Steel bar, bolt, and method for manufacturing bolt

Abstract

Provided are a bar steel 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 producing the bolt. The steel bar (1) has a chemical composition comprising, in mass%, 0.35% to 0.45% of C, 0.10% to 0.50% of Si, 0.50% to 2.00% of Mn, 0.030% or less of P, 0.030% or less of S, 1.00% to 3.00% of Ni, 0.50% to 2.00% of Cr, and 0.10% to 0.50% of Mo, with the remainder 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] This invention relates to steel bars, bolts, and methods for manufacturing bolts. Background Technology

[0002] Patent Document 1 discloses a method for manufacturing large-diameter bolts. In this method, bolts of M25 to M40 are formed from steel wire by cold forging, followed by heating, water quenching, and tempering. The steel wire contains, by weight percent, 0.28–0.38% C, less than 0.10% Si, 0.60–1.20% Mn, and 0.20–0.60% Cr, with a value calculated using a prescribed formula satisfying 30–45. The remainder consists of iron and impurities. This manufacturing method allows for direct cold forging of M25–M40 bolts without softening annealing. Furthermore, it provides sufficient hardenability, preventing cracking during bolt processing, and enables low-cost manufacturing of bolts with a tensile strength of 75 kgf / mm² after quenching and tempering. 2 The above bolts.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: Japanese Patent Application Publication No. 02-166231 Summary of the Invention

[0006] As disclosed in Patent Document 1, bolts undergo quenching and tempering during manufacturing to achieve the desired strength. However, with the increasing size of bolts, it is difficult to obtain a quenched structure inside the bolt. In the bolts described in Patent Document 1, as mentioned above, the maximum diameter is 40 mm (M40). Furthermore, high toughness is also required for bolts. Therefore, there is a need for bar steel suitable as a material for large-diameter, high-strength, and high-toughness bolts, as well as large-diameter, high-strength, and high-toughness bolts and a method for manufacturing such bolts.

[0007] The present invention was made in view of this actual situation, and its object is to provide a bar steel suitable as a material for bolts with large diameter, high strength and high toughness, a bolt with large diameter, high strength and high toughness, and a method for manufacturing the bolt.

[0008] To achieve the above objectives, the present invention relates to steel bars, bolts, and methods for manufacturing bolts as follows.

[0009] 1. A type of bar steel, with a chemical composition (by mass%) of C: 0.35%–0.45%, Si: 0.10%–0.50%, Mn: 0.50%–2.00%, P: less than 0.030%, S: less than 0.030%, Ni: 1.00%–3.00%, Cr: 0.50%–2.00%, and Mo: 0.10%–0.50%, the remainder being Fe and impurities.

[0010] Furthermore, the DI value expressed by the following formula (1) is 420 or higher.

[0011] The diameter is 65mm or more.

[0012] 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)

[0013] Wherein, [C], [Mn], [Si], [Cr], [Mo] and [Ni] are the contents of C, Mn, Si, Cr, Mo and Ni expressed in mass% respectively.

[0014] 2. A bolt, which uses the bar steel described in 1 above,

[0015] The bolts described above have a shaft portion.

[0016] In the section perpendicular to the long side of the shaft, the original γ grain size is 4 to 10. In the metal structure of the above section, the area ratio of tempered martensite is more than 80%.

[0017] 3. A method for manufacturing a bolt, comprising cutting the bar steel described in 1 above, and subjecting the cut bar steel to hot forging, thread rolling, quenching, and tempering, thereby manufacturing a bolt having a shaft portion and a bolt head.

[0018] In the above hot forging process, the bolt head is formed.

[0019] In the aforementioned thread rolling process, the thread teeth of the aforementioned shaft portion are formed.

[0020] In the above quenching process, the heating temperature is set to be above 820℃ and below 1150℃, and the average cooling rate from 800℃ to 300℃ is set to be above 0.5℃ / s.

