Method for manufacturing axle

Abstract

This method for manufacturing an axle (90) involves a material preparation step, a die preparation step, a positioning 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 metal material (10) is reduced using a pair of first dies (21). In the first rotation step, the metal material (10) is rotated and moved in the axial direction. In the second forging step, when the hydrostatic stress acting on the metal material (10) is denoted as σm, the equivalent stress is denoted as σ, and the equivalent plastic strain is denoted as ε, a second reduction step and a second rotation step are alternately repeated after the maximum value in the axial direction of the damage value Df represented by formula (1) reaches a predetermined threshold value D0 satisfying D0 ≤ -0.22. In the second reduction step, the metal material (10) is reduced using the pair of first dies (21) and a pair of second dies (22). In the second rotation step, the metal material (10) is rotated and moved in the axial direction.

Description

Axle manufacturing method

[0001] The present disclosure relates to a method of manufacturing an axle.

[0002] Hot free forging has been known as a method for manufacturing axles. In hot free forging, a metal material such as an ingot, bloom, or billet is heated to a temperature suitable for forging. The metal material has, for example, an axial center. The heated metal material is clamped between a pair of anvils and pressed down. Next, the pair of anvils are separated, and the metal material is rotated and moved axially. This axial movement of the metal material is also called the feed amount. In free forging, this pressing of the metal material and the rotation and axial movement of the metal material are alternately repeated, thereby reducing the diameter of the metal material and stretching it axially.

[0003] During hot free forging, the temperature of the metal material decreases as time passes after the start of forging. If the metal material is cooled excessively, the press load required for forging increases, and in some cases, it may not be possible to form it into the desired shape. To prevent this, the metal material can be reheated. However, in this case, combustion gas is consumed, which increases manufacturing costs. Therefore, in hot free forging, it is desirable to efficiently reduce the diameter of the metal material in a short period of time.

[0004] In hot free forging, in order to improve forging efficiency, four-sided forging, in which a metal material is pressed down with two pairs of anvils (four anvils), is sometimes performed instead of two-sided forging, in which a metal material is pressed down with a pair of anvils (two anvils) (for example, Patent Document 1 and Patent Document 2). When a metal material is pressed down with four-sided forging, the contact area between the metal material and the anvils is larger than when a metal material is pressed down with two-sided forging. Therefore, when a metal material is pressed down with four-sided forging, forging efficiency is improved.

[0005] JP-A-2001-79633 JP-A-4-118142

[0006] Axles are manufactured, for example, by four-sided forging a cylindrical metal material. Metal materials are typically manufactured by casting. During casting, a cavity (void) may occur in the radial center of the metal material. During forging, compressive stress acts inside the metal material, causing the cavity to compress and gradually shrink until it is finally crimped. However, axles manufactured by four-sided forging may have a cavity remaining in the radial center that is not crimped. If a cavity remains in the axle that is not crimped, there is a risk that the results of ultrasonic inspection will not meet the standards. Such axles cannot be shipped as products and are discarded. As a result, manufacturing costs increase.

[0007] An object of the present disclosure is to provide a method for manufacturing an axle that can reduce remaining cavities while ensuring forging efficiency.

[0008] The method for manufacturing an axle according to the present disclosure includes a blank preparation step, an anvil preparation step, a placement step, a first forging step, and a second forging step. In the blank preparation step, a metal blank is prepared. The metal blank has an axis. In the anvil preparation step, a pair of first anvils and a pair of second anvils are prepared. The pair of first anvils face each other. The pair of second anvils face each other in a direction perpendicular to the direction in which the pair of first anvils face each other. In the placement step, the metal blank is placed in the space between the pair of first anvils and the pair of second anvils. In the first forging step, a first reduction step and a first rotation step are alternately repeated. In the first reduction step, the metal blank is reduced using the pair of first anvils. In the first rotation step, the metal blank is rotated around the axis and moved in the axial direction of the metal blank. In the second forging step, the hydrostatic stress acting on the metal blank is expressed as σ ｍ , where σ is the equivalent stress and ε is the equivalent plastic strain, the maximum value of the damage value Df in the axial direction expressed by the following formula (1) is D ０ A predetermined threshold D that satisfies ≦−0.22 ０ After this, a second rolling step and a second rotation step are alternately repeated. In the second rolling step, the metal material is rolled down using a pair of first anvils and a pair of second anvils. In the second rotation step, the metal material is rotated around the axis and moved in the axial direction.

