Hollow member and method for manufacturing hollow member

Abstract

According to the present invention, a hollow member has a circumferential hardness difference section in at least a portion in the longitudinal direction along the center axis thereof. When viewing the circumferential hardness difference section in a cross-section perpendicular to the center axis, a thickness difference, derived by subtracting the minimum value of the thickness in the circumferential direction of the cross-section from the maximum value of the thickness, is no more than 20% of an average value of the thickness over the entire circumference of the cross-section. Furthermore, when the average of the integral of the Vicker's hardness over the entire circumference of the cross-section is used as a hardness threshold, the cross-section includes: a low-strength range in which the Vicker's hardness in the circumferential direction is no more than the hardness threshold; and a high-strength range in which the Vicker's hardness in the circumferential direction exceeds the hardness threshold.

Description

[Technical Field]

[0001] The present disclosure relates to hollow members and methods of manufacturing hollow members. [Background technology]

[0002] It is known that vehicle body members (hollow members) that constitute automobile bodies are manufactured using steel pipes with different wall thicknesses along their longitudinal direction. Patent Document 1 discloses an example of a method for manufacturing this type of steel pipe with different wall thicknesses from a hollow cylindrical mother pipe, which includes: a locking step in which the mother pipe is placed in a die and, while restricting longitudinal movement of the mother pipe, a plug is pushed into one end of the mother pipe to expand the outer shape of the one end and lock it in the die; and an ironing step in which, while releasing the restriction on the mother pipe and maintaining the lock on the mother pipe, the plug is further pushed toward the other end of the mother pipe to form a thin-walled portion by ironing, expanding the inner shape of the mother pipe while maintaining its outer shape. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2017 / 154481 Summary of the Invention [Problem to be solved by the invention]

[0004] On the other hand, in automobile crash safety performance tests, safety against a crash is evaluated based on the degree of deformation of the vehicle body members. It is preferable that the vehicle body members have high robustness in deformation against the crash conditions. Robustness in deformation against the crash conditions means that the deformation mode of the vehicle body members does not change and stable deformation can be obtained even if the crash conditions, such as the crash angle, change slightly.

[0005] One conventional configuration for achieving a stable deformation mode is to pre-bend the carbody member (hollow member) gently at a midpoint in the longitudinal direction. However, this configuration raises concerns that the carbody member may behave unstably and degrade its performance when subjected to a twisting external force during normal operation that does not result in a collision.

[0006] The present disclosure has been made in consideration of the above-mentioned situation, and aims to provide a hollow member with improved robustness without degrading component performance during normal operation, and a hollow member manufacturing method for manufacturing this hollow member. [Means for solving the problem]

[0007] In view of the above circumstances, the present disclosure employs the following aspects. That is, (1) A hollow member according to one embodiment of the present disclosure includes: a circumferential hardness difference portion in at least a portion of the longitudinal direction along the central axis; When the circumferential hardness difference portion is viewed in a cross section perpendicular to the central axis, a thickness difference obtained by subtracting the minimum value of the wall thickness from the maximum value of the wall thickness in the circumferential direction of the cross section is 20% or less of the average value of the wall thickness over the entire circumference of the cross section; The average of the integral of the Vickers hardness around the entire circumference of the cross section is set as a hardness threshold, and the cross section includes a low strength range in which the Vickers hardness along the circumferential direction is equal to or less than the hardness threshold, and a high strength range in which the Vickers hardness along the circumferential direction is greater than the hardness threshold. fruit; The difference between the maximum value of the Vickers hardness and the minimum value of the Vickers hardness in the cross section is 25 HV or more.

[0008] According to the aspect (1) above, by providing a circumferential hardness difference portion, a relatively soft low-strength region and a relatively hard high-strength region are formed in the same cross section of the hollow member in the circumferential direction. During normal operation, both the low-strength region and the high-strength region support external forces within their ranges of elastic deformation, preventing a decrease in component performance. On the other hand, when subjected to an external force, such as an impact force, stronger than that during normal operation, the low-strength region of the circumferential hardness difference portion actively undergoes plastic deformation to absorb energy. As a result, the hollow member undergoes bending deformation such that the low-strength region is on the concave side and the high-strength region is on the convex side. Since the bending direction of the hollow member can be set by the relative positional relationship between the low-strength region and the high-strength region, robustness is high.

[0009] (2) In the hollow member described in (1) above, The proportion of the perimeter of the low strength region to the total perimeter of the cross section may be within a range of 20% to 80%. In the case of (2) above, by setting the lower limit of the ratio to 20%, the circumferential hardness differential portion can be reliably plastically deformed and broken in the low strength range, while by setting the upper limit of the ratio to 80%, the low strength range can be prevented from becoming excessively wide, and the bending direction of the hollow member can be restricted within a predetermined range.

[0010] (3) In the hollow member described in (1) or (2) above, the following configuration may be adopted: the total perimeter of the cross section is Lr (mm); With the minimum hardness position where the Vickers hardness is at its minimum value as a reference, the maximum hardness position where the Vickers hardness is at its maximum value is within the range of 0.3×Lr (mm) to 0.7×Lr (mm) in the circumferential direction. In the case of (3) above, the part with the minimum Vickers hardness and the part with the maximum Vickers hardness can be positioned substantially opposite each other across the central axis of the hollow member, making it easier to control the bending direction of the hollow member.

[0011] (4) In the hollow member according to any one of (1) to (3) above, the following configuration may be adopted: The thickness difference in the cross section is 0.10 mm or less; The difference between the maximum value of the Vickers hardness and the minimum value of the Vickers hardness in the cross section is 15 HV or more. In the case of (4) above, the thickness along the circumferential direction in the cross section is made more uniform, and the difference in Vickers hardness is increased to 15 HV or more, making it easier for the low strength range to undergo plastic deformation and allowing the bending direction of the hollow member to be set more accurately.

[0012] (5) In the hollow member according to any one of (1) to (4), The circumferential hardness difference portion may be formed only in a portion in the longitudinal direction. In the case of (5) above, the difference in Vickers hardness between the high strength range and the low strength range is Even if the difference is small, the hollow member can be broken by reliably plastically deforming the low-strength range. As an example, in the configuration shown in Fig. 17(a) described below, the hollow member can be broken with a smaller difference in Vickers hardness (for example, 10 HV) than in the configuration shown in Fig. 16(a).