[0021] According to the present invention, it is possible to provide bar steel suitable as a material for bolts with large diameter, high strength and high toughness, bolts with large diameter, high strength and high toughness, and a method for manufacturing the bolts. Attached Figure Description

[0022] Figure 1 This is a diagram illustrating an example of the bar steel involved in this embodiment.

[0023] Figure 2 This is a diagram illustrating an example of the bolt involved in this embodiment.

[0024] Figure 3 This is a flowchart illustrating an example of the bolt manufacturing process.

[0025] Figure 4 This is a flowchart illustrating an example of the bolt manufacturing process. Detailed Implementation

[0026] First, a summary of the steel bars, bolts, and bolt manufacturing methods involved in this embodiment will be provided.

[0027] The bar steel involved in this embodiment has the following chemical composition by mass %: C: 0.35% to 0.45%, Si: 0.10% to 0.50%, Mn: 0.50% to 2.00%, P: less than 0.030%, S: less than 0.030%, Ni: 1.00% to 3.00%, Cr: 0.50% to 2.00%, and Mo: 0.10% to 0.50%, with the remainder being Fe and impurities.

[0028] Furthermore, in the bar steel according to this embodiment, the DI value expressed by the following formula (1) is 420 or more. It should be noted that in the following formula (1), [C], [Mn], [Si], [Cr], [Mo], and [Ni] are the contents of C, Mn, Si, Cr, Mo, and Ni, respectively, expressed in mass percent. Additionally, the diameter of the aforementioned bar steel is 65 mm or more.

[0029] 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)

[0030] The aforementioned bar steel is suitable as a material for manufacturing large-diameter bolts. The bolt involved in this embodiment is a bolt made using the aforementioned bar steel, with a shaft portion having an original γ grain size of 4 to 10 in a cross-section perpendicular to the long side, and a tempered martensite area ratio of 80% or more in the microstructure of the metal having the aforementioned cross-section. An example of a method for manufacturing this bolt is: hot forging to form the bolt head, thread rolling to form the thread teeth of the shaft portion, and quenching and tempering are performed on the aforementioned bar steel, thereby producing a bolt having a bolt head and a shaft portion. Furthermore, in the aforementioned quenching, the heating temperature is set to 820°C or higher and 1150°C or lower, and the average cooling rate from 800°C to 300°C is set to 0.5°C / s or higher.

[0031] Figure 1 In this embodiment, bar steel 1 is shown as an example of the bar steel involved. Figure 2 In this embodiment, as an example of a bolt, a bolt 2 is shown having a bolt head 21 and a shaft portion 22 with threaded teeth. The bolt 2 can be formed by cutting a bar steel 1 to a predetermined length, then hot forging to form the bolt head 21 at one end, and rolling the threaded teeth on the shaft portion 22 at the other end, thus achieving the shape shown in the figure. It should be noted that... Figure 2 In this embodiment, the shaft portion 22 is formed entirely with threaded teeth, but in this embodiment, the threaded teeth need to be formed only in at least a portion of the shaft portion.

[0032] The following is a detailed description of the bar steel. It should be noted that, unless otherwise specified, when simply written as "%", it refers to "mass %".

[0033] The steel bar involved in this embodiment is a bar-shaped steel bar. From the viewpoint of steel bar being a suitable material for bolts, the aforementioned steel bar can be, for example, a cylindrical steel bar.

[0034] From the viewpoint of steel bars being suitable as materials for large-diameter bolts, the diameter of the steel bars involved in this invention is set to 65 mm or more. On the other hand, there is no upper limit to the above-mentioned diameter, but it is preferably 100 mm or less.

[0035] As described above, the bar steel involved in this embodiment contains, as a chemical composition, C (carbon), Si (silicon), Mn (manganese), P (phosphorus), S (sulfur), Ni (nickel), Cr (chromium), and Mo (molybdenum), with the remainder being Fe (iron) and impurities. Impurities refer to elements that are permissible within a range that does not adversely affect the effects of the present invention. These impurities include so-called unavoidable impurities that inevitably mix in during the industrial manufacturing of bar steel from raw materials such as ore, waste, or the manufacturing environment.