[0009] According to the manufacturing method of the axle according to the present disclosure, it is possible to reduce the remaining cavities while ensuring forging efficiency.

[0010] FIG. 1 is a schematic diagram of an axle obtained by free forging. FIG. 2 is a schematic diagram showing analysis results. FIG. 3 is a schematic diagram showing analysis results. FIG. 4 is a flow diagram showing a manufacturing method for an axle according to an embodiment. FIG. 5 is a schematic diagram of a metal material before forging. FIG. 6 is a schematic diagram of press equipment used in the manufacturing method according to an embodiment. FIG. 7 is a cross-sectional view showing the arrangement step. FIG. 8 is a cross-sectional view showing the first reduction step. FIG. 9 is a cross-sectional view showing the second reduction step.

[0011] FIG. 1 is a schematic diagram of an axle 90 obtained by free forging. Referring to FIG. 1, the axle 90 has a stepped shape. In other words, the diameter of the axle 90 is not constant but varies depending on the position in the axial direction. The axle 90 includes a large diameter portion 91, which is the part with the largest diameter. The large diameter portion 91 is also called a wheel seat portion.

[0012] If the metal material from which axle 90 is made has a cavity, that cavity is likely to remain in large diameter portion 91 of axle 90. This is because the cross-sectional area reduction rate due to forging is smaller in large diameter portion 91 than in portions of axle 90 that have a smaller diameter than large diameter portion 91, and the cavity is likely to remain unbonded. The cross-sectional area reduction rate is an index that indicates the degree of cross-sectional area reduction due to forging, and is expressed as (S0-S) / S0, where S0 is the cross-sectional area of ​​the metal material before forging cut perpendicular to the axial direction, and S is the cross-sectional area of ​​the metal material during or after forging cut perpendicular to the axial direction.

[0013] The inventors focused on the damage value Df expressed by the following formula (1) as a parameter related to the cavity remaining in the axle 90. In formula (1), σ ｍwhere σ represents the hydrostatic stress acting on the metal material during forging, σ represents the equivalent stress, and ε represents the equivalent plastic strain. When forging the axle 90, compressive force is repeatedly applied to the metal material by the anvil. When forging the axle 90, the damage value Df can be considered as an index showing the compressive work acting inside the metal material. Therefore, it can be assumed that the damage value Df is correlated with the cavities remaining in the axle 90. Note that the damage value Df in the forging of the axle 90 is mainly a negative value, and the smaller the damage value Df (the larger the absolute value of the damage value Df), the greater the compressive work acting inside the metal material.

[0014] 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 (product name: DEFORM, manufactured by Yamanaka Gokin Co., Ltd.). In the analysis, four-sided forging was performed on a cylindrical metal material having a cavity at the radial center.

[0015] FIG. 2 is a schematic diagram showing the analysis results. In FIG. 2, the damage value Df of the axle obtained by free forging is shown in a contour diagram. Referring to FIG. 2, the damage value Df is large (the absolute value of the damage value Df is small) at the radial center of the axle. In particular, the damage value Df is largest at the radial center of the large diameter portion of the axle. From these results, it can be seen that there is a correlation between the cavities remaining in the axle and the damage value Df, and the larger the axle's damage value Df, the more likely the cavities are to remain. After further investigation, the inventors discovered that if the maximum damage value Df of a forged axle is −0.40 or less, the cavities can be sufficiently compressed by forging, and the cavities remaining in the axle can be reduced. In this specification, the maximum damage value Df of an axle or metal material means the maximum value of the damage value Df in the axial direction, unless otherwise specified.