[0013] (6) In the hollow member according to any one of (1) to (4), The circumferential hardness difference portion may be formed over the entire length in the longitudinal direction. In the case of (6) above, the external force required to cause the hollow member to bend and deform can be intentionally set to a high value.

[0014] (7) A method for manufacturing a hollow member according to one aspect of the present disclosure includes: A method for manufacturing a hollow member from a hollow cylindrical mother tube, comprising the steps of: a blank tube placement step of placing the blank tube in a die; an ironing process step of forcing a plug into the mother tube to expand the inner wall of the mother tube while ironing the inner wall so as to feed the wall in a circumferential direction of the inner wall when viewed along the central axis of the mother tube; It has.

[0015] According to the aspect (7) described above, when viewed along the central axis of the blank tube, the ironing process pushes the inner wall material in the circumferential direction of the inner wall. As a result, the ironed inner wall has a high-strength region where the material concentrates and has a high Vickers hardness, and a low-strength region where the material flows out and has a relatively low Vickers hardness. A hollow member having these high-strength and low-strength regions can withstand external forces within the range of elastic deformation during normal operation, preventing a deterioration in component performance. On the other hand, when subjected to an external force, such as an impact force, stronger than during normal operation, the low-strength region actively plastically deforms to absorb energy. As a result, the hollow member bends so that the low-strength region is on the concave side and the high-strength region is on the convex side. Therefore, the bending direction of the hollow member can be determined by the relative positional relationship between the low-strength and high-strength regions, allowing for the manufacture of a highly robust hollow member.

[0016] (8) In the method for manufacturing a hollow member according to (7) above, the following may be adopted: the plug has a tip portion tapered in a pushing direction, and a main body portion connected to a rear end of the tip portion and having a maximum outer dimension in a cross section perpendicular to the pushing direction; A plane including a connecting line between the tip portion and the body portion is inclined relative to a plane perpendicular to the central axis of the plug. In the case of (8) above, because the plane including the connecting line between the front end portion and the main body portion is inclined, the timing at which this connecting line strikes the inner wall of the mother tube in the plug-pushing direction can be varied at different positions in the circumferential direction along the inner wall of the mother tube. That is, among the points on the connecting line, those closer to the front end in the thrust direction strike the inner wall earlier, while those closer to the rear end in the thrust direction strike the inner wall later. As a result, the material that is ironed earlier moves circumferentially toward the area that is ironed later. In this way, the hollow member after ironing has both a high-strength area where the material is concentrated and has a high Vickers hardness, and a low-strength area where the material has flowed out and has a relatively low Vickers hardness.

[0017] (9) In the method for manufacturing a hollow member according to (7) above, the following may be adopted: the plug has a tip portion tapered in a pushing direction, and a main body portion connected to a rear end of the tip portion and having a maximum outer dimension in a cross section perpendicular to the pushing direction; A connecting line between the tip portion and the main body portion, a plurality of first connection points closest to the tip end surface of the plug in a side view; a plurality of second connection points located between the first connection points in a front view and located farther from the tip surface than the first connection points in a side view; Includes: In the case of (9) above, the timing at which each first connection point strikes the inner wall of the blank tube can be made earlier than the timing at which each second connection point passes through the inner wall of the blank tube in the plug-pushing direction. That is, among the points on the connection line, each first connection point located at the leading end in the plug-pushing direction strikes the inner wall earlier, while each second connection point located at the trailing end in the plug-pushing direction strikes the inner wall later. As a result, the material that is ironed earlier moves along the circumferential direction toward the area that is ironed later. In this way, the hollow member after ironing has both a high-strength area where the material is concentrated and has a high Vickers hardness, and a low-strength area where the material has flowed out and has a relatively low Vickers hardness.

[0018] (10) In the method for producing a hollow member according to any one of (7) to (9), a locking step, which is performed after the mother tube arrangement step and before the ironing step, of pushing the plug into the end of the mother tube to enlarge the outer shape of the end, thereby locking the enlarged portion in the die; a drawing process step in which the expanded portion is passed through another die to reduce the shape after the ironing process step; may further comprise: In the case of (10) above, it is possible to manufacture a hollow member that has uniform outer dimensions along the longitudinal direction but has a portion with a circumferential hardness difference at a midpoint in the longitudinal direction.

[0019] (11) In the method for producing a hollow member according to (10), The method may further include a press forming step subsequent to the drawing step, in which the metal sheet is press-formed so that the cross section perpendicular to the central axis has a rectangular shape. In the case of (11) above, a hollow member having a rectangular outer shape can be manufactured.

[0020] (12) In the method for producing a hollow member according to any one of (7) to (9), The method may further include a press forming step subsequent to the drawing step, in which the metal sheet is press-formed so that the cross section perpendicular to the central axis has a rectangular shape. In the case of (12) above, a hollow member having a rectangular outer shape can be manufactured. [Effects of the Invention]