[0036] The following details the chemical composition of steel bars (the content of each element in the steel bars).

[0037] The carbon (C) content is 0.35% to 0.45%. C is an element that affects the hardenability and tensile strength of bar steel. When the C content is less than 0.35%, the tensile strength of the bar steel is insufficient. Therefore, the C content is 0.35% or more, preferably 0.38% or more. On the other hand, when the C content exceeds 0.45%, the toughness of the bar steel decreases. Therefore, the C content is 0.45% or less, preferably 0.43% or less.

[0038] The Si content is 0.10% to 0.50%. Si is an element that affects the hardenability and temper softening resistance of bar steel. When the Si content is less than 0.10%, hardenability is insufficient, and the tensile strength of the bar steel decreases. Therefore, the Si content is 0.10% or more, preferably 0.15% or more. On the other hand, when the Si content exceeds 0.50%, temper softening resistance increases, and the toughness of the bar steel decreases. Therefore, the Si content is 0.50% or less, preferably 0.35% or less.

[0039] The Mn content is 0.50% to 2.00%. Mn is an element that affects the hardenability and martensitic transformation temperature of bar steel. When the Mn content is less than 0.50%, the hardenability is insufficient, and the tensile strength of the bar steel decreases. Therefore, the Mn content is 0.50% or more, preferably 0.70% or more. On the other hand, when the Mn content exceeds 2.00%, the martensitic transformation temperature decreases, retained austenite is easily generated, and the tensile strength of the bar steel is insufficient. Therefore, the Mn content is 2.00% or less, preferably 1.00% or less.

[0040] The phosphorus (P) content is 0.030% or less. P is an element that segregates at grain boundaries and affects the toughness of the bar steel. When the P content exceeds 0.030%, the grain boundary strength decreases, and the toughness of the bar steel decreases. Therefore, the P content is 0.030% or less, preferably 0.020% or less. On the other hand, since the presence of P is usually unavoidable, excessively reducing P may lead to increased refining time and higher refining costs. Therefore, the P content is preferably 0.003% by mass or more, more preferably 0.007% by mass or more.

[0041] The sulfur (S) content is 0.030% or less. S is an element that affects the tensile strength and toughness of bar steel by combining with Mn to form MnS. MnS acts as the initiation point for fracture in the steel structure. When the S content exceeds 0.030%, the toughness of the bar steel decreases. Therefore, the S content is 0.030% or less, preferably 0.015% or less. On the other hand, since the presence of S is usually unavoidable, excessive reduction of S may increase refining costs. Therefore, the S content is preferably 0.003% by mass or more, more preferably 0.007% by mass or more.

[0042] The Ni content is 1.00% to 3.00%. Ni is an element that affects the hardenability and toughness of bar steel. When the Ni content is less than 1.00%, hardenability and toughness decrease. Therefore, the Ni content is 1.00% or more, preferably 1.50% or more. On the other hand, when the Ni content exceeds 3.00%, the effect of improving hardenability and toughness becomes saturated, resulting in high costs. Therefore, the Ni content is 3.00% or less, preferably 2.00% or less.

[0043] The Cr content is 0.50% to 2.00%. Cr is an element that affects the hardenability and martensitic transformation temperature of bar steel. When the Cr content is less than 0.50%, hardenability is insufficient and tensile strength decreases. Therefore, the Cr content is 0.50% or more, preferably 0.70% or more. On the other hand, when the Cr content exceeds 2.00%, the effect of improving hardenability becomes saturated, resulting in high cost. Therefore, the Cr content is 2.00% or less, preferably 1.00% or less.

[0044] The Mo content is 0.10% to 0.50%. Mo is an element that affects the hardenability and temper softening resistance of bar steel. When the Mo content is less than 0.10%, hardenability is insufficient and tensile strength is reduced. Therefore, the Mo content is 0.10% or more. On the other hand, when the Mo content exceeds 0.50%, the effect of improving hardenability becomes saturated. Therefore, the Mo content is 0.50% or less.

