Method for manufacturing a vehicle axle

Abstract

A method for manufacturing an axle (90) includes a material preparation step, an anvil preparation step, a placement step, a first forging step, and a second forging step. In the first forging step, a first reduction step and a first rotation step are alternately repeated. In the first reduction step, a pair of first anvils (21) is used to reduce the metal material (10). In the first rotation step, the metal material (10) is rotated and moved in the axial direction. In the second forging step, when a hydrostatic stress acting on the metal material (10) is σ m , an equivalent stress is σ, and an equivalent plastic strain is ε, a maximum value of a damage value Df expressed by the following equation (1) in the axial direction becomes a prescribed threshold value D0 that satisfies D0 ≦ -0.22, and a second reduction step and a second rotation step are alternately repeated. In the second reduction step, a pair of first anvils (21) and a pair of second anvils (22) are used to reduce the metal material (10). In the second rotation step, the metal material (10) is rotated and moved in the axial direction.

Description

Technical Field

[0001] This disclosure relates to a method for manufacturing an axle. Background Technology

[0002] Hot free forging is a known method for manufacturing axles. In hot free forging, a metal material such as an ingot, billet, or strip is heated to a temperature suitable for forging. The metal material may have a shaft. The heated metal material is clamped and pressed down using a pair of anvils. Then, the anvils are separated, causing the metal material to rotate and move axially. The amount of axial movement of the metal material is also called the feed rate. In free forging, this alternating pressing, rotation, and axial movement of the metal material causes it to shrink in diameter and elongate axially.

[0003] In hot free forging, the temperature of the metal decreases as time passes from the start of forging. If the metal is overcooled, the stamping load required for forging increases, and it may become impossible to form the desired shape. To prevent this, the metal can be reheated. However, this increases manufacturing costs due to the consumption of combustion gases. Therefore, in hot free forging, it is desirable to efficiently reduce the diameter of the metal within a short time.

[0004] In hot free forging, to improve forging efficiency, four-sided forging is sometimes performed using two pairs of anvils (four anvils) to press down the metal material, instead of two-sided forging using one pair of anvils (two anvils) (e.g., Patent Document 1 and Patent Document 2). When pressing down the metal material using four-sided forging, the contact area between the metal material and the anvils is larger compared to two-sided forging. Therefore, if the metal material is pressed down using four-sided forging, the forging efficiency is improved.

[0005] Prior art literature

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2001-79633

[0008] Patent Document 2: Japanese Patent Application Publication No. 4-118142 Summary of the Invention

[0009] The technical problem that the invention aims to solve

[0010] Axles are manufactured, for example, by forging a cylindrical metal material on all four sides. The metal material is typically manufactured by casting. During casting, cavities (pores) are sometimes created at the radial center of the metal material. During forging, due to compressive stress acting on the interior of the metal material, the cavities are compressed and gradually reduced, eventually being pressed together. However, in axles manufactured by four-sided forging, cavities sometimes remain at the radial center in an unpressurized state. If cavities remain unpressurized in an axle, ultrasonic testing results may not meet standards. The axle cannot be shipped as a product and is discarded. As a result, manufacturing costs deteriorate.

[0011] The purpose of this disclosure is to provide a method for manufacturing an axle that can ensure forging efficiency and reduce residual cavities.

