Bolt and bolt manufacturing method

Abstract

Provided are: a steel bar suitable as a material for a bolt having a large diameter, high strength, and high toughness; a bolt having a large diameter, high strength, and high toughness; and a method for producing the bolt. The steel bar 1 has a chemical composition, in mass %, of C: 0.35-0.45% inclusive, Si: 0.10-0.50% inclusive, Mn: 0.50-2.00% inclusive, P: 0.030% or less, S: 0.030% or less, Ni: 1.00-3.00% inclusive, Cr: 0.50-2.00% inclusive, and Mo: 0.10-0.50% inclusive, the balance being Fe and impurities, and has a DI value of 420 or more and a diameter of 65 mm or more.

Description

[Technical Field]

[0001] The present invention relates to a steel bar, a bolt, and a method for manufacturing a bolt. [Background technology]

[0002] Patent Document 1 discloses a method for manufacturing large-diameter bolts. In this manufacturing method, bolts of M25 to M40 are formed by cold forging from a steel wire rod containing, by weight, 0.28 to 0.38% C, 0.10% or less Si, 0.60 to 1.20% Mn, and 0.20 to 0.60% Cr, with the remainder being iron and impurities, with a value calculated by a predetermined formula satisfying 30 to 45. The bolts are then heated, water quenched, and tempered. With this manufacturing method, when manufacturing bolts of M25 to M40, no softening annealing is required, allowing direct cold forging, and the bolts have sufficient hardenability, do not crack during bolt processing, and have a tensile strength of 75 kgf / mm after quenching and tempering. 2 It is said that the above bolts can be manufactured inexpensively. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 02-166231 Summary of the Invention [Problem to be solved by the invention]

[0004] As disclosed in Patent Document 1, bolts are subjected to quenching and tempering treatments during their manufacture to achieve the desired strength. However, as bolts become larger, it becomes more difficult to obtain a quenched structure inside the bolt. The bolt described in Patent Document 1 has a maximum diameter of 40 mm (M40), as described above. High toughness is also required for the bolt. Therefore, there is a demand for a steel bar suitable as a material for a large-diameter, high-strength, high-toughness bolt, a large-diameter, high-strength, high-toughness bolt, and a method for manufacturing such a bolt.

[0005] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a steel bar suitable as a material for a large-diameter, high-strength, and high-toughness bolt, a large-diameter, high-strength, and high-toughness bolt, and a method for manufacturing the bolt. [Means for solving the problem]

[0006] In order to achieve the above object, the steel bar, the bolt, and the method for manufacturing the bolt according to the present invention are as follows.

[0007] 1. Chemical composition in mass % C: 0.35% or more and 0.45% or less, Si: 0.10% or more and 0.50% or less, Mn: 0.50% or more and 2.00% or less, P:0.030% or less, S: 0.030% or less, Ni: 1.00% or more and 3.00% or less, Cr: 0.50% or more and 2.00% or less Mo: 0.10% or more and 0.50% or less The balance is Fe and impurities, The DI value represented by the following formula (1) is 420 or more, Steel bars with a diameter of 65 mm or more. DI=(-23.7×[C] 2 +38.3×[C]+3.54)×(1+3.3×[Mn])×(1+0.7×[Si])×(1+2.16×[Cr])×(1+3×[Mo])×(1+0.36×[Ni]) ···(1) Here, [C], [Mn], [Si], [Cr], [Mo] and [Ni] are the contents of C, Mn, Si, Cr, Mo and Ni expressed in mass %, in that order.

[0008] 2. A bolt using the steel bar described in 1 above, The bolt has a shank, In a cross section perpendicular to the longitudinal direction of the shaft portion, the prior γ grain size is a grain size number of 4 or more and 10 or less, A bolt, wherein the area ratio of tempered martensite in the metal structure of the cross section is 80% or more.