[0045] The DI value expressed by the above formula (1) is the benchmark for hardenability. When the above DI value is low, the hardenability is insufficient. Therefore, the above DI value is set to 420 or above, preferably 450, and more preferably 500. On the other hand, the upper limit of the above DI value is not particularly limited, for example, it can be 8000 or below.

[0046] The bolts involved in this embodiment will be described in detail below.

[0047] The bolt described in this embodiment is a bolt made using the aforementioned bar steel. Furthermore, the bolt has a shaft portion. The outer diameter of the shaft portion is equivalent to the diameter of the bar steel used as the material. Therefore, the outer diameter of the shaft portion can be set to 65 mm or more. Additionally, the outer diameter of the shaft portion is preferably 100 mm or less.

[0048] Next, the microstructure of the bolt's axial portion will be described. The microstructure of the bolt's axial portion described below is observed in a cross-section perpendicular to the long side of the axial portion.

[0049] In the aforementioned metallic microstructure, the original γ grain size (also known as the original austenite grain diameter) is designated as grain size 4 to 10. It should be noted that the grain size designation in this embodiment is based on JIS G 0551 "Steel – Microscopic Test Method for Grain Size," and is measured at a distance d / 4 from the radial center of the shaft outwards. It should be noted that d refers to the outer diameter of the shaft. More specifically, the measurement is performed using the method described in the examples.

[0050] The original gamma particle size affects the toughness of the bar steel. When the original gamma particle size is smaller than particle size number 4, the toughness of the bolt decreases. Therefore, the original gamma particle size is particle size number 4 or higher, preferably particle size number 6 or higher. On the other hand, when the original gamma particle size exceeds particle size number 10, the tensile strength decreases. Therefore, the original gamma particle size is particle size number 10 or lower, preferably particle size number 9 or lower.

[0051] Next, the area ratio of tempered martensite in the microstructure of the aforementioned cross-section will be explained. It should be noted that the area ratio of tempered martensite was measured at the radial center of the shaft portion. More specifically, it was measured using the method described in the examples.

[0052] The area ratio of tempered martensite affects the tensile strength and toughness of bolts. When the area ratio of tempered martensite is small, the tensile strength and toughness of the bolts decrease. Therefore, the area ratio of tempered martensite is 80% or more, preferably 85% or more. On the other hand, there is no upper limit to the area ratio of tempered martensite, and it can be 100%, and the aforementioned metallic microstructure can also consist of tempered martensite. It should be noted that the microstructure of the remaining portion is not limited, and it can be one or more microstructures selected from ferrite, pearlite, bainite, and retained austenite.

[0053] By increasing the tensile strength of the aforementioned shaft portion, a suitable bolt can be made. Therefore, the tensile strength of the aforementioned shaft portion is preferably 1040 MPa or higher. It should be noted that the tensile strength of the shaft portion is measured according to JIS Z 2241 at a distance of d / 4 from the radial center outward. More specifically, it is measured using the method described in the embodiments.

[0054] By improving the toughness of the aforementioned shaft portion, it becomes a suitable bolt for use. Therefore, the Charpy absorption energy of the aforementioned shaft portion is 27 J or higher. It should be noted that the Charpy absorption energy of the shaft portion is measured using the Charpy impact test specified in JIS Z2242, at a distance d / 4 from the radial center outwards. More specifically, it is measured using the method described in the examples.

[0055] The manufacturing method of bolts is described in detail below.

[0056] Figure 3An example of a bolt manufacturing process is shown. In the bolt manufacturing process, steel is used to manufacture the bar steel and bolts involved in this embodiment.

[0057] The bolt manufacturing process includes: a bar manufacturing process S1, which uses steel to manufacture bars that will serve as the material for bolts, and a bolt forming process S3, which processes the bars manufactured in the bar manufacturing process S1 into bolts. This bolt manufacturing process may include an intermediate process S2 that transports the bars from the workshop or factory where the bar manufacturing process S1 is performed to the workshop or factory where the bolt forming process S3 is performed.

