quenching and tempering is required in the bolt manufacturing process. However, there is a problem that high-strength bolts obtained by processes including heat treatment are prone to delayed fracture. Since the higher the strength of a bolt, the more prone it is to delayed fracture, it is common for bolts used in components for automobiles and the like to have a tensile strength of less than 1200 MPa.

[0003] On the other hand, in recent automobile engines, the combustion pressure is increasing to improve performance, so high axial force is required for the bolts that fasten parts such as connecting rods, and furthermore, to make the engine lighter and more compact, the bolts need to have a smaller diameter. Therefore, the adoption and development of high-strength bolts with a tensile strength of 1200 MPa or more is being promoted, and for example, Patent Document 1 proposes a technology to obtain high-strength bolts with excellent resistance to delayed fracture by specifying the composition of the steel for the bolts. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2006-219709 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, even if the high-strength bolt described in Patent Document 1 is used, there is a demand for improvements in corrosion resistance and fatigue strength, which are required performance when applied to high-output engines that require even higher output.For example, for fastening bolts used in reciprocating engines having connecting rod mechanisms or multi-link mechanisms, there is a demand for high-strength bolts that are stronger than ever before and have excellent corrosion resistance and fatigue strength.

[0006] SUMMARY OF THE PRESENT EMBODIMENT An object of the present invention is to provide a high-strength bolt that is excellent in corrosion resistance and fatigue strength. [Means for solving the problem]

[0007] The present inventors have conducted extensive research to achieve the above object, and as a result, have found that the above object can be achieved by controlling the surface roughness of a high-strength bolt using bolt steel having a specific composition within a specific range and by performing a specific surface treatment on the surface of the bolt, thereby completing the present invention.

[0008] That is, the present invention relates to a high-strength bolt having a tensile strength of 1500 MPa or more, the bolt being made of bolt steel containing 0.50 to 0.65 mass% C, 1.5 to 2.5 mass% Si, 1.0 to 1.6 mass% Cr, 0.4 mass% or less Mn, and 1.5 to 2.2 mass% Mo, the bolt having a surface roughness Ra of 0.48 μm or less, and having a surface treatment film selected from a chemical conversion coating, a plating coating, or a paint coating on the surface of the bolt. Effect of the Invention

[0009] According to the high-strength bolt of the present invention, excellent tensile strength is imparted by using a bolt steel having a specified composition. In addition, by controlling the surface roughness, excellent fatigue strength can be achieved even in high-strength bolts where the surface roughness tends to affect fatigue strength as notches. In addition, corrosion resistance can be improved by forming a specified surface treatment film. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, a high-strength bolt according to one embodiment of the present invention will be described.

[0011] [High strength bolts] One embodiment of the present invention is a high-strength bolt having a tensile strength of 1500 MPa or more, the bolt being made using bolt steel containing 0.50 to 0.65 mass% C, 1.5 to 2.5 mass% Si, 1.0 to 1.6 mass% Cr, 0.4 mass% or less Mn, and 1.5 to 2.2 mass% Mo, the bolt having a surface roughness Ra of 0.48 μm or less, and having a surface treatment film selected from a chemical conversion coating, a plating coating, or a paint coating on a surface of the bolt.

[0012] (Bolt steel) A high-strength bolt according to one embodiment of the present invention is made using bolt steel containing 0.50 to 0.65 mass% C, 1.5 to 2.5 mass% Si, 1.0 to 1.6 mass% Cr, 0.4 mass% or less Mn, and 1.5 to 2.2 mass% Mo.

[0013] Preferably, the bolt steel contains 0.50 to 0.65 mass% C, 1.5 to 2.5 mass% Si, 1.0 to 1.6 mass% Cr, 0.4 mass% or less Mn, 1.5 to 2.2 mass% Mo, and the balance being Fe and unavoidable impurities.