[0021] According to the present disclosure, it is possible to provide a hollow member having improved robustness without degrading component performance during normal operation, and a method for manufacturing the hollow member. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 2 is a side view of a hollow member according to an embodiment of the present disclosure, showing the circumferential Vickers hardness distribution by gradation. [Figure 2] 2A and 2B are diagrams showing the main part of the hollow member, in which (a) is an enlarged view of part A in FIG. 1 and (b) is a cross-sectional view taken along line BB in (a). The Vickers hardness distribution in the circumferential direction is shown by gradation. [Figure 3] 1 is a graph showing an example of the Vickers hardness distribution of the hollow member, in which the horizontal axis indicates the measurement position in the circumferential direction and the vertical axis indicates the Vickers hardness at each position. [Figure 4] 1A and 1B are side views showing the hollow member when a load is applied, in which (a) shows the state before the load is applied and (b) shows the state after the load is applied, and the Vickers hardness distribution in the circumferential direction is shown by a gradation. [Figure 5]1A and 1B are diagrams showing the head portion of a plug used in the hollow member manufacturing method of the same embodiment, in which (a) is a perspective view seen from the tip side, and (b) is a perspective view seen from the rear end side. [Figure 6] 2A and 2B are diagrams showing the head portion, in which (a) is a side view and (b) is a front view. [Figure 7] 1(a) to 1(c) are cross-sectional views showing the first half of the method for manufacturing a hollow member using a plug having a head portion in chronological order, where 1(a) shows the mother tube arrangement step, 1(b) shows the locking step, and 1(c) shows the ironing step. [Figure 8] 7(c) are cross-sectional views showing the second half of the hollow member manufacturing method in chronological order (a) and (b), where (a) shows the start of the drawing process and (b) shows the completion. [Figure 9] 1 is a graph illustrating a variation of the hollow member, showing the Vickers hardness distribution of the hollow member, where the horizontal axis represents the measurement position in the circumferential direction and the vertical axis represents the Vickers hardness at each position. [Figure 10] 10A and 10B are side views showing another modified example of a hollow member, in which (a) shows a case where a circumferential distribution of Vickers hardness is imparted over the entire length, and (b) shows a case where a circumferential distribution of Vickers hardness is imparted to half of the longitudinal direction. The circumferential Vickers hardness distribution is shown by a gradation. [Figure 11] 1 is a graph showing another modified example of the hollow member, in which the horizontal axis indicates the measurement position in the circumferential direction, and the vertical axis indicates the Vickers hardness at each position. [Figure 12] 12 is a front view of a plug used in manufacturing the hollow member of FIG. 11. FIG. [Figure 13] 1 is a side view showing a first embodiment of the present disclosure, in which hollow members T2, T4, and T5 are examples and hollow members T1 and T3 are comparative examples. Note that the Vickers hardness distribution in the circumferential direction is shown by gradation. [Figure 14]These are side views showing the test conditions when applying external force to hollow members T1 to T5, where (a) shows Model A in which hollow members T1 to T5 are supported on a plane perpendicular to their central axis CL, and (b) shows Model B in which hollow members T1 to T5 are supported on a plane that forms an inclination angle of 15 degrees with respect to their central axis CL. [Figure 15] FIG. 7 is a diagram illustrating a second embodiment of the present disclosure, and is a graph showing the Vickers hardness distribution in the circumferential direction of a hollow member obtained by a method for manufacturing a hollow member using a plug having the head portion shown in FIG. 6. Here, the horizontal axis represents the measurement position in the circumferential direction, and the vertical axis represents the Vickers hardness at each position. [Figure 16] 10A and 10B are side views showing a third embodiment of the present disclosure, each showing a Vickers hardness distribution over the entire length. (a) shows the state before impact force is applied, and (b) and (c) show the state after impact force is applied. (b) shows the case where the difference in Vickers hardness between the strong and weak portions is 15 HV, and (c) shows the case where the difference in Vickers hardness between the strong and weak portions is 10 HV. The circumferential Vickers hardness distribution is shown by a gradation. [Figure 17] This is a side view showing a fourth embodiment of the present disclosure, in which a Vickers hardness distribution is provided at the center position in the longitudinal direction. Here, (a) shows the state before the impact force is applied, and (b) and (c) show the state after the impact force is applied. (b) shows the case where the difference in Vickers hardness between the strong and weak parts is 10 HV, and (c) shows the case where the difference in Vickers hardness between the strong and weak parts is 5 HV. The Vickers hardness distribution in the circumferential direction is shown by a gradation. DETAILED DESCRIPTION OF THE INVENTION

[0023] Embodiments and variations of a hollow member and a method for manufacturing a hollow member according to the present disclosure will be described below with reference to the drawings. Note that in each figure, the size and shape of each part may be exaggerated as appropriate to facilitate understanding. Also, in each figure, hatching or symbols may be omitted for convenience. Furthermore, the direction along the central axis of a hollow member may be referred to as the longitudinal direction, and the direction along the circumference of the inner or outer wall surface of the hollow member around the central axis may be referred to as the circumferential direction. Furthermore, unless otherwise specified, such as "average of maximum and minimum values," each average value for Vickers hardness refers to the average of the integral in the circumferential direction of the cross section.

[0024] First, the hollow member of this embodiment will be described with reference to Figures 1 to 4. Next, the head portion of the plug that irons the mother tube when manufacturing this hollow member will be described with reference to Figures 5 and 6. Next, a method for manufacturing a hollow member using this plug will be described with reference to Figures 7 and 8. Finally, a modified example of this embodiment will be described with reference to Figures 9 and 10.

[0025] <Hollow member> Fig. 1 is a side view of a hollow member 10 according to this embodiment, showing the circumferential Vickers hardness distribution using gradation. Fig. 2 is a diagram showing a main part of the hollow member 10, where (a) is an enlarged view of part A in Fig. 1 and (b) is a cross-sectional view taken along line BB of (a). Fig. 3 is a graph showing an example of the Vickers hardness distribution of the hollow member 10, where the horizontal axis indicates the measurement position in the circumferential direction and the vertical axis indicates the Vickers hardness at each position.

[0026] As shown in Figures 1 and 2, the hollow member 10 has a linear central axis CL and is a metal tubular body that is elongated along the central axis CL. The hollow member 10 is a circular tube whose cross section is circular at every position along its entire length, and whose outer diameter, inner diameter, and wall thickness are the same at every position along the length. Specific dimensions include an outer diameter D of 20 mm to 180 mm and a wall thickness t of 0.4 mm to 10 mm. Here, the wall thickness t is uniform in the circumferential direction, and the thickness difference, which is the difference between the maximum and minimum dimensions, is preferably 20% or less, and more preferably 10% or less, of the average wall thickness in the circumferential direction at the same cross section.

[0027] However, in the case of a hollow member 10 manufactured using a welded pipe formed by processing a flat plate into a tubular shape as a base pipe, the wall thickness at the welded portion is likely to be non-uniform compared to the surrounding area. Therefore, the wall thickness t and wall thickness difference must be set excluding the welded portion. Specifically, in the case of a hollow member having a seam weld, it is preferable to set the wall thickness t and wall thickness difference over 80% of the cross section perpendicular to the center axis CL, excluding both 10° counterclockwise and 10° clockwise ranges from a line connecting the center axis CL and the widthwise center of the seam weld. The same applies to welds other than seam welds. Naturally, in the case of a hollow member manufactured using a base pipe without a weld, it is preferable to set the wall thickness t and wall thickness difference over 100% of the circumferential direction of the cross section.