[0012] Technical means for solving technical problems

[0013] The disclosed method for manufacturing an axle includes a material preparation step, an anvil preparation step, an arrangement step, a first forging step, and a second forging step. In the material preparation step, a metal material is prepared. The metal material has a shaft center. In the anvil preparation step, a pair of first anvils and a pair of second anvils are prepared. The pair of first anvils are opposite each other. The pair of second anvils are opposite each other in a direction perpendicular to the direction in which the pair of first anvils are opposite each other. In the arrangement step, the metal material is arranged in the space between the pair of first anvils and the pair of second anvils. In the first forging step, a first pressing step and a first rotating step are repeated alternately. In the first pressing step, the metal material is pressed down using the pair of first anvils. In the first rotating step, the metal material is rotated about the shaft center and moved axially along the metal material. In the second forging process, when the hydrostatic stress acting on the metal material is set as σm, the equivalent stress is set as σ, and the equivalent plastic strain is set as ε, the maximum value of the damage value Df in the axial direction, expressed by the following formula (1), becomes the specified threshold D0 that satisfies D0≦-0.22. The second pressing process and the second rotation process are then repeated alternately. In the second pressing process, a pair of first anvils and a pair of second anvils are used to press down the metal material. In the second rotation process, the metal material is rotated about the axis and moved axially.

[0014] [Formula 1]

[0015]

[0016] Invention Effects

[0017] The axle manufacturing method disclosed herein can ensure forging efficiency and reduce residual cavities. Attached Figure Description

[0018] Figure 1 A schematic diagram of an axle obtained by free forging.

[0019] Figure 2 This is a schematic diagram representing the analysis results.

[0020] Figure 3 This is a schematic diagram representing the analysis results.

[0021] Figure 4 This is a flowchart illustrating the manufacturing method of the axle according to the embodiment.

[0022] Figure 5 This is a schematic diagram of the metal material before forging.

[0023] Figure 6 This is a schematic diagram of the pressure device used in the manufacturing method of the embodiment.

[0024] Figure 7 This is a cross-sectional view showing the configuration process.

[0025] Figure 8 This is a cross-sectional view showing the first pressing process.

[0026] Figure 9 This is a cross-sectional view showing the second pressing process. Detailed Implementation

[0027] Figure 1 This is a schematic diagram of a 90mm axle obtained through free forging. (Refer to...) Figure 1 The axle 90 has a stepped shape. In short, the diameter of the axle 90 is not constant and varies depending on its position along the axis. The axle 90 includes the largest diameter portion, the large-diameter section 91. The large-diameter section 91 is also called the hub seat section.

[0028] When the metal material used as the axle 90 has a cavity, this cavity tends to remain in the large-diameter portion 91 of the axle 90. This is because, in the large-diameter portion 91, compared to the portion of the axle 90 with a diameter smaller than the large-diameter portion 91, the reduction rate of cross-sectional area caused by forging is smaller, and the cavity tends to remain in an unpressed state. The reduction rate of cross-sectional area is an indicator of the degree to which the cross-sectional area is reduced due to forging. If we define the cross-sectional area of ​​the metal material cut perpendicular to the axial direction before forging as S0, and the cross-sectional area of ​​the metal material cut perpendicular to the axial direction during or after forging as S, it is expressed as (S0-S) / S0.

[0029] As a parameter related to the cavity remaining in the axle 90, the inventors focus on the damage value Df expressed by the following equation (1), in which σ mσ refers to the hydrostatic stress acting on the metal material during forging, σ refers to the equivalent stress, and ε refers to the equivalent plastic strain. In the forging of axle 90, compressive force is repeatedly applied to the metal material through an anvil. In the forging of axle 90, the damage value Df can be considered as an indicator of the compressive work acting on the interior of the metal material. Therefore, it can be inferred that the damage value Df is correlated with the cavity remaining in axle 90. Furthermore, the damage value Df in the forging of axle 90 is mainly negative; the smaller the damage value Df (the larger the absolute value of damage value Df), the greater the compressive work acting on the interior of the metal material.

[0030] [Equation 2]

[0031]

[0032] The inventors first investigated the relationship between the cavity remaining in the axle 90 and the damage value Df. Specifically, a simulation analysis of free forging was performed using general-purpose structural analysis software (trade name: DEFORM, Yamanaka Alloy Co., Ltd.). In the analysis, a cylindrical metal material with a cavity at the radial center was forged on all four sides.