[0009] 3. A method for manufacturing a bolt, comprising cutting the steel bar described in 1 above, and subjecting the cut steel bar to hot forging, thread rolling, quenching, and tempering to manufacture a bolt having a shank and a bolt head, The bolt head is formed in the hot forging. In the thread rolling, a thread is formed on the shank portion, A method of manufacturing a bolt, wherein the heating temperature during the quenching is 820°C or higher and 1150°C or lower, and the average cooling rate from 800°C to 300°C is 0.5°C / s or higher. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a steel bar suitable as a material for a large-diameter, high-strength, and high-toughness bolt, a large-diameter, high-strength, and high-toughness bolt, and a method for manufacturing the bolt. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram showing an example of a steel bar according to an embodiment of the present invention. [Figure 2] FIG. 2 is a diagram illustrating an example of a bolt according to the present embodiment. [Figure 3] FIG. 1 is a flow chart showing an example of a bolt manufacturing process for manufacturing a bolt. [Figure 4] FIG. 1 is a flow chart showing an example of a bolt manufacturing process for manufacturing a bolt. DETAILED DESCRIPTION OF THE INVENTION

[0012] First, an outline of the steel bar, the bolt, and the method for manufacturing the bolt according to this embodiment will be described.

[0013] The steel bar according to this embodiment is Chemical composition in mass %: C: 0.35% or more and 0.45% or less, Si: 0.10% or more and 0.50% or less, Mn: 0.50% or more and 2.00% or less, P:0.030% or less, S: 0.030% or less, Ni: 1.00% or more and 3.00% or less, Cr: 0.50% or more and 2.00% or less Mo: 0.10% or more and 0.50% or less The balance is Fe and impurities.

[0014] The steel bar according to this embodiment has a DI value of 420 or more as expressed by the following formula (1). In the following formula (1), [C], [Mn], [Si], [Cr], [Mo], and [Ni] represent the contents of C, Mn, Si, Cr, Mo, and Ni, respectively, expressed in mass%. The diameter of the steel bar is 65 mm or more.

[0015] DI=(-23.7×[C] 2 +38.3×[C]+3.54)×(1+3.3×[Mn])×(1+0.7×[Si])×(1+2.16×[Cr])×(1+3×[Mo])×(1+0.36×[Ni]) ···(1)

[0016] The above steel bar is suitable as a material for manufacturing large-diameter bolts. A bolt according to this embodiment uses the above steel bar and has a shank portion in which, in a cross section perpendicular to the longitudinal direction, the prior γ grain size is between 4 and 10 inclusive, and the area ratio of tempered martensite in the metal structure of the cross section is 80% or more. One example of a method for manufacturing this bolt is a bolt manufacturing method in which the above steel bar is subjected to hot forging to form a bolt head, thread rolling to form threads in the shank, and quenching and tempering to obtain a bolt having a bolt head and a shank. Furthermore, in the quenching, the heating temperature is 820°C or more and 1150°C or less, and the average cooling rate from 800°C to 300°C is 0.5°C / s or more.

[0017] FIG. 1 shows a steel bar 1 as an example of a steel bar according to this embodiment. FIG. 2 shows a bolt 2 having a bolt head 21 and a shank 22 on which threads are formed as an example of a bolt according to this embodiment. The bolt 2 can be formed into the shape shown in the figure by cutting the steel bar 1 to a predetermined length, forming the bolt head 21 at one end by hot forging, and then forming threads on the shank 22 at the other end by rolling. Note that although the shank 22 in FIG. 2 is shown with threads formed on the entirety, in this embodiment, it is sufficient that the threads are formed on at least a portion of the shank.

[0018] Steel bars will be described in detail below. In the following description, when "%" is simply written, it means "% by mass" unless otherwise specified.

[0019] The steel bar according to the present embodiment is a steel bar in a rod shape. From the viewpoint of making the steel bar suitable as a material for a bolt, the steel bar may be, for example, a cylindrical steel bar.

[0020] The steel bar according to the present invention has a diameter of 65 mm or more in order to be suitable as a material for large-diameter bolts. On the other hand, although there is no upper limit to the diameter, it is preferably 100 mm or less.

[0021] As described above, the steel bar according to this embodiment has a chemical composition containing C (carbon), Si (silicon), Mn (manganese), P (phosphorus), S (sulfur), Ni (nickel), Cr (chromium), and Mo (manganese), with the balance being Fe (iron) and impurities. Impurities are elements that are permitted to be contained within a range that does not adversely affect the effects of the present invention. These impurities include so-called inevitable impurities that are inevitably mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of steel bars.