[0014] (Carbon (C): 0.50~0.65% by mass) If the carbon content is less than 0.50% by mass, sufficient temper softening resistance is not obtained, and high-temperature tempering, which will be described later, cannot be performed, so that delayed fracture resistance is not excellent.If the carbon content exceeds 0.65% by mass, the amount of cementite that accumulates hydrogen increases significantly, so that delayed fracture resistance is not excellent.

[0015] (Silicon (Si): 1.5 to 2.5% by mass) If the silicon content is less than 1.5 mass%, sufficient temper softening resistance is not obtained, and high-temperature tempering, which will be described later, cannot be performed, resulting in poor delayed fracture resistance. In addition, by increasing the silicon content, the diffusion coefficient of hydrogen in the steel is reduced, and the concentration of hydrogen that causes delayed fracture can be suppressed. However, if the silicon content exceeds 2.5 mass%, the forgeability is significantly deteriorated, making it impossible to form a specified bolt.

[0016] (Chromium (Cr): 1.0 to 1.6% by mass) If the chromium content is less than 1.0 mass%, sufficient tempering softening resistance is not obtained, and high-temperature tempering, which will be described later, cannot be performed, so delayed fracture resistance is not excellent. If the chromium content exceeds 1.6 mass%, the amount of coarse alloy carbides containing chromium increases, and delayed fracture resistance may decrease. The chromium content is preferably 1.4 mass% or less.

[0017] (Manganese (Mn): 0.4% by mass or less) If the manganese content exceeds 0.4 mass%, the grain boundary segregation of the grain boundary segregation components is promoted, so that the grain boundary strength is significantly reduced, and the delayed fracture resistance is not excellent. The manganese content is not particularly limited, but is preferably more than 0 mass%, and more preferably 0.2 mass% or more.

[0018] (Molybdenum (Mo): 1.5 to 2.2% by mass) If the molybdenum content is less than 1.5 mass%, the amount of molybdenum-based carbides that render hydrogen harmless is not sufficient, and the delayed fracture resistance is not excellent. On the other hand, if the molybdenum content exceeds 2.2 mass%, the amount of coarse alloy carbides containing molybdenum increases, and there is a risk of the delayed fracture resistance being reduced.

[0019] (Bolt composition) The bolt according to this embodiment preferably contains 0.50 to 0.65 mass% C, 1.5 to 2.5 mass% Si, 1.0 to 1.6 mass% Cr, 0.4 mass% or less Mn, and 1.5 to 2.2 mass% Mo, with the balance being Fe and unavoidable impurities.

[0020] (Bolt tensile strength) The bolt according to this embodiment has a tensile strength of 1500 MPa or more. If the tensile strength is less than 1500 MPa, the strength required for application to a high-power engine or the like is insufficient.

[0021] The tensile strength of the bolt can be controlled within the above range by appropriately controlling the composition of the steel for the bolt and appropriately performing heat treatment such as quenching and tempering in the process of manufacturing the bolt. For example, the technology described in JP 2016-50329 A can be appropriately adopted. The tensile strength of the bolt can be measured by the method described in the examples below.

[0022] (Bolt surface roughness) The bolt according to this embodiment has a surface roughness of 0.48 μm or less in arithmetic mean roughness Ra. If the surface roughness of the bolt exceeds 0.48 μm in Ra, sufficient fatigue strength cannot be obtained and the bolt cannot be used as a high-strength bolt. The smaller the surface roughness of the bolt, the more preferable it is from the viewpoint of increasing fatigue strength, and the lower limit is not particularly limited, but is substantially 0.2 μm. The surface roughness of the bolt refers to the surface roughness of the bolt bearing surface portion after the surface treatment film is formed.