[0028] As shown in FIG. 1, the hollow member 10 has a first region 11, a circumferential hardness difference region 12, and a second region 13, which are arranged in this order from left to right on the page along the central axis CL. That is, in this embodiment, the circumferential hardness difference region 12 is formed only in the central region in the longitudinal direction. Note that in FIGS. 1 and 2, the boundary between the first region 11 and the second region 13 is indicated by a solid line to clearly indicate the position of the circumferential hardness difference region 12. However, in reality, such a boundary is often not visible by visual inspection alone, and can be confirmed by measuring the Vickers hardness distribution in each region.

[0029] In the present disclosure, Vickers hardness is measured in accordance with JIS Z 2244:2020 on a cross section perpendicular to the central axis CL. The indentation load during measurement is 1 kgf. However, if the indentation load is too large and the spacing between indentations does not meet the standards specified in JIS Z 2244:2020, the indentation load may be 100 gf. The measurement interval is set to 10° or less along the circumferential direction or 5 mm or less, as long as the spacing between indentations meets the standards specified in JIS Z 2244:2020. However, if the spacing between indentations does not meet the standards specified in JIS Z 2244:2020, measurements are made on two cross sections as follows. That is, first, a portion of the hollow member 10 is cut out to obtain a cut section, and the two cut surfaces on both sides of this cut section are used as measurement targets. On one of the cut surfaces, the Vickers hardness was measured at intervals of 0° to 20° in the circumferential direction, and on the other cut surface, the Vickers hardness was measured at intervals of 10° to 20° in the circumferential direction. The measurement positions were basically the center of the plate thickness.

[0030] For hollow members 10 manufactured from mother pipes having welds, it is preferable to measure the Vickers hardness in a cross section perpendicular to the center axis CL over 80% of the range, excluding both 10° counterclockwise and 10° clockwise, based on a line connecting the center axis CL and the circumferential center position of the weld, in the same manner as in the measurement of the wall thickness t described above. For hollow members manufactured from mother pipes without welds, the Vickers hardness is measured over 100% of the circumferential direction in a cross section perpendicular to the center axis CL.

[0031] Returning to the explanation of FIG. 1, the first region 11 has uniform outer diameter, inner diameter, thickness and Vickers hardness at each position in the longitudinal and circumferential directions. Regarding the thickness, at each longitudinal position of the first region 11, the difference in thickness, calculated by subtracting the minimum from the maximum, in the circumferential direction of a cross section perpendicular to the central axis CL, is 20% or less of the average thickness in the circumferential direction of the same cross section. Furthermore, the Vickers hardness is uniform, with no hardness distribution in the circumferential direction. That is, in the first region 11, the average Vickers hardness in the circumferential direction of a cross section perpendicular to the central axis CL is constant at each longitudinal position. Note that, if the first region 11 includes a welded portion, the explanations regarding the thickness and Vickers hardness exclude that portion.

[0032] The first region 11 does not have a regular hardness distribution. The material of the first region 11 is the same as the material of the circumferential hardness difference region 12. The first region 11 has a higher deformation resistance than the circumferential hardness difference region 12 and is therefore less likely to deform when an axial load is applied. The Vickers hardness and thickness of the first region 11 are not particularly limited as long as they provide a higher deformation resistance than the circumferential hardness difference region 12. The Vickers hardness and thickness of the first region 11 may be uniform or non-uniform in the longitudinal direction. The deformation resistance can be evaluated, for example, by cutting out a measurement location from the first region 11 and measuring its ease of deformation when an axial load is applied. Therefore, the ease of deformation of the first region 11 and the circumferential hardness difference region 12 are calculated and compared. The region that is relatively less likely to deform can be evaluated as having a high relative deformation resistance, and the region that is relatively more likely to deform can be evaluated as having a low relative deformation resistance.

[0033] In the circumferential direction of the first region 11, the maximum value of Vickers hardness is HV1 max The minimum Vickers hardness is HV1 min In this case, HV1 max From HV1 min The difference ΔHV1 obtained by subtracting this is, for example, less than 15 HV. The difference ΔHV1 may be 10 HV or less. In addition, in the circumferential direction of the circumferential hardness difference portion 12 described later, the maximum value of Vickers hardness is HV2max The minimum Vickers hardness is HV2 min And then HV2 max From HV2 min The difference obtained by subtracting ΔHV1 from ΔHV2 is set to ΔHV2. In this case, ΔHV1 is smaller than ΔHV2. The difference obtained by subtracting ΔHV1 from ΔHV2 may be set to preferably 3HV or more, more preferably 5HV or more, and most preferably 10HV or more.

[0034] The thickness of the first region 11 in the circumferential direction may be the same as the thickness of the circumferential hardness difference portion 12 in the circumferential direction, or may be larger than the thickness of the circumferential hardness difference portion 12 in the circumferential direction. The thickness of the first region 11 in the circumferential direction may be uniform. Specifically, the maximum thickness of the first region 11 in the circumferential direction may be set to T1 max The minimum thickness is T1 min In this case, T1 max From T1 min The difference obtained by subtracting the above may be 0.50 mm or less. On the other hand, the thickness of the first region 11 may vary in the longitudinal direction. The cross-sectional shape of the first region 11 (the shape of a cross section perpendicular to the longitudinal direction) is not particularly limited, but examples include circles such as a perfect circle and an ellipse, and polygons such as a rectangle. The polygon referred to here includes not only a strict polygon but also a shape in which the parts corresponding to the corners of a polygon are arc-shaped.

[0035] The second region 13 has the same configuration as the first region 11. That is, the second region 13 also has a uniform outer diameter, inner diameter, wall thickness, and Vickers hardness at each longitudinal and circumferential position. Regarding the wall thickness, at each longitudinal position of the second region 13, the difference in wall thickness, calculated by subtracting the minimum wall thickness from the maximum wall thickness in the circumferential direction of a cross section perpendicular to the central axis CL, is 20% or less of the average wall thickness in the circumferential direction of the same cross section. Furthermore, the Vickers hardness is uniform and does not have a hardness distribution in the circumferential direction. That is, in the second region 13, the average Vickers hardness in the circumferential direction of a cross section perpendicular to the central axis CL is constant at each longitudinal position. The descriptions of the wall thickness and Vickers hardness exclude welded sections, if any.