[0016] Next, the inventors investigated the transition of the damage value Df of the metal material during forging. Specifically, a simulation analysis was performed using the above-mentioned structural analysis software. In the analysis, the metal material was heated to 1277°C and then left until the surface temperature of the metal material reached 1070°C. The metal material was then free-forged using an anvil. The analysis was performed for three cases: a case in which only two-sided forging was performed on the metal material (Case 1), a case in which only four-sided forging was performed (Case 2), and a case in which two-sided forging was performed followed by four-sided forging (Case 3). The numerical conditions used in this analysis are shown below.・Number of reductions: 10 ・Reduction amount: 25 mm ・Feed rate: 0 mm ・Rotation angle: 40.000° (for two-sided forging), 13.846° (for four-sided forging) ・Diameter of metal material before forging: 409 mm ・Width of anvil: 110 mm ・Reduction speed of anvil: 100 mm / s ・Temperature of anvil: 250°C ・Coulomb friction coefficient: 0.2 ・Shear friction coefficient: 0.4

[0017] FIG. 3 is a schematic diagram showing the analysis results. In FIG. 3, the vertical axis represents the damage value Df, and the horizontal axis represents the cross-sectional area reduction rate (S0-S) / S0 of the metal material. Referring to FIG. 3, the results of Cases 1 and 2 show that the damage value Df decreases the more the forging is performed in both two-side forging and four-side forging. However, two-side forging can reduce the damage value Df more efficiently than four-side forging. Therefore, from the perspective of crimping the internal cavity of the metal material, two-side forging is superior to four-side forging.

[0018] In two-side forging, the surface area of ​​the metal material constrained by the anvil during rolling is smaller than in four-side forging, so the metal material tends to expand in the non-constrained direction. When the metal material is rotated around its axis and the expanded part of the metal material is rolled down, the amount of reduction is relatively large. Therefore, in two-side forging, a larger compressive stress can be applied to the interior of the metal material than in four-side forging. For this reason, two-side forging is considered to be superior in the effect of crimping the internal cavity of the metal material.

[0019] However, when manufacturing axles simply by two-side forging, the contact area between the metal material and the anvil is smaller than in four-side forging, resulting in lower dimensional accuracy. Therefore, it is difficult to ensure the roundness of the axle with two-side forging. To ensure the roundness of the axle with two-side forging, it is necessary to reduce the rotation angle and feed rate of the metal material and increase the number of reductions. In this case, the forging efficiency decreases.

[0020] Therefore, the inventors came up with the idea of ​​performing two-sided forging in the first half of forging, and then four-sided forging in the second half of forging. However, the timing of switching from two-sided forging to four-sided forging needs to be appropriately set so that the cavity inside the metal material can be sufficiently crimped. As described above, if the maximum value of the damage value Df in the axle after forging is −0.40 or less, the cavity can be sufficiently crimped by forging. As a result of careful consideration of the timing of switching from two-sided forging to four-sided forging, the inventors found that if the switching is performed when the damage value Df is −0.22 or less, the maximum value of the damage value Df in the axle after forging will be −0.40 or less.

[0021] In Case 3 of FIG. 3 , two-face forging is switched to four-face forging when the damage value Df reaches −0.22 (cross-sectional area reduction rate of 0.33). Typically, the cross-sectional area reduction rate of the large diameter portion of an axle when a metal material is formed into an axle is, for example, 0.65 or more. From the results of Case 3, the damage value Df at a cross-sectional area reduction rate of 0.65 is −0.42. Therefore, if two-face forging is switched to four-face forging when the maximum value of the damage value Df of the metal material during forging reaches −0.22, the damage value Df of the large diameter portion of the axle after four-face forging can be set to −0.40 or less. Furthermore, if two-face forging is switched to four-face forging when the maximum value of the damage value Df of the metal material during forging is less than −0.22, two-face forging can be performed longer than in Case 3, and the damage value Df of the large diameter portion of the axle after four-face forging will be less than −0.40.

[0022] From the viewpoint of crimping the internal cavity of the metal material, the timing of switching from two-face forging to four-face forging is not particularly limited as long as the maximum value of the damage value Df of the metal material during forging is −0.22 or less. For example, the timing of switching from two-face forging to four-face forging may be when the maximum value of the damage value Df of the metal material during forging becomes −0.40. If switching to four-face forging is performed when the damage value Df of the metal material becomes −0.40 due to two-face forging, the damage value Df of the axle after four-face forging will naturally be −0.40 or less. From the results of Case 1 in FIG. 3, the cross-sectional area reduction rate when the damage value Df becomes −0.40 during two-face forging is approximately 0.50. Therefore, the timing of switching from two-face forging to four-face forging may be when the cross-sectional area reduction rate becomes 0.50.