[0033] Figure 2 This is a schematic diagram representing the analysis results. Figure 2 In the diagram, the damage value Df of the axle obtained through free forging is represented by a contour plot. (Refer to...) Figure 2 At the radial center of the axle, the damage value Df increases (while the absolute value of Df decreases). Specifically, the damage value Df is largest at the radial center of the large-diameter portion of the axle. This result indicates a correlation between the residual cavity in the axle and the damage value Df; the larger the damage value Df, the more likely a cavity is to remain. Further investigation by the inventors revealed that in forged axles, if the maximum damage value Df is below -0.40, the cavity can be sufficiently pressed together through forging, reducing the amount of residual cavity in the axle. In this specification, unless otherwise specified, the maximum damage value Df of the axle or metal material refers to the maximum axial value of Df.

[0034] Next, the inventors investigated the shift in the damage value Df of the metal material during forging. Specifically, simulation analysis was performed using the aforementioned structural analysis software. In the analysis, the metal material was heated to 1277°C and then left to stand until its surface temperature reached 1070°C. Afterward, the metal material was freely forged using an anvil. The analysis was conducted for three cases: forging only on two sides (Case 1), forging only on four sides (Case 2), and forging on four sides after forging on two sides (Case 3). The numerical conditions used in this analysis are shown below.

[0035] • Number of presses: 10

[0036] • Compressive force: 25mm

[0037] • Feed rate: 0mm

[0038] • Rotation angle: 40,000° (for two-sided forging), 13,846° (for four-sided forging)

[0039] • Diameter of the metal material before forging: 409mm

[0040] • Width of the anvil: 110mm

[0041] • Anvil pressing speed: 100mm / s

[0042] • Anvil temperature: 250°C

[0043] • Coulomb friction coefficient: 0.2

[0044] • Shear friction coefficient: 0.4

[0045] Figure 3 This is a schematic diagram representing the analysis results. Figure 3 In the diagram, the vertical axis represents the damage value Df, and the horizontal axis represents the reduction rate of the cross-sectional area of ​​the metallic material (S0-S) / S0. (Refer to...) Figure 3 Based on the results of cases 1 and 2, in both two-sided and four-sided forging, the damage value Df decreases with increasing forging depth. However, two-sided forging is more efficient at reducing the damage value Df compared to four-sided forging. Therefore, from the viewpoint of pressing together the internal cavities of the metal material, two-sided forging is superior to four-sided forging.

[0046] Compared to four-sided forging, in two-sided forging, the smaller surface area of ​​the metal material constrained by the anvil during pressing allows it to easily expand in the unconstrained direction. If the metal material is rotated around an axis and the expanded portion is pressed down, the reduction is relatively large. Therefore, compared to four-sided forging, two-sided forging can apply greater compressive stress to the interior of the metal material. Thus, it can be considered that two-sided forging has a superior effect in pressing together the internal cavities of the metal material.

[0047] However, when manufacturing axles using only two-sided forging, the dimensional accuracy decreases compared to four-sided forging due to the smaller contact area between the metal and the anvil. Therefore, it is difficult to ensure the roundness of the axle in two-sided forging. To ensure the roundness of the axle in two-sided forging, the rotation angle and feed rate of the metal need to be set smaller, and the number of presses needs to be increased. In this case, forging efficiency decreases.

[0048] Therefore, the inventors conceived of performing two-sided forging in the first half of the forging process and four-sided forging in the second half. However, the timing of switching from two-sided forging to four-sided forging needs to be appropriately set so as to adequately press the internal cavities of the metal material. As mentioned above, if the maximum damage value Df in the forged axle is below -0.40, the cavity can be adequately pressed through forging. The inventors conducted in-depth research on the timing of switching from two-sided forging to four-sided forging and found that if the switch is performed when the damage value Df is below -0.22, the maximum damage value Df of the forged axle becomes below -0.40.