[0036] Like the first region 11, the second region 13 does not have a regular hardness distribution. The material of the second region 13 is the same as the material of the circumferential hardness difference region 12. The second region 13 has higher rigidity than the circumferential hardness difference region 12, making it less susceptible to deformation when an axial load is applied. The Vickers hardness and thickness of the second region 13 are not particularly limited as long as they provide higher deformation resistance than the circumferential hardness difference region 12. The Vickers hardness and thickness of the second region 13 may be uniform or non-uniform in the longitudinal direction. The deformation resistance can be evaluated, for example, by cutting out a measurement target portion of the second region 13 and measuring its ease of deformation when an axial load is applied. Therefore, the ease of deformation of each of the second region 13 and the circumferential hardness difference region 12 can be determined and compared, and the one that is relatively less likely to deform can be evaluated as having a high relative deformation resistance, and the one that is relatively easier to deform can be evaluated as having a low relative deformation resistance.

[0037] In addition, the maximum value of Vickers hardness in the circumferential direction of the second region 13 is HV3 max The minimum Vickers hardness is HV3 min In this case, HV3 max From HV3 minThe difference ΔHV3 obtained by subtracting this is, for example, less than 15 HV. The difference ΔHV3 may be 10 HV or less. Furthermore, in comparison with the circumferential hardness difference portion 12 described below, ΔHV3 is smaller than ΔHV2. The difference obtained by subtracting ΔHV3 from ΔHV2 may be preferably 3 HV or more, more preferably 5 HV or more, and most preferably 10 HV or more.

[0038] The thickness of the second region 13 in the circumferential direction may be the same as the thickness of the circumferential hardness difference portion 12 in the circumferential direction, or may be larger than the thickness of the circumferential hardness difference portion 12 in the circumferential direction. The thickness of the second region 13 in the circumferential direction may be uniform. max The minimum thickness is T3 min In this case, T3 max From T3 min The difference obtained by subtracting the above may be 0.50 mm or less. On the other hand, the thickness of the second region portion 13 may vary in the longitudinal direction. The cross-sectional shape of the second region 13 (the shape of a cross section perpendicular to the longitudinal direction) is not particularly limited, but examples include circles such as a perfect circle and an ellipse, and polygons such as a rectangle. The polygon referred to here includes not only a strict polygon but also a shape in which the parts corresponding to the corners of a polygon are arc-shaped.

[0039] The circumferential hardness difference portion 12 may have the same outer diameter, inner diameter, and thickness as the first region 11 and the second region 13, but have a different Vickers hardness distribution. That is, the thickness difference of the circumferential hardness difference portion 12, obtained by subtracting the minimum value from the maximum value of the circumferential thickness of a cross section perpendicular to the central axis CL, at each position in the longitudinal direction, is 20% or less (preferably 10% or less) of the average thickness over the entire circumference of the cross section. Note that the average value referred to here does not mean the average of the maximum and minimum values, but is a value obtained by determining the thickness distribution along the entire circumference in the circumferential direction, integrating, and averaging the thickness distribution. When the outer diameter of the hollow member 10 is, for example, 20 mm to 180 mm and the average wall thickness is, for example, 0.4 to 10 mm, the wall thickness difference is preferably 0.10 mm or less, more preferably 0.05 mm or less, and most preferably 0.03 mm or less.

[0040] 1 and 2(a) and (b), the Vickers hardness of the circumferential hardness difference portion 12 is such that the lower part of the drawing is a relatively soft low-strength portion and the upper part of the drawing is a relatively hard high-strength portion. In this way, the circumferential hardness difference portion 12 has a hardness distribution along the circumferential direction at each position in the longitudinal direction.

[0041] An example of the hardness distribution in the circumferential hardness difference portion 12 is shown in Fig. 3. The measurement positions shown on the horizontal axis of Fig. 3 are the positions P where the minimum value was obtained when the Vickers hardness was measured at each position along the circumferential direction in the cross section shown in Fig. 2(b). min is set as the reference (0°), and the angle θ from this reference is expressed using 0°≦θ≦360°. As will be described later, the measurement position can also be expressed using the total circumferential length Lr (mm) of the circumferential hardness difference portion 12. For example, at position P in FIG. min is 0×Lr(mm)=0(mm), position P max can be expressed as 0.5 × Lr (mm). In Figure 3, in addition to the measurement positions using the angle θ, the measurement positions using the total circumference Lr (mm) are also shown.

[0042] As shown in FIG. 3, the hardness distribution in this embodiment is min ) and the minimum Vickers hardness (HV min ), and the Vickers hardness increases with increasing angle θ, and the angle θ=180°(P max ) and the maximum Vickers hardness (HV max ) As the angle θ increases beyond 180°, the Vickers hardness decreases, and min ) and the minimum value (HV min) in the circumferential hardness difference portion 12. In this way, the circumferential hardness difference portion 12 has a hardness distribution in which the Vickers hardness changes regularly in the circumferential direction. In the circumferential hardness difference portion 12, the maximum value HV of the Vickers hardness max to minimum value HV min The hardness difference ΔHV obtained by subtracting the above is 15 HV or more.

[0043] Due to the hardness distribution, as shown in FIGS. 2(a) and 2(b), the circumferential hardness difference portion 12 has low strength regions 12A and high strength regions 12B at each position in the longitudinal direction. The low strength range 12A is defined as a hardness threshold HV, which is the average integral of the Vickers hardness in the circumferential direction of a cross section perpendicular to the central axis CL. av When the Vickers hardness is av It is defined as a range that is: In contrast, the high strength area 12B has a Vickers hardness that is greater than the hardness threshold HV av It is defined as a range that is greater than or equal to 1.

[0044] As shown in Figure 3, the low strength range 12A is the measurement position P where the Vickers hardness is the minimum. min The Vickers hardness gradually increases from the starting point to one end of the high strength region 12B, and the Vickers hardness gradually decreases from the other end of the high strength region 12B to the measurement position P min and a portion returning to The high intensity range 12B is measured at a measurement position P max The maximum Vickers hardness is HV max And, this measurement position P max The Vickers hardness gradually decreases from the center toward the left and right in the circumferential direction, and continues into the low strength range 12A.

[0045] Measurement position P in the low intensity range 12A min and the measurement position P in the high intensity range 12B. max These will be explained in more detail below. The total outer circumferential length of the circumferential hardness difference portion 12 is Lr (mm), and the position P where the Vickers hardness is the minimum value in the low strength range 12A is min When the Vickers hardness is set as a standard, the position P where the Vickers hardness becomes maximum is within the high strength range 12B, which is in the range of 0.3×Lr (mm) to 0.7×Lr (mm) (more preferably, in the range of 0.4×Lr (mm) to 0.6×Lr (mm)). max In this embodiment, as shown in FIG. min and position P max 3 shows an example in which the measurement positions are set at 0° (0×Lr) or 360° (1.0×Lr). min is set, and the position P is set at 180° (0.5 × Lr). max is set.