[0023] The surface roughness of the bolt can be controlled, for example, by performing a shot blasting process before or after heat treatment such as quenching or tempering in the manufacturing process of the bolt. Oxide scale is usually present on the surface of the base material made of steel, and may have a certain level of surface roughness, but the surface roughness can be controlled to a predetermined level by performing a shot blasting process. In this case, the surface roughness can be adjusted by appropriately adjusting the type, shape, particle size, and hardness of the projectile used in shot blasting. In high-strength bolts, roughness caused by shot blasting or the like may become notches, which may cause a decrease in the fatigue strength of the bolt, but high fatigue strength can be ensured by setting the surface roughness of the bolt within the above range. The surface roughness of the bolt can be measured by the method described in the examples below.

[0024] (Surface treatment film) The bolt according to the present embodiment has a surface treatment film selected from a chemical conversion film, a plating film, or a paint film on the surface. This can improve the corrosion resistance and fatigue strength of the bolt. The surface treatment film may be formed on at least a part of the surface of the bolt, but is preferably formed on the entire surface of the bolt. Furthermore, it is preferable that the surface treatment film is uniformly adhered to the entire surface of the bolt. The formation of the surface treatment film and its film thickness can be confirmed, for example, by cutting the bolt and observing the bolt surface using the cross section under an electron microscope.

[0025] The chemical conversion coating can be formed by applying a treatment agent to the surface of the base material made of the above-mentioned bolt steel to cause a chemical reaction, and for example, the formation of a coating essentially consisting of zinc phosphate and zinc iron phosphate (zinc phosphate coating), a coating essentially consisting of manganese phosphate and manganese iron phosphate (manganese phosphate coating), etc. can be used. That is, a zinc phosphate coating, a manganese phosphate coating, etc. can be preferably used.

[0026] The plated coating is a coating formed by adsorbing a thin metal film onto the surface of the base material made of the steel for bolts. The plated coating is not particularly limited, and can be formed by electrolytic plating, non-electrolytic plating, hot-dip plating, vacuum plating, etc. The metal forming the plated coating is also not particularly limited, but zinc is preferred.

[0027] The coating film is a surface treatment film formed by applying a coating agent to the surface of the base material made of the above-mentioned bolt steel, baking it as necessary, and drying it. The coating film can be formed by either powder coating or solvent coating. There is no particular limitation on the coating film, but a zinc-aluminum composite coating can be preferably used.

[0028] The thickness of the surface treatment film is not particularly limited, but is, for example, 1 to 20 μm, preferably 1 to 10 μm, and more preferably 2 to 10 μm. In the case of a chemical conversion coating or a paint coating, the thickness is more preferably 6 μm or less, and particularly preferably 5 μm or less. When the thickness of the surface treatment film is 1 μm or more, the surface treatment film is formed stably without disappearing early, and no part without the coating is formed. In addition, it is preferable because the surface treatment film can be formed with high accuracy. On the other hand, it is preferable that the thickness is 20 μm or less, particularly 10 μm or less, because no trouble occurs during fastening.

[0029] The surface treatment film is preferably a zinc phosphate film, a manganese phosphate film, a zinc plating film, or a zinc-aluminum composite film, which can further improve the corrosion resistance of the bolt.

[0030] (Zinc phosphate coating) The zinc phosphate coating is one of the chemical coatings, and can be formed by chemical conversion treatment that forms a phosphate coating on the metal surface. For example, after preferably performing alkaline degreasing and cleaning on a high-strength bolt obtained by the manufacturing method described below, the surface roughness is adjusted by shot blasting or the like, surface conditioning (chemical surface preparation performed before chemical conversion treatment) is performed, and the coating can be formed by immersing the bolt in a zinc phosphate bath under controlled conditions and chemically reacting the bolt. The thickness of the coating is not particularly limited, but is preferably 1 to 6 μm, and more preferably 2 to 5 μm. If the thickness is 1 μm or more, the zinc phosphate coating is unlikely to disappear early and no part is likely to be formed without the coating. By appropriately managing the reaction time and suppressing the variation in the surface state, a coating of an appropriate thickness can be formed on the entire surface of the high-strength bolt.