[0049] exist Figure 3 In Case 3, the switch from two-sided forging to four-sided forging is performed when the damage value Df reaches -0.22 (cross-sectional area reduction rate is 0.33). Typically, when metal material is formed into an axle, the cross-sectional area reduction rate of the major diameter portion of the axle is, for example, 0.65 or higher. Based on the results of Case 3, the damage value Df at a cross-sectional area reduction rate of 0.65 is -0.42. Therefore, by switching from two-sided forging to four-sided forging when the maximum damage value Df of the metal material during forging reaches -0.22, the damage value Df of the major diameter portion of the four-sided forged axle can be kept below -0.40. Furthermore, by switching from two-sided forging to four-sided forging when the maximum damage value Df of the metal material during forging is less than -0.22, two-sided forging can be performed for a longer period than in Case 3, thus the damage value Df of the major diameter portion of the four-sided forged axle is less than -0.40.

[0050] From the perspective of pressing together the internal cavity of the metal material, as long as the maximum damage value Df of the metal material during forging is below -0.22, there is no particular limitation on the timing of switching from two-sided forging to four-sided forging. For example, the timing of switching from two-sided forging to four-sided forging could also be when the maximum damage value Df of the metal material during forging becomes -0.40. If the switch to four-sided forging is performed when the damage value Df of the metal material becomes -0.40 through two-sided forging, then naturally the damage value Df of the axle after four-sided forging will be below -0.40. Figure 3 As shown in Case 1, the reduction rate of cross-sectional area is approximately 0.50 when the damage value Df becomes -0.40 in two-sided forging. Therefore, the timing for switching from two-sided forging to four-sided forging can also be when the reduction rate of cross-sectional area becomes 0.50.

[0051] The method for manufacturing the axle according to the embodiments of this disclosure is based on the above insights.

[0052] The axle manufacturing method of the embodiment includes a material preparation step, an anvil preparation step, an arrangement step, a first forging step, and a second forging step. In the material preparation step, a metal material is prepared. The metal material has an axle center. In the anvil preparation step, a pair of first anvils and a pair of second anvils are prepared. The pair of first anvils are opposite each other. The pair of second anvils are opposite each other in a direction perpendicular to the direction in which the pair of first anvils are opposite each other. In the arrangement step, the metal material is arranged in the space between the pair of first anvils and the pair of second anvils. In the first forging step, a first pressing step and a first rotating step are repeated alternately. In the first pressing step, the metal material is pressed down using the pair of first anvils. In the first rotating step, the metal material is rotated about the axle center and moved axially along the metal material. In the second forging process, when the hydrostatic stress acting on the metal material is set as σm, the equivalent stress is set as σ, and the equivalent plastic strain is set as ε, the maximum value of the damage value Df in the axial direction, as expressed by the above formula (1), becomes the specified threshold D0 that satisfies D0≦-0.22. The second pressing process and the second rotation process are then repeated alternately. In the second pressing process, a pair of first anvils and a pair of second anvils are used to press down the metal material. In the second rotation process, the metal material is rotated about the axis and moved axially (first configuration).

[0053] In the manufacturing method of the first configuration, the metal material is forged through a first forging step and a second forging step. In the first forging step, two-sided forging is performed using a pair of first anvils. In the second forging step, four-sided forging is performed using a pair of first anvils and a pair of second anvils. The timing for switching from the first forging step to the second forging step is when the maximum value of the damage value Df reaches a predetermined threshold D0. The threshold D0 is -0.22 or less. If the switch is performed at this timing, the maximum value of the damage value Df of the forged axle becomes -0.40 or less. Therefore, the cavity can be sufficiently pressed together by forging. Furthermore, since the second forging step (four-sided forging) is performed in the manufacturing method of the first configuration, the forging efficiency is better compared to forging the metal material only through two-sided forging. Thus, according to the manufacturing method of the first configuration, forging efficiency can be ensured and cavities can be reduced.