[0023] The method for manufacturing an axle according to an embodiment of the present disclosure has been completed based on the above findings.

[0024] The manufacturing method of an axle according to the embodiment includes a blank preparation step, an anvil preparation step, a positioning step, a first forging step, and a second forging step. In the blank preparation step, a metal blank is prepared. The metal blank has an axis. In the anvil preparation step, a pair of first anvils and a pair of second anvils are prepared. The pair of first anvils face each other. The pair of second anvils face each other in a direction perpendicular to the direction in which the pair of first anvils face each other. In the positioning step, the metal blank is positioned in the space between the pair of first anvils and the pair of second anvils. In the first forging step, a first reduction step and a first rotation step are alternately repeated. In the first reduction step, the metal blank is reduced using the pair of first anvils. In the first rotation step, the metal blank is rotated around the axis and moved in the axial direction of the metal blank. In the second forging step, the hydrostatic stress acting on the metal blank is expressed as σ ｍ , where σ is the equivalent stress and ε is the equivalent plastic strain, the maximum value of the damage value Df in the axial direction expressed by the above formula (1) is D ０ A predetermined threshold D that satisfies ≦−0.22 ０After the first rolling step and the second rotation step are alternately repeated. In the second rolling step, the metal material is rolled down using a pair of first and second anvils. In the second rotation step, the metal material is rotated around the axis and moved in the axial direction (first configuration).

[0025] In the manufacturing method according to the first aspect, a metal material is forged by a first forging process and a second forging process. In the first forging process, two-sided forging is performed using a pair of first anvils. In the second forging process, 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 process to the second forging process is determined when the maximum value of the damage value Df reaches a predetermined threshold value D ０ This is when the threshold D ０ is -0.22 or less. If the switching is performed at this timing, the maximum damage value Df in the forged axle will be -0.40 or less. Therefore, the cavity can be sufficiently crimped by forging. Furthermore, since the manufacturing method according to the first configuration performs the second forging process (four-sided forging), the forging efficiency is better than when forging a metal material using only two-sided forging. As described above, the manufacturing method according to the first configuration can reduce cavities while ensuring forging efficiency.

[0026] In the manufacturing method of the first configuration, the threshold value D ０ is preferably -0.70≦D ０ (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 time required for four-sided forging can be extended to some extent, thereby improving forging efficiency.

[0027] In the manufacturing method of the above configuration, the threshold value D ０ is preferably -0.40≦D ０ (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 time required for four-sided forging can be extended, further improving forging efficiency.

[0028] In the manufacturing method configured as described above, the metal material may be formed into a stepped shape in the second forging step (fourth configuration).

[0029] A specific example of the manufacturing method of the axle of this embodiment will be described below with reference to the drawings. In the drawings, the same or corresponding parts are designated by the same reference numerals and description thereof will not be repeated.

[0030] Figure 4 is a flow diagram showing the manufacturing method of the axle according to this embodiment. As shown in Figure 4, the manufacturing method according to this embodiment includes a material preparation step (#5), an anvil preparation step (#10), a placement step (#15), a first forging step (#20), and a second forging step (#25). Each step shown in Figure 4 will be described in detail below.

[0031] [Material Preparation Step (#5)] In the material preparation step (#5), a metal material is prepared. Fig. 5 is a schematic diagram of a metal material 10 before forging. Referring to Fig. 5, the metal material 10 has an axis 10a. In this embodiment, the metal material 10 has a cylindrical shape.

[0032] The material of the metallic material 10 is not particularly limited. The material of the metallic material 10 is, for example, carbon steel, stainless steel, or alloy steel. The material of the metallic material 10 may also be a low-plasticity material. The main chemical composition of the metallic material 10 may be, in mass %, C: 0.22 to 0.59%, Si: 0.15 to 0.50%, Mn: 0.50 to 1.60%, P: 0.045% or less, S: 0.050% or less, and V: 0.02 to 0.10%.