[0022] The chemical composition of the steel bar (the content of each element in the steel bar) will be described in detail below.

[0023] The C content is 0.35% or more and 0.45% or less. C is an element that affects the hardenability and tensile strength of a steel bar. If the C content is less than 0.35%, the tensile strength of the steel bar will be insufficient. Therefore, the C content is 0.35% or more, preferably 0.38% or more. On the other hand, if the C content exceeds 0.45%, the toughness of the steel bar will decrease. Therefore, the C content is 0.45% or less, preferably 0.43% or less.

[0024] The Si content is 0.10% or more and 0.50% or less. Si is an element that affects the hardenability and temper softening resistance of steel bars. If the Si content is less than 0.10%, the hardenability is insufficient and the tensile strength of the steel bars decreases. Therefore, the Si content is 0.10% or more, preferably 0.15% or more. On the other hand, if the Si content exceeds 0.50%, the temper softening resistance increases and the toughness of the steel bars decreases. Therefore, the Si content is 0.50% or less, preferably 0.35% or less.

[0025] The Mn content is 0.50% or more and 2.00% or less. Mn is an element that affects the hardenability and martensitic transformation temperature of steel bars. If the Mn content is less than 0.50%, the hardenability will be insufficient and the tensile strength of the steel bars will decrease. Therefore, the Mn content is 0.50% or more, preferably 0.70% or more. On the other hand, if the Mn content exceeds 2.00%, the martensitic transformation temperature will decrease, retained austenite will be more likely to be formed, and the tensile strength of the steel bars will be insufficient. Therefore, the Mn content is 2.00% or less, preferably 1.00% or less.

[0026] The P content is 0.030% or less. P is an element that segregates at grain boundaries and affects the toughness of steel bars. If the P content exceeds 0.030%, the grain boundary strength decreases, and the toughness of the steel bars decreases. Therefore, the P content is 0.030% or less, and preferably 0.020% or less. On the other hand, since the inclusion of P is usually unavoidable, excessively low P content may lead to an increase in refining time and refining costs. Therefore, the P content is preferably 0.003% by mass or more, and more preferably 0.007% by mass or more.

[0027] The S content is 0.030% or less. S is an element that combines with Mn to form MnS, which affects the tensile strength and toughness of steel bars. MnS acts as a fracture initiation point in the steel structure. If the S content exceeds 0.030%, the toughness of the steel bars decreases. Therefore, the S content is 0.030% or less, preferably 0.015% or less. On the other hand, since the inclusion of S is usually unavoidable, excessively reducing the S content may increase refining costs. Therefore, the S content is preferably 0.003% by mass or more, more preferably 0.007% by mass or more.

[0028] The Ni content is 1.00% or more and 3.00% or less. Ni is an element that affects the hardenability and toughness of steel bars. If the Ni content is less than 1.00%, the hardenability and toughness decrease. Therefore, the Ni content is 1.00% or more, and preferably 1.50% or more. On the other hand, if the Ni content exceeds 3.00%, the effect of improving hardenability and toughness saturates, resulting in high costs. Therefore, the Ni content is 3.00% or less, and preferably 2.00% or less.

[0029] The Cr content is 0.50% or more and 2.00% or less. Cr is an element that affects the hardenability and martensitic transformation temperature of steel bars. If the Cr content is less than 0.50%, the hardenability is insufficient and the tensile strength decreases. Therefore, the Cr content is 0.50% or more, preferably 0.70% or more. On the other hand, if the Cr content exceeds 2.00%, the effect of improving hardenability saturates and the cost becomes high. Therefore, the Cr content is 2.00% or less, preferably 1.00% or less.

[0030] The Mo content is 0.10% or more and 0.50% or less. Mo is an element that affects the hardenability and temper softening resistance of steel bars. If the Mo content is less than 0.10%, the hardenability is insufficient and the tensile strength decreases. Therefore, the Mo content is 0.10% or more. On the other hand, if the Mo content exceeds 0.50%, the effect of improving hardenability saturates. Therefore, the Mo content is 0.50% or less.

[0031] The DI value represented by the above formula (1) is a criterion for hardenability. If the DI value is low, the hardenability will be insufficient. Therefore, the DI value is set to 420 or more, preferably 450, and more preferably 500. On the other hand, the upper limit of the DI value is not particularly limited, but may be, for example, 8000 or less.