[0054] In the manufacturing method of the first configuration, the threshold D0 preferably satisfies -0.70 ≤ D0 (second configuration). In the second configuration, the switch from the first forging process (two-sided forging) to the second forging process (four-sided forging) is performed when the maximum value of the damage value Df is between -0.70 and -0.22. In this case, the forging efficiency is improved because the time for four-sided forging can be extended to a certain extent.

[0055] In the manufacturing method described above, the threshold D0 is preferably -0.40 ≤ D0 (third configuration). In the third configuration, the switch from the first forging process (two-sided forging) to the second forging process (four-sided forging) is performed when the maximum value of the damage value Df is between -0.40 and -0.22. In this case, the forging efficiency is further improved because the time for four-sided forging can be further extended.

[0056] In the manufacturing method of the above structure, the metal material can also be formed into a stepped shape in the second forging process.

[0057] Hereinafter, with reference to the accompanying drawings, a specific example of the manufacturing method of the axle according to this embodiment will be described. The same or equivalent reference numerals in the drawings will not be repeated in their description.

[0058] Figure 4 This is a flowchart illustrating the manufacturing method of the axle according to this embodiment. For example... Figure 4 As shown, the manufacturing method of this embodiment includes a material preparation step (#5), an anvil preparation step (#10), a configuration step (#15), a first forging step (#20), and a second forging step (#25). The following is a detailed description. Figure 4 The processes shown are as follows.

[0059] [Material preparation process (#5)]

[0060] In the material preparation process (#5), prepare the metal materials. Figure 5 This is a schematic diagram of the metal material 10 before forging. (Refer to...) Figure 5 The metal material 10 has a central axis 10a. In this embodiment, the metal material 10 is cylindrical.

[0061] The material of the metallic material 10 is not particularly limited. The material of the metallic material 10 may be, for example, carbon steel, stainless steel, or alloy steel. The material of the metallic material 10 may also be a refractory material. The main chemical composition of the metallic material 10, in mass percent, may be C: 0.22~0.59%, Si: 0.15~0.50%, Mn: 0.50~1.60%, P: less than 0.045%, S: less than 0.050%, and V: 0.02~0.10%.

[0062] In this embodiment, the axle is manufactured by hot free forging. In this case, during the material preparation step (#5), the metal material 10 can also be heated using a gas atmosphere furnace or the like. The temperature of the metal material 10 can be any temperature suitable for forging, for example, between 800°C and 1300°C.

[0063] [Anvil preparation process (#10)]

[0064] In the anvil preparation step (#10), a pressure device for forging metal material 10 is prepared. Specifically, in the anvil preparation step (#10), a pair of first anvils and a pair of second anvils are prepared. Figure 6 This is a schematic diagram of the pressure device 20 used in the manufacturing method of this embodiment. (Refer to...) Figure 6 A pair of first anvils 21 are positioned opposite each other. The pair of first anvils 21 are movable in a manner that allows them to approach or separate from each other. A pair of second anvils 22 are positioned opposite each other in a direction substantially perpendicular to the direction in which the pair of first anvils 21 are positioned opposite each other. The pair of second anvils 22 are movable in a manner that allows them to approach or separate from each other.

[0065] The pressure device 20 used in the manufacturing method of this embodiment also has an auxiliary device (not shown). The auxiliary device is typically a robotic arm. The auxiliary device is capable of holding one end of the metal material 10, causing the metal material 10 to rotate about the axis 10a. The auxiliary device is also capable of causing the metal material 10 to move slightly in the vertical or horizontal direction during rotation. Furthermore, the auxiliary device is capable of causing the metal material 10 to move axially.

[0066] [Configuration process (#15)]

[0067] In the configuration process (#15), such as Figure 7 As shown, a metal material 10 is arranged in the space between a pair of first anvils 21 and a pair of second anvils 22. Figure 7 This is a cross-sectional view showing the configuration process (#15). Figure 7 The cross section shown is perpendicular to the axis of the metallic material 10.