[0058] In the bar manufacturing process S1, steel with the above-mentioned chemical composition and DI value is hot-rolled or hot-forged (rolling process S11) to produce bars of a specified diameter (e.g., bars with a diameter of 65 mm or more). The steel can be, for example, a bloom cast by continuous casting or a steel ingot cast by ingot casting. The bars undergo specified quality inspections as required.

[0059] In intermediate process S2, the bar steel is shipped (shipping process S21) and transported (transportation process S22) in the workshop or factory where the bolt forming process S3 is performed, and then received in the workshop or factory where the bolt forming process S3 is performed (receiving process S23).

[0060] In the bolt forming process S3, the received bar steel is cut into a specified size (cutting process S31). Then, the cut bar steel is subjected to hot forging to form the bolt head (hot forging process S32), thread rolling to form the thread teeth (thread rolling process S33), followed by quenching (quenching process S34) and tempering (tempering process S35) to obtain the bolt as a product.

[0061] Figure 4 Other examples illustrating the process of manufacturing bolts are shown below. For example... Figure 4 As shown, in the bolt forming process S3, rolling (thread rolling process S33) can be performed after quenching (quenching process S34) and tempering (tempering process S35).

[0062] After bolt forming process S3, the bolts can also be coated with paint, electroplated, or otherwise treated.

[0063] The following details the quenching (quenching process S34) and tempering (tempering process S35). Regarding bolts, the process conditions for obtaining the desired tensile strength and toughness are explained. It should be noted that in the following process descriptions, the temperature of the steel forming the bolt shape is expressed as the temperature at the radial center of the shaft portion.

[0064] The quenching in quenching process S34 is performed at a heating temperature of 820°C to 1150°C and at an average cooling rate of 0.5°C / s or higher from 800°C to 300°C. If the heating temperature is below 820°C, the microstructure of the shaft portion cannot become a single-phase austenitic structure. Therefore, the heating temperature is 820°C or higher. On the other hand, if the heating temperature rises too high, it leads to grain coarsening and reduced toughness; therefore, the heating temperature is 1150°C or lower, preferably 900°C or lower.

[0065] By setting the cooling rate to an average cooling rate of 0.5°C / s or more from 800°C to 300°C, phase transformation to structures other than martensite, such as ferrite and pearlite, is suppressed. When the average cooling rate is slow, the desired microstructure 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 below, martensitic phase transformation can occur. Therefore, the cooling stop temperature is preferably 150°C or below.

[0066] In the tempering process S35, the tempering is preferably carried out by heating to 500°C or higher and holding for 30 minutes or more. That is, the holding temperature is preferably 500°C or higher, and the holding time is preferably 30 minutes or more. After holding, it is preferably cooled to room temperature. When performing this cooling, in order to avoid embrittlement caused by grain boundary segregation of P, it is preferable to accelerate the cooling rate as much as possible.

[0067] Example

[0068] The following describes an example. In the following example, in order to evaluate the relationship between the properties of the bar steel and the properties of the bolt made from the bar steel, the bar steel was quenched and tempered without forming thread teeth and bolt heads, thereby creating a specimen simulating the shaft portion of the bolt, and the specimen was used for evaluation.

[0069] Molten steel (steel No. A to V) with the composition (remaining components being Fe and impurities) and DI value shown in Table 1 was cast using a continuous casting machine to produce castings with a cross-section of 300 mm × 400 mm. It should be noted that in Table 1, underlines on the values ​​indicate that the value is outside the range of composition or DI value specified in this embodiment.

[0070]

[0071] Next, the casting is homogenized at 1250°C for 30 minutes, and then hot-rolled into steel sheets with a rectangular cross-section of 140 mm on each side. Furthermore, each steel sheet is hot-rolled to the diameters shown in Table 2, becoming bar steel (hot-rolled billet) of bar steel No. 1 to 30. Here, steel sheets other than steel No. J are hot-rolled into bars with a diameter of 65 mm to 100 mm. Steel sheets of steel No. J are hot-rolled into bars with a diameter of 40 mm. Bar steel No. 1 to 30 are cylindrical.