[0033] In this embodiment, the axle is manufactured by hot free forging. In this case, in the material preparation step (#5), the metal material 10 may be heated in a gas atmosphere furnace or the like. The temperature of the metal material 10 may be any temperature that allows for forging, for example, 800°C or higher and 1300°C or lower.

[0034] [Anvil Preparation Step (#10)] In the anvil preparation step (#10), press equipment for forging the 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. FIG. 6 is a schematic diagram of press equipment 20 used in the manufacturing method according to this embodiment. Referring to FIG. 6, the pair of first anvils 21 face each other. The pair of first anvils 21 are movable so as to move toward or away from each other. The pair of second anvils 22 face each other in a direction substantially perpendicular to the direction in which the pair of first anvils 21 face each other. The pair of second anvils 22 are movable so as to move toward or away from each other.

[0035] The press equipment 20 used in the manufacturing method according to this embodiment further includes ancillary equipment (not shown). The ancillary equipment is typically a manipulator. The ancillary equipment is capable of gripping one end of the metal material 10 and rotating the metal material 10 around the axis 10a. When rotating the metal material 10, the ancillary equipment can also slightly move the metal material 10 in the vertical or horizontal direction. Furthermore, the ancillary equipment can move the metal material 10 in the axial direction.

[0036] [Placement Step (#15)] In the placement step (#15), as shown in Fig. 7, the metal material 10 is placed in the space between the pair of first anvils 21 and the pair of second anvils 22. Fig. 7 is a cross-sectional view showing the placement step (#15). Fig. 7 shows a cross section perpendicular to the axial direction of the metal material 10.

[0037] 7 , the pair of first anvils 21 are spaced apart from each other. The direction in which the pair of first anvils 21 face each other is substantially perpendicular to the axial direction of the metal material 10. Similarly, the pair of second anvils 22 are spaced apart from each other. The direction in which the pair of second anvils 22 face each other is substantially perpendicular to both the axial direction of the metal material 10 and the direction in which the pair of first anvils 21 face each other. The position of the metal material 10 is fixed by an auxiliary device. After the placement of the metal material 10 is complete, one first anvil 21, one second anvil 22, the other first anvil 21, and the other second anvil 22 are arranged in this order around the metal material 10 at 90° intervals.

[0038] In the manufacturing method according to this embodiment, after the placement step (#15), the metal material 10 is forged in a first forging step (#20) and a second forging step (#25) to obtain a stepped axle 90 as shown in Figure 1. The axle 90 manufactured by the manufacturing method according to this embodiment is used, for example, in railway vehicles. The first forging step (#20) and the second forging step (#25) will be described in detail below.

[0039] [First Forging Step (#20)] In the first forging step (#20), two-side forging is performed on the metal material 10. In the first forging step (#20), a first reduction step (#20A) and a first rotation step (#20B) are alternately repeated.

[0040] 8 is a cross-sectional view showing the first rolling step (#20A). In the first rolling step (#20A), the metal material 10 is rolled down using a pair of first anvils 21. At this time, the pair of second anvils 22 are not used. That is, in the first rolling step (#20A), the pair of first anvils 21 are moved closer to each other, but the pair of second anvils 22 are not moved.

[0041] In the first rotation step (#20B), the pair of first anvils 21 are first moved away from each other and returned to the position shown in FIG. 7. Then, in the first rotation step (#20B), the metal material 10 is rotated around the axis 10a. At the same time, the metal material 10 is moved in the axial direction. The rotation and movement of the metal material 10 are performed by ancillary equipment.

[0042] In the first forging step (#20), the reduction, rotation, and axial movement of the metallic material 10 are each performed multiple times. In this embodiment, the reduction amount of the metallic material 10 is constant in the first forging step (#20). Furthermore, in the first forging step (#20), the rotation angle and feed amount of the metallic material 10 are typically constant. In the first rotation step (#20B), the rotation angle of the metallic material 10 is, for example, 5° to 85°, and the feed amount of the metallic material 10 is, for example, 1 mm to 200 mm.

[0043] [Second Forging Step (#25)] In the second forging step (#25), four-sided forging is performed on the metal material 10 after the first forging step (#20). In the second forging step (#25), a second rolling step (#25A) and a second rotating step (#25B) are alternately repeated.