[0046] 3, the low-strength region 12A and the high-strength region 12B are connected at two points a and b where the Vickers hardness at both ends of the low-strength region 12A and the Vickers hardness at both ends of the high-strength region 12B are equal. In this embodiment, the measurement positions in the range of 0° (0×Lr) to 108° (0.3×Lr) and the range of 252° (0.7×Lr) to 360° (1.0×Lr) are set as the low-strength region 12A. The range greater than 108° (0.3×Lr) and less than 252° (0.7×Lr) is set as the high-strength region 12B.

[0047] 2(b), the ratio of the circumferential length La (mm) of the low-strength region 12A to the total circumferential length Lr (mm) is in the range of 20% to 80%, and a ratio of 30% to 70% is preferable because a more significant effect can be obtained. Therefore, the ratio of the circumferential length Lb (mm) of the high-strength region 12B to the total circumferential length Lr (mm) is the value obtained by subtracting the ratio of the circumferential length La (mm) from 100%. The above description of the thickness and Vickers hardness of the circumferential hardness difference portion 12 excludes the welded portion if it is included.

[0048] The hollow member 10 of this embodiment having the above configuration can improve robustness without degrading component performance during normal operation. This will be explained below with reference to Figures 4(a) and 4(b). Figure 4 is a side view showing the application of a load F to the hollow member 10, where (a) shows the state before the load F is applied and (b) shows the state after the load F is applied.

[0049] As shown in Figures 4(a) and 4(b), the hollow member 10 includes a first region 11, a circumferential hardness difference region 12, and a second region 13, arranged in this order along its longitudinal direction. As shown in Figure 4(a), a load F is applied to the tip of the first region 11 of the hollow member 10 along the central axis CL. If the load F is a value during normal operation, the hollow member 10 can withstand the load without bending. The circumferential hardness difference region 12 has a relatively soft low-strength region 12A and a relatively hard high-strength region 12B along its circumferential direction. However, both the low-strength region 12A and the high-strength region 12B can withstand external forces within the range of elastic deformation during normal operation, preventing a decrease in component performance. Therefore, the hollow member 10 maintains its linear shape as shown in Figure 4(a) and continues to function as a strength member.

[0050] On the other hand, when the load F applied in FIG. 4(a) is an impact force stronger than that during normal operation, as shown in FIG. 4(b), the low-strength region 12A of the circumferential hardness difference portion 12 breaks and deforms preferentially over the high-strength region 12B. At this time, the low-strength region 12A, which has a relatively lower Vickers hardness than the high-strength region 12B, actively undergoes plastic deformation. As a result, the hollow member 10 bends and deforms so that the low-strength region 12A is on the concave side and the high-strength region 12B is on the convex side, and energy is absorbed during this plastic deformation process. In this way, the bending direction of the hollow member 10 can be determined by the relative positions of the low-strength region 12A and the high-strength region 12B. Therefore, even if the direction of application of the load F is slightly angled with respect to the central axis CL, the position of application of the load F is slightly shifted with respect to the central axis CL, or the direction of the reaction force against the load F varies slightly, the hollow member 10 can be bent and deformed in the desired direction. As a result, a stable folding deformation mode can be achieved, which increases the robustness of the deformation against collision conditions.

[0051] As described above, in order to increase the robustness of deformation under collision conditions, conventionally, measures such as pre-bending the hollow member have been considered, but these measures may result in disadvantages such as a decrease in part performance during normal operation, an increase in the number of manufacturing steps, and limitations on the use of the part. In contrast, the hollow member 10 of this embodiment does not require bending or other deformation, thereby achieving a stable bending deformation mode. Furthermore, because the thickness of the circumferential hardness difference portion 12 of this hollow member 10 is uniform along the circumferential direction, the component rigidity during normal operation is also uniform along the circumferential direction, which has the advantage of preventing deformation from concentrating in one location. For example, if the wall thickness of the hollow member is nonuniform along the circumferential direction, when subjected to torsion as an external force, differences in thickness may occur between different portions, such as large deflection in thin portions and almost no deflection in thick portions. In particular, when out-of-plane deformation is involved, the ease of deformation increases as the cube of the difference in wall thickness, making it easy for out-of-plane deformation to concentrate in thin portions. On the other hand, the hollow member 10 of this embodiment avoids such inconvenience because the wall thickness of the circumferential hardness difference portion 12 is uniform along the circumferential direction.

[0052] The hollow member 10 described above is difficult to distinguish from its external appearance alone, but can be confirmed by the following method. First, the presence or absence of a circumferential hardness difference portion 12 having a hardness difference and a uniform thickness in the circumferential direction is confirmed. Here, the "circumferential direction" refers to the direction along the outer periphery of the cylindrical shape in a cross section perpendicular to the longitudinal direction. Also, the "hardness difference" refers to the distribution of Vickers hardness, which will be explained below. First, the Vickers hardness is measured along the circumferential direction over the entire circumference (360°). Then, as shown in FIG. 3, a graph is created with the measurement position (circumferential angle θ) on the horizontal axis and the Vickers hardness on the vertical axis. This graph may be created by approximating the plotted data with a quadratic curve or a linear line. From the created graph, the circumferential length La (mm) that falls within the low strength range 12A described above is determined. Finally, the ratio of the circumferential length La (mm) to the total circumferential length Lr (mm) is determined, and a hardness difference is defined when this ratio is within the range of 20% to 80%. Note that the "hardness difference" is defined excluding welded portions (e.g., seams in electric resistance welded pipes).

[0053] In addition, in the circumferential direction of the circumferential hardness difference portion 12, the maximum value of Vickers hardness HV max and the minimum Vickers hardness value HV min The difference ΔHV is 25HV It may be 30HV or more, or 30HV or more. If ΔHV is too small, the folding direction may not be determined, and good robustness may not be obtained. On the other hand, the upper limit of ΔHV may be, for example, 120HV. max and HV min The values ​​of are not particularly limited, but may be, for example, 80 HV or more, 150 HV or more, or 200 HV or more, respectively, as long as the above-mentioned ΔHV can be ensured between them.