[0068] Reference Figure 7 The pair of first anvils 21 are separated from each other. The orientation of the pair of first anvils 21 is substantially perpendicular to the axial direction of the metal material 10. Similarly, the pair of second anvils 22 are separated from each other. The orientation of the pair of second anvils 22 is substantially perpendicular to both the axial direction of the metal material 10 and the orientation of the pair of first anvils 21. The position of the metal material 10 is fixed by an auxiliary device. After the metal material 10 is configured, one first anvil 21, one second anvil 22, another first anvil 21, and another second anvil 22 are arranged sequentially around the metal material 10 at 90° intervals.

[0069] In the manufacturing method of this embodiment, after the configuration step (#15), the metal material 10 is forged through a first forging step (#20) and a second forging step (#25) to obtain... Figure 1The stepped-shaped axle 90 is shown. The axle 90 manufactured by the manufacturing method of this embodiment is used, for example, in railway vehicles. Hereinafter, the first forging process (#20) and the second forging process (#25) will be specifically described.

[0070] [First forging process (#20)]

[0071] In the first forging process (#20), the metal material 10 is forged on both sides. In the first forging process (#20), the first pressing process (#20A) and the first rotating process (#20B) are repeated alternately.

[0072] Figure 8 This is a cross-sectional view showing the first pressing process (#20A). In the first pressing process (#20A), a pair of first anvils 21 are used to press down the metal material 10. At this time, a pair of second anvils 22 are not used. That is, in the first pressing process (#20A), the pair of first anvils 21 are brought close to each other, but the pair of second anvils 22 are not moved.

[0073] In the first rotation process (#20B), firstly, the pair of first anvils 21 are separated from each other and returned. Figure 7 The position is shown. Then, in the first rotation process (#20B), the metal material 10 is rotated about the axis 10a. At the same time, the metal material 10 is moved axially. The rotation and movement of the metal material 10 are performed by an auxiliary device.

[0074] In the first forging step (#20), the metal material 10 is pressed down, rotated, and moved axially multiple times. In the example of this embodiment, the amount of metal material 10 pressed down in the first forging step (#20) is constant. Furthermore, the rotation angle and feed rate of the metal material 10 are typically constant in the first forging step (#20). In the first rotation step (#20B), the rotation angle of the metal material 10 is, for example, 5° to 85°, and the feed rate of the metal material 10 is, for example, 1 mm to 200 mm.

[0075] [Second forging process (#25)]

[0076] In the second forging process (#25), the metal material 10 after the first forging process (#20) is forged on all four sides. In the second forging process (#25), the second pressing process (#25A) and the second rotating process (#25B) are alternately repeated.

[0077] Figure 9This is a cross-sectional view showing the second pressing process (#25A). In the second pressing process (#25A), a pair of first anvils 21 and a pair of second anvils 22 are used to press down the metal material 10. That is, unlike the first pressing process (#20A), both the pair of first anvils 21 and the pair of second anvils 22 are used. In the second pressing process (#25A), the pair of first anvils 21 are brought close together, and the pair of second anvils 22 are brought close together at the same time.

[0078] In the second rotation process (#25B), the pair of first anvils 21 are separated from each other and returned. Figure 7 The position shown. Similarly, the pair of second anvils 22 are separated from each other and returned. Figure 7 The position is shown. Then, in the second rotation process (#25B), the metal material 10 is rotated about the axis 10a. At the same time, the metal material 10 is moved axially. The rotation and movement of the metal material 10 are performed by an auxiliary device.

[0079] In the second forging process (#25), the metal material 10 is pressed down, rotated, and moved axially multiple times. In the second forging process (#25), the rotation angle and feed rate of the metal material 10 are typically constant. In the second rotation process (#25B), the rotation angle of the metal material 10 is, for example, 5° to 40°, and the feed rate of the metal material 10 is, for example, 1 mm to 200 mm.