[0031] (Manganese phosphate coating) The manganese phosphate coating is one of the chemical coatings, and can be formed by chemical conversion treatment that forms a phosphate coating on the metal surface. The high-strength bolt is preferably subjected to alkaline degreasing and cleaning, and then the surface roughness is adjusted by shot blasting or the like, surface conditioning (chemical surface preparation performed before chemical conversion treatment), and the bolt is immersed in a manganese phosphate bath under controlled conditions to cause a chemical reaction to form a coating. The coating thickness is preferably 1 to 6 μm, more preferably 2 to 5 μm. If the thickness is 1 μm or more, the manganese phosphate coating is unlikely to disappear early, and no part is likely to be formed without the coating. By appropriately controlling the reaction time and suppressing the variation in the surface state, a coating of an appropriate thickness can be formed on the entire surface of the high-strength bolt. In addition, the metal surface has a strong etching resistance.

[0032] (Zinc plating) The zinc plating film is a plating film formed by an electric plating method. As the plating bath, either an acid bath or an alkaline bath can be used. As the alkaline bath, a cyanide bath or a zincate bath can be mentioned, and zinc plating using a cyanide bath has excellent uniform electrodeposition, smoothness, and flexibility of the plating film. As the acid bath, a chloride bath or a sulfate bath can be mentioned, and as the chloride bath, a zinc chloride ammonium bath, a zinc chloride potassium bath, or a zinc chloride ammonium potassium bath can be mentioned. The zinc plating film can be formed by a conventional plating method, for example, in a plating bath containing a metal salt, a conductivity imparting agent, a hydrogen ion concentration regulator, an additive, etc., using the metal substrate as the cathode and depositing zinc on the surface of the metal substrate at an appropriate current density. In a preferred embodiment, the high-strength bolt is subjected to alkaline degreasing and cleaning, and then the surface roughness is adjusted by shot blasting or the like, and then the zinc plating film is formed.

[0033] (Zinc-aluminum composite coating) The zinc-aluminum composite coating is formed, for example, by degreasing the high-strength bolt with an alkali, adjusting the surface roughness preferably by shot blasting, and then coating and baking (drying) a coating liquid mainly composed of metal flakes and a silicon-based inorganic binder. The coating method for the zinc-aluminum composite coating can be either the dip spin method or the spray method.

[0034] The amount of coating of the zinc-aluminum composite coating is not particularly limited, but it is generally controlled by the number of times of painting and baking. Usually, 2C2B (2 coats, 2 bakes) is selected, but 1C1B (1 coat, 1 bake) may be selected if a thinner coating is required.

[0035] (Bolt manufacturing method) The manufacturing method of the high-strength bolt of this embodiment is not particularly limited. For example, a conventionally known method such as the method described in JP 2016-50329 A can be appropriately adopted. For example, the above-mentioned steel for high-strength bolts is first subjected to cold forging, then to heat treatment in which quenching is performed at 900°C or higher, and tempering (high-temperature tempering) is performed at 570°C or higher, and then thread rolling is performed, thereby obtaining a high-strength bolt. The order of the above heat treatment (quenching and tempering) and the above thread rolling may be reversed. In addition, when tempering at a high temperature such as 570°C or higher is performed, the schistose cementite on the grain boundary, which is a cause of embrittlement, becomes spheroidal, and the grain boundary strength can be improved.

[0036] Preferably, a step of adding compressive residual stress (shot blasting, etc.) is performed before or after the heat treatment. In this case, by controlling the surface roughness of the bolt to be within a predetermined range, excellent fatigue strength can be obtained in the high-strength bolt. In addition, since the oxide scale on the surface of the steel material can be removed, a surface treatment film can be easily formed. As a result, a surface treatment film that is in close contact with the bolt surface can be obtained, and a bolt with excellent corrosion resistance that can be used even in a corrosive environment can be obtained. The conditions for shot blasting are not particularly limited as long as the predetermined surface roughness can be obtained, and conventionally known knowledge can be referred to as appropriate.