[0072] Table 2

[0073]

[0074] Next, the hot-rolled bars described above are quenched using the heating temperature, cooling method, and average cooling rate shown in Table 3. In Table 3, the "Heating Temperature" field indicates the temperature at which the bars are heated during quenching. The "Cooling Method" refers to the cooling method performed after the heating used for quenching, and the "Average Cooling Rate" indicates the average cooling rate from 800°C to 300°C. It should be noted that in Table 3, underlines on the values ​​indicate that the value is outside the range specified in the manufacturing method of this embodiment.

[0075]

[0076] During quenching, the bar steel is first heated to the radial center temperature shown in Table 3, held at this temperature for 30 minutes, and then further cooled using the cooling method and average cooling rate shown in Table 3. It should be noted that the oil used in the oil cooling is high-speed quenching oil manufactured by Nippon Lubricating Grease Co., Ltd. The oil or water temperature during cooling is set to room temperature. Except for products No. 15 and No. 18, the average cooling rate from 800°C to 300°C is set to 1.5–9.2°C / s. For products No. 15 and No. 18, the average cooling rates from 800°C to 300°C are set to 6.7°C / s and 0.3°C / s, respectively.

[0077] After the above quenching, the bar steel is heated to 540°C at the radial center and held at that temperature for 120 minutes before being further tempered by water cooling.

[0078] For the samples of products No.1 to No.30 obtained as described above after quenching and tempering, the samples were used as the shaft of the bolt, and the cross-sectional microstructure was observed, the original γ grain size was determined, and tensile and Charpy impact tests were performed.

[0079] Tissue observation was performed on a section perpendicular to the long side of the specimen. The specimen was cut crosswise along its long side to expose the cross section. This cross section was then etched with a 1% nitric acid-ethanol solution and observed using an optical microscope.

[0080] The observation location was set within a 4mm × 4mm rectangle around the center of the sample (the radial center of the specimen), where the cooling rate was slowest and it was difficult to obtain martensite. Three randomly selected fields of view were observed at 400x magnification, and the tempered martensite and other microstructures were visually identified. The area fraction of the tempered martensite was then calculated based on this identification. The area fraction was the average of the three fields of view. Table 3 shows the area fraction of the tempered martensite. In Table 3, for specimens where the area fraction of the tempered martensite was not 100%, the observed microstructures other than martensite are shown in the "Remaining Microstructure" section.

[0081] The original γ grain size was determined on a cross section perpendicular to the long side, similar to the section observed in the tissue study. The original γ grain boundaries on this cross section were visualized using a picric acid aqueous solution and observed using an optical microscope. The observation position was set at a distance d / 4 oz from the center of the sample, with the sample diameter as d. The grain size designation was determined according to JIS G 0551 and compared with a standard diagram. The results of the original γ grain size determination are shown in Table 3.

[0082] The tensile test was conducted according to JIS Z 2241. The specimens for the tensile test were collected at a distance of d / 4 as described above. The specimen shape was JIS size 4 (diameter of the parallel section was 14 mm). The test was performed at a tensile speed of 3 mm / min. The tensile strength was determined by dividing the maximum load shown in the tensile test by the cross-sectional area of ​​the specimen measured before the test.

[0083] To evaluate toughness, Charpy impact tests were performed according to JIS Z 2242. The sampled specimens (cubic prisms with a square cross-section of 10 mm on each side) for the Charpy impact tests were collected at a distance of d / 4 from the center of the square cross-section of the specimen. The Charpy impact test specimens were collected with one face along the long side of the specimen perpendicular to the radial center of the specimen (a perpendicular line extending from the face along the long side of the specimen and opposite the radial center of the steel overlaps with the center of the specimen). The notch shape of the Charpy impact test specimen was set to 2 mmV (a V-shape with a depth of 2 mm, a V-notch). The notch was machined on the face perpendicular to the radial center of the specimen during collection. The Charpy impact tests were performed at -40°C. The absorbed energy obtained from these Charpy impact tests is shown in Table 3.