[0032] The bolt according to this embodiment will be described in detail below.

[0033] The bolt according to this embodiment uses the steel bar described above. The bolt has a shank. The outer diameter of the shank corresponds to the diameter of the steel bar used as the material. Therefore, the outer diameter of the shank can be set to 65 mm or more. The outer diameter of the shank is preferably set to 100 mm or less.

[0034] Next, the metal structure of the shank of the bolt will be described. The metal structure of the shank of the bolt described below is the structure observed in a cross section perpendicular to the longitudinal direction of the shank.

[0035] In the metal structure, the grain size of the prior γ grain size (also referred to as prior austenite grain size) is a grain size number of 4 or more and 10 or less. The grain size number in this embodiment is based on the provisions of JIS G 0551 "Steel - Microscopic test method for grain size" and is measured at a position d / 4 away from the radial center of the shaft portion. d refers to the outer diameter of the shaft portion. More specifically, it is measured by the method described in the examples.

[0036] The prior γ grain size affects the toughness of steel bars. If the prior γ grain size is less than grain size number 4, the toughness of the bolt decreases. Therefore, the prior γ grain size is grain size number 4 or more, preferably grain size number 6 or more. On the other hand, if the prior γ grain size is more than grain size number 10, the tensile strength decreases. Therefore, the prior γ grain size is grain size number 10 or less, preferably grain size number 9 or less.

[0037] Next, the area ratio of tempered martensite in the metal structure of the cross section will be described. The area ratio of tempered martensite is measured at the radial center of the shaft portion. More specifically, it is measured by the method described in the examples.

[0038] The area fraction of tempered martensite affects the tensile strength and toughness of the bolt. If the area fraction of tempered martensite is small, the tensile strength and toughness of the bolt decrease. Therefore, the area fraction of tempered martensite is 80% or more, preferably 85% or more. On the other hand, there is no upper limit to the area fraction of tempered martensite, and it may be 100%, and the metal structure may consist of tempered martensite. The structure of the remaining portion is not limited, and may be one or more structures selected from the group consisting of ferrite, pearlite, bainite, and retained austenite.

[0039] By improving the tensile strength of the shank, the bolt can be made suitable for use. Therefore, the tensile strength of the shank is preferably 1040 MPa or more. The tensile strength of the shank is measured at a distance of d / 4 from the radial center to the outside, based on JIS Z 2241. More specifically, it is measured by the method described in the examples.

[0040] By improving the toughness of the shank, the bolt can be made suitable for use. Therefore, the Charpy absorbed energy of the shank is preferably 27 J or more. The Charpy absorbed energy of the shank is measured at a distance of d / 4 from the radial center to the outside by the Charpy impact test specified in JIS Z 2242. More specifically, it is measured by the method described in the examples.

[0041] The method for manufacturing the bolt will be described in detail below.

[0042] An example of a flow of a bolt manufacturing process for manufacturing a bolt is shown in Figure 3. In the bolt manufacturing process, a steel bar and a bolt according to this embodiment are manufactured using a steel material.

[0043] The bolt manufacturing process includes a steel bar manufacturing process S1 in which steel is used to manufacture steel bars that serve as the material for bolts, and a bolt forming process S3 in which the steel bars manufactured in the steel bar manufacturing process S1 are processed into bolts. This bolt manufacturing process may include an intermediate process S2 in which the steel bars are transported from the workplace or business establishment where the steel bar manufacturing process S1 is carried out to the workplace or business establishment where the bolt forming process S3 is carried out.

[0044] In the steel bar manufacturing process S1, a steel material having the above-mentioned chemical composition and DI value is hot-rolled or hot-forged (rolling process S11) to form a steel bar of a predetermined diameter (e.g., a steel bar with a diameter of 65 mm or more). Examples of the steel material that can be used include blooms cast by a continuous casting method and steel ingots cast by an ingot casting method. The steel bar undergoes predetermined quality inspections as necessary.

[0045] In the intermediate process S2, the steel bars are shipped (shipping process S21) and transported (transporting process S22) to the workshop or business where the bolt forming process S3 is performed, and then received at the workshop or business where the bolt forming process S3 is performed (receiving process S23).