[0037] As conditions for shot blasting, for example, a projection material such as glass can be used. The shape of the projection material is not particularly limited, but is preferably spherical. The particle size of the projection material is not particularly limited, but is preferably 20 to 60 μm. The hardness of the projection material is not particularly limited, but is, for example, Hv450 to 650. The shot time is not particularly limited, but is, for example, 1 to 100 minutes.

[0038] Next, a surface treatment film can be formed on the surface of the bolt by performing a surface treatment. The specific form of the surface treatment film is as described above.

[0039] The high-strength bolt of the present embodiment is not particularly limited, but can be suitably applied to a lower link in a reciprocating engine having a multi-link mechanism. In a reciprocating engine having a multi-link mechanism, the strength required for the lower link is higher than that of a reciprocating engine having a general single-link mechanism. In addition, from the viewpoint of improving fuel efficiency, it is also desired to make the lower link more compact and lightweight. The lower link usually has a structure in which divided lower link components are fastened with a high-strength bolt. In order to obtain a lower link that satisfies the high performance requirements described above, a high-strength bolt that is excellent in corrosion resistance and fatigue strength is required. The high-strength bolt of the present embodiment is not particularly limited, but is particularly suitable for fastening such lower link components. EXAMPLES

[0040] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0041] (Examples 1 to 4, Comparative Examples 1 to 5) A steel for a high-strength bolt, containing 0.59% by mass of C, 1.9% by mass of Si, 1.2% by mass of Cr, 0.3% by mass of Mn, and 1.6% by mass of Mo, with the remainder being Fe and unavoidable impurities, was subjected to cold forging and then thread rolling. Thereafter, a heat treatment was performed in which the steel was quenched at 900°C or higher and tempered at 570°C or higher to obtain a high-strength bolt. The high-strength bolt after the heat treatment was subjected to alkaline degreasing and then shot blasting according to Table 1 below. The bolts of Examples 1 to 4 and Comparative Example 5 were then subjected to the following surface treatment to obtain the high-strength bolts (M11×1.0, neck length 26 mm) of Examples 1 to 4 and Comparative Example 1 to 5. Table 1 shows the shot blasting conditions and the type of surface treatment film.

[0042] (Manganese phosphate coating treatment in Example 1 and Comparative Example 5) The heat-treated high-strength bolt was subjected to alkaline degreasing and cleaning, and then shot blasting was performed under the conditions in Table 1 below. Thereafter, the surface was adjusted using a commercially available surface conditioner, and the bolt was immersed in a manganese phosphate bath for reaction to form a manganese phosphate coating with a thickness of 3 μm. However, in Comparative Example 5, the coating did not adhere uniformly to the entire bolt. This is thought to be because the oxide scale on the bolt surface could not be removed in the bolt of Comparative Example 5. If the oxide scale cannot be removed, the reaction of the chemical conversion treatment does not proceed sufficiently, and the coating does not adhere well to the bolt.

[0043] (Zinc phosphate coating treatment in Example 2) The heat-treated high-strength bolts were subjected to alkaline degreasing and cleaning, and then shot blasting was carried out under the conditions shown in Table 1 below. Thereafter, the surface was conditioned using a commercially available surface conditioner, and the bolts were immersed in a zinc phosphate bath for reaction to form a zinc phosphate coating with a thickness of 3 μm.

[0044] (Zinc-aluminum composite coating treatment of Example 3) The heat-treated high-strength bolts were degreased with an alkali and then shot blasted under the conditions in Table 1 below. After that, a coating solution containing aluminum and zinc metal flakes and a silicon-based inorganic binder as the main components was applied to the bolts, baked (dried), and then washed with water. This formed a zinc-aluminum composite coating with a thickness of 8 μm on the surface of the bolt.