[0054] Vickers hardness (HV) may be converted to tensile strength (TS) based on JIS Handbook for Steel I. For steels in the range of 100 HV to 400 HV, conversion can also be performed using the approximate formula TS [MPa] ≒ 3.12 × HV + 16. In this case, the maximum value of the tensile strength in the circumferential direction of the circumferential hardness difference portion 12 is TS max The minimum tensile strength is TS min TS max and T.S. min The difference (ΔTS) is, for example, 94MPa On the other hand, ΔTS is, for example, 390 MPa or less. max and T.S. min The values ​​of are not particularly limited, but may be, for example, 270 MPa or more, 490 MPa or more, or 680 MPa or more, respectively, as long as the above-mentioned ΔTS can be ensured between them.

[0055] As mentioned above, the measurement position when showing the hardness distribution is determined based on the minimum Vickers hardness (HV min ) at position P min Instead of defining the angle θ with respect to the reference (0°), the total circumferential length Lr (mm) of the outer periphery of the circumferential hardness difference portion 12 may be used. In this case, the measurement position is set to the position P min It can be defined as x × Lr (0≦x≦1) with x as the reference (0 mm). Specifically, the Vickers hardness is measured all around the circumferential hardness difference portion 12 along the circumferential direction, thereby min The change in Vickers hardness at each measurement position is calculated, starting from the starting point (measurement position 0 mm). For example, Figure 3 shows the change in Vickers hardness at each position of x × Lr (0≦x≦1), and there is one peak in Vickers hardness. In this case, the position of the peak is where the Vickers hardness reaches its maximum value HV max Position P maxFurthermore, there may be multiple Vickers hardness peaks. For example, in FIG. 11 described later, there are two Vickers hardness peaks. When there are multiple peaks, the Vickers hardnesses may be the same as each other as shown in FIG. 11, or may be different from each other. In the latter case, the position of the peak with the highest Vickers hardness is P max This becomes:

[0056] In Figure 3, measurement position P min With reference to the reference value, the Vickers hardness peak is located at 0.5×Lr. From the viewpoint of symmetry, as described above, it is preferable to set the position of the Vickers hardness peak at a position equal to or greater than 0.3×Lr and equal to or less than 0.7×Lr. On the other hand, the position of the Vickers hardness peak may be located at a position smaller than 0.3×Lr or a measurement position larger than 0.7×Lr. For example, in FIG. 9 described later, P min Based on this, the position of the Vickers hardness peak is at 0.75×Lr.

[0057] The cross-sectional shape of the circumferential hardness difference portion 12 (the shape of a cross section perpendicular to the longitudinal direction) is not particularly limited, and examples thereof include circles such as a perfect circle and an ellipse, and polygons such as a rectangle. The polygon referred to here includes not only a strict polygon but also a shape in which the portions corresponding to the corners of a polygon are arc-shaped.

[0058] <plug> Next, the plug 20 used to iron a blank tube when manufacturing the hollow member 10 described above will be described below. The plug 20 has a head portion 20H shown in Figs. 5 and 6 and a shaft portion 20S shown in Fig. 7. Fig. 5 is a diagram showing the head portion 20H of the plug 20, where (a) is a perspective view seen from the front end side and (b) is a perspective view seen from the rear end side. Fig. 6 is a diagram showing the head portion 20H, where (a) is a side view and (b) is a front view. As shown in Fig. 7, the head portion 20H is coaxially fixed to the front end of the shaft portion 20S.

[0059] The head portion 20H shown in Figures 5 and 6 includes a tapered portion (tip portion) 20a having a tip surface 21 and a parallel portion (main body portion) 20b. The parallel portion 20b typically has an outer dimension that is larger than the inner dimension of a mother tube (described later) and smaller than the inner dimension of a first structural portion 40a of a die 40 (described later). In this disclosure, if the mother tube and hollow member 10 have a circular cross section, the dimensions are indicated as "inner diameter dimension" and "outer diameter dimension." However, the mother tube and hollow member 10 of this disclosure are not limited to circular cross sections. Therefore, in this specification, the terms "inner dimension" and "outer dimension" may be used to refer to dimensions that include cross sections other than circular, such as rectangular cross sections.

[0060] As shown in FIG. 6(a), the head portion 20H has a longitudinal direction D along the central axis CL from the tip end surface 21 to the taper start point t in a side view. A The length L at each position in the circumferential direction of the head portion 20H is different from one another. max The taper start point is t1, and the length L is the shortest L min The taper start point is t2. max and L min By providing a difference between the taper start points t1 and t2 in the side view, B But in the longitudinal direction D A It intersects diagonally, not perpendicularly.

[0061] The head portion 20H has a tapered portion 20a that tapers toward the pushing direction, and a parallel portion 20b that is continuous with the rear end of the tapered portion 20a and has a maximum outer dimension (outer diameter) in a cross section perpendicular to the pushing direction. In the side view shown in FIG. 6(a), a connection surface CS including a connection line between the tapered portion 20a and the parallel portion 20b is inclined with respect to an imaginary plane VS that is perpendicular to the central axis CL of the head portion 20H. Furthermore, as shown in FIG. 6(b), taper start points t1 and t2 are located on the connection line, facing each other with the central axis CL therebetween. In the side view shown in Figure 6(a), direction D B and direction D AThe angle (acute angle) therebetween is, for example, 50° or less, and may be 45° or less. In the side view shown in Figure 6(a), the taper angle θ at each taper start point t is the same at each position in the circumferential direction of the head portion 20H.

[0062] By using the head portion 20H having the above-described tapered shape for the plug 20, it is possible to cause the wall to move in the circumferential direction during ironing of the mother tube. Specifically, the wall of the mother tube's inner wall at the taper start point t2 moves from the taper start point t2 toward the taper start point t1 along the circumferential direction of the mother tube's inner wall. As a result, the wall reduction rate is low around the taper start point t2, forming a thin-walled portion with low Vickers hardness. On the other hand, around the taper start point t1, the wall of the mother tube's inner wall gathers from the surrounding area, resulting in a high wall reduction rate and forming a thin-walled portion with high Vickers hardness. Therefore, by ironing a mother tube using the plug 20 having this head portion 20H, it is possible to form a circumferential hardness difference portion 12 with a hardness distribution in the circumferential direction in the manufactured hollow member 10. Meanwhile, the outer dimensions of the parallel portion 20b are constant regardless of the position of the taper start point t. 7, the distance between the outer peripheral surface of the parallel portion 20b and the inner peripheral surface of the die 40 is constant in the circumferential direction about the central axis CL. Therefore, the circumferential hardness difference portion 12 having a uniform thickness in the circumferential direction can be formed in the hollow member 10.