[0080] In this embodiment, stepped forming is performed in the second forging step (#25). In short, in the second forging step (#25), the metal material 10 is formed into a stepped shape. During stepped forming, the amount of metal material 10 reduced is not constant and varies each time it is reduced. However, stepped forming can be performed at least in the latter half of the second forging step (#25), and does not need to be performed throughout the entire second forging step (#25). That is, the amount of metal material 10 reduced can also be constant in the first half of the second forging step (#25).

[0081] In the manufacturing method of this embodiment, during the forging of the metal material 10, a switch is made from the first forging step (#20) to the second forging step (#25). Regarding the timing of the switch from the first forging step (#20) to the second forging step (#25), the hydrostatic stress acting on the metal material is set as σ. mWhen the equivalent stress is set to σ and the equivalent plastic strain is set to ε, the maximum value of the damage value Df in the axial direction, as expressed by the above formula (1), becomes the specified threshold D0. That is, the first forging process (#20) is carried out until the maximum value of the damage value Df of the metal material 10 becomes the threshold D0, and the second forging process (#25) is carried out after the maximum value of the damage value Df of the metal material 10 becomes the threshold D0.

[0082] The threshold D0 satisfies D0 ≤ -0.22. That is, in the manufacturing method of this embodiment, the switch from the first forging process (#20) to the second forging process (#25) is performed when the damage value Df becomes -0.22 or less. In this case, the maximum value of the damage value Df of the forged axle 90 becomes -0.40 or less.

[0083] From the viewpoint of reducing the damage value Df of the forged axle 90 and thus pressing together the internal cavity, the lower limit of the threshold D0 is not particularly limited. However, when the threshold D0 is small, the switching from the first forging process (#20) to the second forging process (#25) becomes slower. Thus, the forging efficiency deteriorates because the time for the first forging process (#20) (two-sided forging) becomes longer and the time for the second forging process (#25) (four-sided forging) becomes shorter. Therefore, the threshold D0 is preferably -0.70 ≤ D0. When the threshold D0 is -0.70 ≤ D0, the switching from two-sided forging to four-sided forging occurs when the maximum value of the damage value Df is between -0.70 and -0.22. In this case, the forging efficiency is improved because the time for four-sided forging can be extended to some extent. More preferably, the threshold D0 is -0.40 ≤ D0. When the threshold D0 satisfies -0.40 ≤ D0, the switch from two-sided forging to four-sided forging is performed when the maximum damage value Df is between -0.40 and -0.22. In this case, the forging efficiency is further improved because the time for four-sided forging can be extended further.

[0084] [Effect]

[0085] In the manufacturing method of this embodiment, the metal material 10 is forged through a first forging step (#20) and a second forging step (#25). In the first forging step (#20), two-sided forging is performed using a pair of first anvils 21. In the second forging step (#25), four-sided forging is performed using a pair of first anvils 21 and a pair of second anvils 22. The timing for switching from the first forging step (#20) to the second forging step (#25) is when the maximum value of the damage value Df becomes a predetermined threshold D0. The threshold D0 is -0.22 or less. If the switch is performed at this timing, the maximum value of the damage value Df of the forged axle 90 becomes -0.40 or less. Therefore, the cavity can be sufficiently pressed together by forging. Furthermore, since the second forging step (#25) (four-sided forging) is performed in the manufacturing method of this embodiment, the forging efficiency is better than that of forging the metal material 10 using only two-sided forging. Therefore, the manufacturing method according to this embodiment can ensure forging efficiency and reduce cavities.

[0086] Generally speaking, compared to two-sided forging, four-sided forging results in a larger contact area between the metal material and the anvil, thus achieving higher dimensional accuracy. In the manufacturing method of this embodiment, a second forging process (#25) (four-sided forging) is performed in the latter half of the forging of the metal material 10. Therefore, the roundness of the final axle 90 can be ensured.