[0044] 9 is a cross-sectional view showing the second rolling down step (#25A). In the second rolling down step (#25A), the metal material 10 is rolled down using a pair of first anvils 21 and a pair of second anvils 22. That is, unlike the first rolling down step (#20A), both the pair of first anvils 21 and the pair of second anvils 22 are used. In the second rolling down step (#25A), the pair of first anvils 21 are brought close to each other, and the pair of second anvils 22 are brought close to each other.

[0045] In the second rotation step (#25B), the pair of first anvils 21 are moved away from each other and returned to the position shown in FIG. 7. Similarly, the pair of second anvils 22 are moved away from each other and returned to the position shown in FIG. 7. Then, in the second rotation step (#25B), the metal material 10 is rotated around the axis 10a. At the same time, the metal material 10 is moved in the axial direction. The rotation and movement of the metal material 10 are performed by auxiliary equipment.

[0046] In the second forging step (#25), the metal material 10 is subjected to a plurality of reductions, rotations, and axial movements. In the second forging step (#25), the rotation angle and feed rate of the metal material 10 are typically constant. In the second rotation step (#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.

[0047] In this embodiment, step 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. When step forming is performed, the reduction amount of the metal material 10 is not constant, but changes each time the metal material 10 is reduced. However, step forming only needs to be performed in at least the latter half of the second forging step (#25), and does not necessarily have to be performed throughout the entire second forging step (#25). In other words, in the first half of the second forging step (#25), the reduction amount of the metal material 10 may be constant.

[0048] In the manufacturing method according to the present embodiment, a switch is made from the first forging process (#20) to the second forging process (#25) when forging the metallic material 10. The timing of the switch from the first forging process (#20) to the second forging process (#25) is determined by the hydrostatic stress acting on the metallic material 10 being σ ｍ , where σ is the equivalent stress and ε is the equivalent plastic strain, the maximum value of the damage value Df in the axial direction expressed by the above formula (1) is a predetermined threshold value D ０ That is, when the maximum value of the damage value Df of the metal material 10 is equal to or exceeds the threshold value D ０ The first forging step (#20) is performed until the maximum value of the damage value Df of the metal material 10 reaches the threshold value D ０ After this, the second forging step (#25) is carried out.

[0049] Threshold D ０ is D ０ ≦-0.22. In other words, in the manufacturing method according to this embodiment, when the damage value Df becomes -0.22 or less, the first forging process (#20) is switched to the second forging process (#25). In this case, the maximum value of the damage value Df in the forged axle 90 becomes -0.40 or less.

[0050] From the viewpoint of reducing the damage value Df in the forged axle 90 and crimping the internal cavity, the threshold value D ０ However, the lower limit of the threshold D ０ If the threshold value D is small, the switching from the first forging process (#20) to the second forging process (#25) will be delayed. As a result, the time required for the first forging process (#20) (two-side forging) will be long and the time required for the second forging process (#25) (four-side forging) will be short, resulting in a deterioration in forging efficiency. ０ -0.70≦D ０ It is preferable to satisfy the following threshold value D ０ -0.70≦D ０ When the condition is satisfied, the switch from two-side forging to four-side forging is performed when the maximum value of the damage value Df is not less than -0.70 and not more than -0.22. In this case, the time for four-side forging can be lengthened to some extent, thereby improving the forging efficiency. More preferably, the threshold value D ０ -0.40≦D ０ fulfill。 Threshold D ０ -0.40≦D ０ When the above formula is satisfied, switching from two-sided forging to four-sided forging is performed when the maximum value of the damage value Df is −0.40 or more and −0.22 or less. In this case, the time for four-sided forging can be made longer, further improving forging efficiency.

[0051] [Effect] In the manufacturing method according to this embodiment, the metal material 10 is forged by a first forging process (#20) and a second forging process (#25). In the first forging process (#20), two-sided forging is performed using a pair of first anvils 21. In the second forging process (#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 process (#20) to the second forging process (#25) is determined when the maximum value of the damage value Df reaches a predetermined threshold value D ０ This is when the threshold D ０ is -0.22 or less. If the switching is performed at this timing, the maximum value of the damage value Df in the forged axle 90 will be -0.40 or less. Therefore, the cavity can be sufficiently crimped by forging. Furthermore, since the manufacturing method according to this embodiment performs the second forging process (#25) (four-sided forging), the forging efficiency is better than when the metal material 10 is forged by only two-sided forging. As described above, according to the manufacturing method according to this embodiment, the cavity can be reduced while ensuring forging efficiency.