[0046] In the bolt forming process S3, the received steel bar is cut to a predetermined size (cutting process S31). Then, the cut steel bar is hot forged to form a bolt head (hot forging process S32), threads are formed by rolling (thread rolling process S33), and then quenched (quenching process S34) and tempered (tempering process S35) to obtain a bolt as a product.

[0047] Another example of the flow of the process for manufacturing a bolt is shown in Fig. 4. As shown in Fig. 4, in the bolt forming process S3, rolling (thread rolling process S33) may be performed after quenching (quenching process S34) and tempering treatment (tempering process S35).

[0048] After the bolt forming step S3, the bolt may be subjected to a coating treatment such as painting or plating.

[0049] The quenching (quenching step S34) and tempering treatment (tempering step S35) are described in detail below. The process conditions for obtaining the desired tensile strength and toughness of the bolt are explained. In the following process description, the temperature of the steel used to form the bolt is indicated as the temperature at the radial center of the shank.

[0050] The quenching in the quenching step S34 is performed at a heating temperature of 820°C or higher and 1150°C or lower, and at an average cooling rate of 0.5°C / s or higher from 800°C to 300°C. If the heating temperature is lower than 820°C, the structure of the shaft portion cannot be made into an austenite single-phase structure. Therefore, the heating temperature is 820°C or higher. On the other hand, if the heating temperature is raised too high, the crystal grains will become coarse and toughness will decrease, so the heating temperature is 1150°C or lower, and preferably 900°C or lower.

[0051] By setting the cooling rate at an average cooling rate of 0.5°C / s or more from 800°C to 300°C, transformation into structures other than martensite, such as ferrite and pearlite, is suppressed. If the average cooling rate is slow, the desired structure cannot be obtained. Therefore, the average cooling rate from 800°C to 300°C is set to 0.5°C / s or more. Furthermore, by setting the cooling stop temperature to 150°C or less, martensitic transformation can be promoted. Therefore, the cooling stop temperature is preferably 150°C or less.

[0052] The tempering in the tempering step S35 is preferably performed by heating to 500°C or higher and then holding for 30 minutes or longer. That is, the holding temperature is preferably 500°C or higher. The holding time is preferably 30 minutes or longer. After holding, it is preferably cooled to room temperature. When performing this cooling, it is desirable to increase the cooling rate as much as possible to avoid embrittlement due to grain boundary segregation of P. [Example]

[0053] In the following examples, in order to evaluate the relationship between the properties of a steel bar and the properties of a bolt manufactured from the steel bar, a sample simulating the shank of a bolt was created by quenching and tempering the steel bar without forming a thread or a bolt head, and evaluation was performed using the sample.

[0054] Molten steels (Steel Nos. A to V) having the compositions (the balance being Fe and impurities) and DI values ​​shown in Table 1 were cast by a continuous casting machine to form slabs with a cross section of 300 mm × 400 mm. In Table 1, the underlined values ​​indicate that the values ​​are outside the range of the compositions or DI values ​​specified in this embodiment.

[0055] [Table 1]

[0056] Next, this slab was soaked at 1250°C for 30 minutes and then hot rolled to form a slab with a rectangular cross section with one side measuring 140 mm. Furthermore, each slab was hot rolled to the diameters shown in Table 2 to form steel bars Nos. 1 to 30 (as-hot-rolled materials). Here, the slabs other than Steel No. J were hot rolled to form steel bars with diameters of 65 mm to 100 mm. The slab of Steel No. J was hot rolled to form a steel bar with a diameter of 40 mm. Steel bars Nos. 1 to 30 were cylindrical.

[0057] [Table 2]

[0058] Next, the as-hot-rolled steel bars were quenched at the heating temperature, cooling method, and average cooling rate shown in Table 3. In Table 3, the "heating temperature" item refers to the temperature during heating for quenching. Furthermore, the "cooling method" refers to the cooling method performed after heating for quenching, and the "average cooling rate" refers to the average cooling rate from 800°C to 300°C. In Table 3, the underlines next to each value indicate that the value is outside the range of conditions for the manufacturing method defined in this embodiment.