[0087] When metal material is pressed down by forging, the metal material elongates axially. Typically, in two-sided forging, the metal material mainly elongates axially along the radially inward side. The end face of an axle manufactured by two-sided forging has a shape where the radially inward side extends axially. Conversely, in four-sided forging, the metal material mainly elongates axially along the radially outward side. The end face of an axle manufactured by four-sided forging has a shape where the radially outward side extends axially. In contrast, as shown in the manufacturing method of this embodiment, if both the first forging step (#20) (two-sided forging) and the second forging step (#25) (four-sided forging) are performed, the end face shape of the resulting axle 90 is nearly flush. Therefore, according to the manufacturing method of this embodiment, the yield rate can be improved.

[0088] The embodiments of this disclosure have been described above. However, the above embodiments are merely illustrative examples for implementing this disclosure. Therefore, this disclosure is not limited to the above embodiments, and appropriate modifications can be made to the above embodiments without departing from its spirit.

[0089] In the example of the above embodiment, the reduction amount of the metal material 10 in the first forging step (#20) is constant. That is, step forming is not performed in the first forging step (#20). However, step forming may also be performed in the first forging step (#20) in addition to the second forging step (#25). In this case, step forming is performed in at least the latter half of the first forging step (#20).

[0090] In the manufacturing method of the above embodiment, after the configuration step (#15), the metal material 10 is forged through a first forging step (#20) (two-sided forging) and a second forging step (#25) (four-sided forging). However, four-sided forging can also be performed after the configuration step (#15) and before the first forging step (#20). In this case, the forging of the metal material 10 is performed in the order of four-sided forging, two-sided forging, and four-sided forging. Comparison Figure 3 The results for both Case 1 and Case 2 show that in the early stages of forging, i.e., when the reduction rate of cross-sectional area is small, the difference between the damage value Df in two-sided forging and the damage value Df in four-sided forging is small. Therefore, even if four-sided forging is performed before the first forging process (#20), it does not have a significant impact on the damage value Df of the forged axle 90. In addition, by performing four-sided forging before the first forging process (#20), the time for performing the first forging process (#20) (two-sided forging) is shortened, thus further improving the overall forging efficiency.

[0091] Explanation of reference numerals in the attached figures

[0092] 10: Metallic Materials

[0093] 10a: Axis

[0094] 21: First Anvil

[0095] 22: Second Anvil

[0096] 90: Axle

Claims

1. A method for manufacturing an axle, comprising the following steps: Material preparation process: Prepare metal materials with a shaft; The anvil preparation process involves preparing a pair of first anvils that are opposite each other and a pair of second anvils that are opposite each other in a direction perpendicular to the direction in which the pair of first anvils are opposite each other. The configuration process involves placing the metal material in the space between the pair of first anvils and the pair of second anvils; The first forging process alternately repeats the first pressing process and the first rotation process. In the first pressing process, the metal material is pressed down using the pair of first anvils. In the first rotation process, the metal material is rotated about the axis and moved along the axial direction of the metal material. In the second forging process, the hydrostatic stress acting on the metal material is set as σ. m When the equivalent stress is set to σ and the equivalent plastic strain is set to ε, the maximum value of the damage value Df in the axial direction, as expressed by the following formula (1), becomes the specified threshold D0 that satisfies D0≦-0.

22. Then, the second pressing process and the second rotation process are repeated alternately. In the second pressing process, the metal material is pressed down using the pair of first anvils and the pair of second anvils. In the second rotation process, the metal material is rotated about the axis and moved along the axial direction. [Formula 1] 。 2. The manufacturing method as described in claim 1, The threshold D0 satisfies -0.70≦D0.

3. The manufacturing method as described in claim 1, The threshold D0 satisfies -0.40≦D0.

4. The manufacturing method according to any one of claims 1 to 3, In the second forging process, the metal material is formed into a stepped shape.

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