[0052] Generally, four-sided forging has a larger contact area between the metal material and the anvil than two-sided forging, resulting in higher dimensional accuracy. In the manufacturing method according to this embodiment, the second forging step (#25) (four-sided forging) is performed in the latter half of the forging of the metal material 10. Therefore, the circularity of the final axle 90 can be ensured.

[0053] When a metal material is pressed down by forging, the metal material elongates in the axial direction. Normally, in two-surface forging, the radially inner portion of the metal material elongates in the axial direction. The end face of an axle manufactured by two-surface forging has a shape in which the radially inner portion protrudes in the axial direction. In addition, in four-surface forging, the radially outer portion of the metal material elongates in the axial direction. The end face of an axle manufactured by four-surface forging has a shape in which the radially outer portion protrudes in the axial direction. In contrast, when both the first forging process (#20) (two-surface forging) and the second forging process (#25) (four-surface forging) are performed as in the manufacturing method of this embodiment, the end face shape of the resulting axle 90 approaches a flush surface. Therefore, the manufacturing method of this embodiment can improve yield.

[0054] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

[0055] In the above embodiment, the reduction amount of the metal material 10 is constant in the first forging step (#20). In other words, no stepped forming is performed in the first forging step (#20). However, stepped forming may be performed in the first forging step (#20) in addition to the second forging step (#25). In this case, stepped forming is performed in at least the latter half of the first forging step (#20).

[0056] In the manufacturing method according to the above embodiment, after the placement step (#15), the metal blank 10 is forged through a first forging step (#20) (two-face forging) and a second forging step (#25) (four-face forging). However, four-face forging may also be performed after the placement step (#15) and before the first forging step (#20). In this case, the metal blank 10 is forged in the order of four-face forging, two-face forging, and four-face forging. Comparing the results of Case 1 and Case 2 in FIG. 3, at the initial stage of forging, i.e., when the cross-sectional area reduction rate is small, the difference between the damage value Df in two-face forging and the damage value Df in four-face forging is relatively small. Therefore, even if four-face forging is performed before the first forging step (#20), it does not significantly affect the damage value Df of the forged axle 90. Furthermore, by performing four-sided forging before the first forging step (#20), the time required to perform the first forging step (#20) (two-sided forging) is shortened, thereby further improving the overall forging efficiency.

[0057] 10: Metal material 10a: Axle center 21: First anvil 22: Second anvil 90: Axle

Claims

1. A method for manufacturing an axle, comprising: a material preparation step of preparing a metal material having an axial center; an anvil preparation step of preparing a pair of first anvils facing each other and a pair of second anvils facing each other in a direction perpendicular to the direction in which the pair of first anvils face each other; an arrangement step of arranging the metal material in a space between the pair of first anvils and between the pair of second anvils; a first pressing step of pressing the metal material using the pair of first anvils; and a first rotation step of rotating the metal material around the axial center and moving it in the axial direction of the metal material; ｍ , where σ is the equivalent stress and ε is the equivalent plastic strain, the maximum value of the damage value Df in the axial direction expressed by the following formula (1) is D ０ A predetermined threshold D that satisfies ≦−0.22 ０ and a second forging process in which a second rolling process is performed in which the metal material is rolled down using the pair of first anvils and the pair of second anvils after the metal material has been rolled down, and a second rotation process is performed in which the metal material is rotated around the axis and moved in the axial direction, and these processes are repeated alternately.

2. The manufacturing method according to claim 1, wherein the threshold value D ０ -0.70≦D ０ Manufacturing method that meets the above requirements.

3. The manufacturing method according to claim 1, wherein the threshold value D ０ -0.40≦D ０ Manufacturing method that meets the above requirements.

4. A manufacturing method according to any one of claims 1 to 3, wherein in the second forging step, the metal material is formed into a stepped shape.

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