[0059] [Table 3]

[0060] During quenching, the steel bars were first heated until the radial center reached the heating temperature shown in Table 3, and then held at this temperature for 30 minutes. After that, they were further cooled using the cooling method and average cooling rate shown in Table 3. The oil used for oil quenching was High Speed ​​Quench Oil manufactured by Nippon Grease Co., Ltd. The oil or water temperature during cooling was room temperature. Except for Product No. 15 and Product No. 18, the average cooling rate during cooling from 800°C to 300°C was 1.5 to 9.2°C / s. For Product No. 15 and Product No. 18, the average cooling rate during cooling from 800°C to 300°C was 6.7°C / s and 0.3°C / s, respectively.

[0061] After the above quenching, the steel bar was heated to 540°C at the radial center, held at this temperature for 120 minutes, and then subjected to a tempering treatment in which it was further water-cooled.

[0062] For the quenched and tempered product samples No. 1 to No. 30 obtained as described above, the samples were used as the shank of a bolt and were subjected to cross-sectional structure observation, measurement of prior γ grain size, tensile testing, and Charpy impact testing.

[0063] The microstructure was observed on a cross section perpendicular to the longitudinal direction of the sample. The cross section was exposed by cutting the sample across the longitudinal direction. The cross section was then corroded with 1% nital solution and observed using an optical microscope.

[0064] The observation position was within a 4 mm x 4 mm rectangular area in the center (the center in the radial direction of the sample), where the cooling rate was slowest and martensite structure was least likely to be obtained. Three randomly selected fields were observed at 400x magnification, and tempered martensite and other structures were distinguished by visual observation. The area fraction of tempered martensite structure was then calculated based on the results of this distinction. The average value of the three fields was used as the area fraction. Table 3 shows the area fraction of tempered martensite. In Table 3, for each sample where the area fraction of tempered martensite structure was not 100%, the observed structure other than martensite structure is shown in the "Remaining structure" section.

[0065] The prior γ grain size was measured on a cross section perpendicular to the longitudinal direction, as in the structure observation. The prior γ grain boundaries in the cross section were revealed using a picric acid aqueous solution, and the cross section was observed using an optical microscope. The observation position was a distance of d / 4 from the center of the sample to the outside, where d is the diameter of the sample. The grain size number was determined by comparison with a standard diagram based on JIS G 0551. Table 3 also shows the measurement results of the prior γ grain size.

[0066] The tensile test was carried out in accordance with JIS Z 2241. The test specimens used for the tensile test were taken at a distance of d / 4 as described above. The test specimen shape was JIS No. 4 (diameter of the parallel part was 14 mm). The test was carried out at a tensile speed of 3 mm / min. The tensile strength was calculated by dividing the maximum load shown during the tensile test by the cross-sectional area of ​​the test specimen measured before the test.

[0067] To evaluate toughness, Charpy impact tests were performed in accordance with JIS Z 2242. The test specimens (rectangular prisms with square cross sections measuring 10 mm on a side) were taken at the distance d / 4 described above, with the center of the square cross section of the test specimen positioned at that point. The Charpy impact test specimens were taken so that one longitudinal face of the specimen was perpendicular to the radial center of the specimen (i.e., the perpendicular line extending from the longitudinal face of the specimen facing the radial center of the steel overlaps with the center of the specimen). The notch shape of the Charpy impact test specimens was 2 mm V (V-shaped, 2 mm deep, V-notch). The notch was made on the surface perpendicular to the radial center of the specimen when the specimen was taken from the specimen. The Charpy impact test was performed at -40°C. The absorbed energy determined from this Charpy impact test is also shown in Table 3.

[0068] Of product Nos. 1 to 30 shown in Table 3, product Nos. 1 to 14 correspond to invention examples. Product Nos. 16 to 30 do not satisfy the requirements defined in this embodiment and correspond to comparative examples. Product No. 15 is a reference example that uses a steel bar with a diameter of 40 mm as the material.

[0069] For use as a bolt, a tensile strength of 1040 MPa or more is sufficient (i.e., pass), and an absorbed energy of 27 J or more is sufficient (i.e., pass).