[0084] Of the products No. 1 to 30 shown in Table 3, products No. 1 to 14 correspond to the inventive examples. Products No. 16 to 30 do not meet the requirements specified in this embodiment and are corresponding to comparative examples. Product No. 15 is a reference example using a 40mm diameter steel bar as the material.

[0085] For applications involving bolts, a tensile strength of 1040 MPa or higher is sufficient (i.e., acceptable), and an energy absorption of 27 J or higher is sufficient (i.e., acceptable).

[0086] As shown in Table 3, although the specimens of products No. 1 to 14 have large diameters of 65 mm or more, their tensile strengths are all above 1040 MPa, and their absorbed energy is all above 27 J. Therefore, they possess the strength and toughness required for bolts. Specifically, the specimens of products No. 1 to 14 have tensile strengths and absorbed energy equivalent to those of product No. 15, which has a diameter of 40 mm, comparable to that of conventional technology. Furthermore, compared to the specimens of products No. 15 to 30, which do not meet the requirements specified in this embodiment, the tensile strengths and absorbed energy of the specimens of products No. 1 to 14 are significantly higher.

[0087] In other words, it is effective to use bar steel with the chemical composition and DI value described above in order to manufacture bolts with high tensile strength and toughness. Furthermore, by ensuring that bolts made from the aforementioned bar steel meet the requirements of the original γ grain size and the area ratio of tempered martensite, high tensile strength and toughness can be achieved.

[0088] It should be noted that the configurations disclosed in the above embodiments (including other embodiments, the same below) can be combined with the configurations disclosed in other embodiments as long as there is no contradiction. In addition, the embodiments disclosed in this specification are exemplary, and the embodiments of the present invention are not limited thereto, and can be appropriately modified within the scope of the purpose of the present invention.

[0089] Industrial availability

[0090] This invention applies to steel bars, bolts, and methods for manufacturing bolts.

[0091] Symbol Explanation

[0092] 1: Bar steel

[0093] 2: Bolts

[0094] 21: Bolt head

[0095] 22: Shaft

[0096] S1: Bar steel manufacturing process

[0097] S11: Rolling process

[0098] S2: Intermediate process

[0099] S21: Shipping Process

[0100] S22: Transportation Process

[0101] S23: Receiving Procedure

[0102] S3: Bolt forming process

[0103] S31: Cutting process

[0104] S32: Hot forging process

[0105] S33: Thread rolling process

[0106] S34: Quenching process

[0107] S35: Tempering process

Claims

1. A type of bar steel, with the following chemical composition (by mass%): C: 0.35%–0.45%, Si: 0.10%–0.50%, Mn: 0.50%–2.00%, P: less than 0.030%, S: less than 0.030%, Ni: 1.00%–3.00%, Cr: 0.50%–2.00%, and Mo: 0.10%–0.50%, the remainder being Fe and impurities. Furthermore, the DI value expressed by the following formula (1) is 420 or higher. Diameter of 65mm or more OF=(-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) in, [C], [Mn], [Si], [Cr], [Mo], and [Ni] represent the contents of C, Mn, Si, Cr, Mo, and Ni, respectively, expressed in mass percent.

2. A bolt, which uses the bar steel as described in claim 1, The bolt has a shaft portion. In the cross-section perpendicular to the long side of the shaft, the original γ particle size was particle size number 4 to 10. In the metal microstructure of the cross section, the area fraction of tempered martensite is more than 80%.

3. A method for manufacturing a bolt, comprising cutting the bar steel as described in claim 1, and subjecting the cut bar steel to hot forging, thread rolling, quenching, and tempering, thereby manufacturing a bolt having a shaft portion and a bolt head. In the hot forging process, the bolt head is formed. During the thread rolling process, thread teeth are formed on the shaft portion. In the quenching process, the heating temperature is set to above 820°C and below 1150°C, and the average cooling rate from 800°C to 300°C is set to above 0.5°C / s.

Images