[0045] (Zinc plating coating treatment in Example 4) The heat-treated high-strength bolts were subjected to alkaline degreasing and cleaning, and then shot blasting was performed under the conditions shown in Table 1 below. After that, they were immersed in an alkaline bath containing 13 g / L of metallic zinc, 140 g / L of sodium hydroxide, and 9 g / L of brightener at a bath temperature of 30°C, and a current density of 3 A / dm 2 The plate was then washed with water to form a zinc plating film with a thickness of 5 μm.

[0046] (Bolt tensile strength) The tensile strength of the bolts produced in each Example and Comparative Example was measured according to JIS B 1051:2014 Mechanical properties of carbon steel and alloy steel fasteners - Bolts, machine screws and stud bolts - Coarse threads and fine threads, which specify the strength classification. The tensile strength of the bolts produced in each Example and Comparative Example was 1600 MPa.

[0047] (Bolt surface roughness) The surface roughness of the bolts produced in each of the examples and comparative examples was measured according to the surface roughness measurement method of JIS B 1071: 2010. The results are shown in Table 1 below.

[0048] (Corrosion test) The high-strength bolts produced in each of the examples and comparative examples were used to carry out a composite corrosion test to evaluate the corrosion resistance. Specifically, a salt spray treatment using a 5% by mass aqueous solution of sodium chloride was carried out at room temperature of 35°C for 4 hours, followed by a drying treatment at room temperature of 60°C and a relative humidity of 30% or less for 2 hours, and then a wet treatment at room temperature of 50°C and a relative humidity of 95±5%RH for 2 hours. After carrying out 18 cycles of a composite corrosion test for 8 hours (total of 144 hours), the appearance corrosion condition was observed from the viewpoint of the occurrence of corrosion pits. The results of the corrosion test of the high-strength bolts of each of the examples and comparative examples are shown in Table 1 below. In Table 1, "OK" indicates that no corrosion pits occurred on the bolt, and "NG" indicates that corrosion pits occurred on the bolt.

[0049] (Fatigue Test) The high-strength bolts produced in each of the Examples and Comparative Examples were mounted on an axial fatigue testing machine at room temperature (25°C) and in the air, and fatigue tests were carried out at a constant stress by repeatedly applying a tensile load, based on JIS B 1081:1997 Threaded parts - Tensile fatigue test - Test method and evaluation of the results, to compare the fracture life of the bolts. The results are shown in Table 1 below. In Table 1, the fracture life is shown up to one decimal place as a relative ratio (= time to fracture for each of the Examples and Comparative Examples / time to fracture for Comparative Example 1), with the time to fracture for Comparative Example 1 being taken as 1.0.

[0050] [Table 1]

[0051] From the results in Table 1 above, it can be seen that the high-strength bolts of Examples 1 to 4, which have a predetermined surface treatment film and a surface roughness of 0.48 μm or less, are high-strength bolts with excellent corrosion resistance and fatigue strength. On the other hand, the bolts of Comparative Examples 1 to 5, which do not have a surface treatment film or have a surface roughness of more than 0.48 μm, have insufficient corrosion resistance and fatigue strength.

Claims

1. In a high-strength bolt having a tensile strength of 1500 MPa or more, The bolt is made of a steel for a bolt containing C: 0.50 to 0.65 mass%, Si: 1.5 to 2.5 mass%, Cr: 1.0 to 1.6 mass%, Mn: more than 0 mass% and 0.4 mass% or less, and Mo: 1.5 to 2.2 mass%, A high-strength bolt, characterized in that the bolt has a surface that has been shot blasted with glass having a particle size of 20 to 60 μm and a hardness of Hv 450 to 650, and has a surface treatment film selected from a chemical conversion coating, a plating coating, or a paint coating, and the bolt on which the surface treatment film has been formed has a surface roughness Ra of 0.48 μm or less.

2. 2. The high-strength bolt according to claim 1, wherein the surface treatment film is a zinc phosphate coating, a manganese phosphate coating, a zinc plating coating, or a zinc-aluminum composite coating.

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