[0070] As is clear from Table 3, samples Nos. 1 to 14 have a tensile strength of 1040 MPa or more and an absorbed energy of 27 J or more, despite having a large diameter of 65 mm or more. Therefore, they have the strength and toughness required for bolts. Specifically, samples Nos. 1 to 14 have tensile strengths and absorbed energy equivalent to those of samples such as sample No. 15, which has a diameter of 40 mm, equivalent to the prior art. Furthermore, samples Nos. 1 to 14 have significantly higher tensile strengths and absorbed energy than samples Nos. 15 to 30, which do not meet the requirements of this embodiment.

[0071] That is, to manufacture a bolt having high tensile strength and toughness, it is effective to use a steel bar having the chemical composition and DI value described above. Furthermore, when a bolt using the steel bar satisfies the requirements for the prior γ grain size and the area fraction of tempered martensite described above, high tensile strength and toughness can be achieved.

[0072] The configurations disclosed in the above embodiments (including other embodiments, the same applies below) can be applied in combination with configurations disclosed in other embodiments, as long as no contradiction arises. Furthermore, the embodiments disclosed in this specification are examples, and the embodiments of the present invention are not limited to these, and can be modified as appropriate within the scope that does not deviate from the purpose of the present invention. [Industrial Applicability]

[0073] The present invention is applicable to steel bars, bolts, and methods for manufacturing bolts. [Explanation of symbols]

[0074] 1: Steel bar 2: Bolt 21: Bolt head 22: Shaft S1: Steel bar manufacturing process S11: Rolling process S2: Intermediate process S21: Shipping process S22: Transport process S23: Receiving Process S3: Bolt forming process S31: Cutting process S32: Hot forging process S33: Thread rolling process S34: Quenching process S35: Tempering process

Claims

1. A bolt made of steel bar, The steel bar is Chemical composition in mass %: C: 0.35% or more and 0.45% or less, Si: 0.10% or more and 0.50% or less, Mn: 0.50% or more and 2.00% or less, P: 0.030% or less, S: 0.030% or less, Ni: 1.00% or more and 3.00% or less, Cr: 0.50% or more and 2.00% or less; Mo: 0.10% or more and 0.50% or less, The balance is Fe and impurities, The DI value represented by the following formula (1) is 420 or more, The diameter is 65 mm or more, The bolt has a shank, In a cross section perpendicular to the longitudinal direction of the shaft portion, the prior γ grain size is a grain size number of 4 or more and 10 or less, A bolt, wherein the area ratio of tempered martensite in the metal structure of the cross section is 80% or more. DI=(-23.7×[C] 2 +38.3×[C]+3.54)×(1+3.3×[Mn])×(1+0.7×[Si])×(1+2.16×[Cr])×(1+3×[Mo])×(1+0.36×[Ni]) ・・・(1) Here, [C], [Mn], [Si], [Cr], [Mo] and [Ni] are the contents of C, Mn, Si, Cr, Mo and Ni expressed in mass %, in that order.

2. A method for manufacturing a bolt, comprising cutting a steel bar, and subjecting the cut steel bar to hot forging, thread rolling, quenching, and tempering to manufacture a bolt having a shank and a bolt head, The steel bar is Chemical composition in mass %: C: 0.35% or more and 0.45% or less, Si: 0.10% or more and 0.50% or less, Mn: 0.50% or more and 2.00% or less, P: 0.030% or less, S: 0.030% or less, Ni: 1.00% or more and 3.00% or less, Cr: 0.50% or more and 2.00% or less; Mo: 0.10% or more and 0.50% or less, The balance is Fe and impurities, The DI value represented by the following formula (1) is 420 or more, The diameter is 65 mm or more, The bolt head is formed in the hot forging. In the thread rolling, a thread is formed on the shank portion, A method of manufacturing a bolt, wherein the heating temperature during the quenching is 820°C or higher and 1150°C or lower, and the average cooling rate from 800°C to 300°C is 0.5°C / s or higher. DI=(-23.7×[C] 2 +38.3×[C]+3.54)×(1+3.3×[Mn])×(1+0.7×[Si])×(1+2.16×[Cr])×(1+3×[Mo])×(1+0.36×[Ni]) ・・・(1) Here, [C], [Mn], [Si], [Cr], [Mo] and [Ni] are the contents of C, Mn, Si, Cr, Mo and Ni expressed in mass %, in that order.

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