[0063] <Hollow member manufacturing method> Next, a method for manufacturing a hollow member 10 from a hollow cylindrical mother tube 30 using the plug 20 described above will be described with reference to FIGS. 7 and 8. FIG. 7 is a cross-sectional view showing the first half of the method for manufacturing a hollow member using a plug 20 having a head portion 20H, in chronological order from (a) to (c). Here, (a) shows the mother tube arrangement step, (b) shows the locking step, and (c) shows the ironing step. FIG. 8 is a cross-sectional view showing the second half of the method for manufacturing a hollow member, following FIG. 7(c), in chronological order from (a) to (b). Here, (a) shows the start of the drawing step, and (b) shows its completion.

[0064] The mother pipe 30 used in this embodiment preferably has a tensile strength of 290 MPa or more. For example, the mother pipe 30 may have a tensile strength of 440 MPa or 980 MPa. The material of the mother pipe 30 is not limited to steel, and may be other metals such as aluminum. The mother pipe 30 is, for example, a hollow cylindrical metal pipe (including a steel pipe). It is particularly preferable that the mother pipe 30 is a round steel pipe. Examples of round steel pipes include seamless steel pipes, UO pipes, spiral pipes, and electric resistance welded steel pipes. The cross-sectional shape of the mother pipe 30 perpendicular to the longitudinal direction may be circular, elliptical, rectangular, etc.

[0065] In the blank tube placement step shown in FIG. 7(a), first, the blank tube 30 is placed in a die 40, and then a stopper 50 restricts longitudinal movement of the blank tube 30. The die 40 has a first structural portion 40a having an inner dimension (inner diameter) corresponding to the outer dimension of the blank tube 30. In the present disclosure, "inner dimension corresponding to the outer dimension of the blank tube" refers to the outer dimension of the blank tube plus a clearance large enough to allow the blank tube to be inserted or removed. Furthermore, the die 40 has a second structural portion 40b having an inner dimension (inner diameter) larger than the outer dimension of the blank tube 30, for enlarging the outer shape of one end 30x of the blank tube 30.

[0066] Next, in the locking process shown in Figure 7(b), the head portion 20H of the plug 20 is pressed into the mother tube 30 from one end 30x, expanding the outer shape of the mother tube 30 at the one end 30x to form an expanded portion 30a, and the mother tube 30 is locked in the die 40 by this expanded portion 30a. The head portion 20H of the plug 20 has a leading end surface 21 that is smaller than the inner dimensions of the mother tube 30. The expanded portion 30a is formed by pressing the head portion 20H of the plug 20 into the mother tube 30, but this process is a tube expansion process, not an ironing process. Therefore, the expanded portion 30a does not have a circumferential hardness distribution as shown in Figure 3.

[0067] Next, in the ironing process shown in Figure 7(c), the stopper 50 is removed from the die 40, and the restriction on longitudinal movement of the mother tube 30 is released. Furthermore, while maintaining the mother tube 30 in a locked position, the head portion 20H of the plug 20 is pressed from one end 30x toward the other end 30y of the mother tube 30, thereby performing an ironing process to expand the inner shape of the mother tube 30. This reduces the thickness of the mother tube 30, forming a thin-walled portion 30b. The thin-walled portion 30b has a uniform thickness and a uniform hardness distribution in the circumferential direction. Meanwhile, the portion of the mother tube 30 that has not been ironed becomes an unprocessed portion 30c.

[0068] 8(a) is a schematic cross-sectional view of an intermediate body W1 obtained through the steps shown in FIGS. 7(a) to 7(c), taken along a cross section including the central axis CL. This intermediate body W1 has, along its longitudinal direction DL, portions corresponding to the first region 11, the circumferential hardness difference portion 12, and the second region 13. This intermediate body W1 may be used as the hollow member 10.

[0069] Alternatively, as shown in Figures 8(a) and (b), a drawing process may be further performed in which the enlarged portion 30a in the intermediate body W1 is reduced in size by passing it through a die (another die) 60 to return the outer dimensions of the enlarged portion 30a to their original state. The die 60 has an inside dimension (inner diameter) corresponding to the outside dimension (outer diameter d1) of the mother tube 30. This inside dimension is smaller than the outside diameter of the expanded portion 30a, so when the preform W1 is passed through the die 60, it passes through the second region 13 and the circumferential hardness difference portion 12 without getting caught, but is squeezed so that the outside diameter of the expanded portion 30a becomes smaller.

[0070] As a result, as shown in FIG. 8(b), a hollow member 10 can be obtained in which the outer shape is uniform along the entire longitudinal length and the inner shape is partially expanded. Here, the thick-walled portion obtained by shrinking the original expanded portion 30a becomes the first region 11, the thin-walled portion connected to this first region 11 and ironed becomes the circumferential hardness difference portion 12, and the thick-walled portion connected to this circumferential hardness difference portion 12 and having a thickness greater than that of the circumferential hardness difference portion 12 becomes the second region 13. Thus, the circumferential hardness difference portion 12 is thinner than the adjacent first region 11 and second region 13, and therefore has the thinnest wall thickness within the hollow member 10. Furthermore, a tapered portion is formed on the inner wall surface of the first region 11 at the end where it connects to the circumferential hardness difference portion 12. Similarly, a tapered portion is formed on the inner wall surface of the second region 13 at the end where it connects to the circumferential hardness difference portion 12. These tapered portions absorb the difference in the internal dimensions of the inner wall surface of the hollow member 10 .

[0071] The product obtained in FIG. 8(b) is considered to be an intermediate body W1, and this intermediate body W1 may be press-formed to have a cross-sectional shape perpendicular to the longitudinal direction other than a circle. For example, a press-forming step may be further performed to press-form the cross-sectional shape perpendicular to the longitudinal direction of the intermediate body W1 into a rectangle. In this case, a hollow member 10 having a rectangular cross-sectional shape at each position in the longitudinal direction can be manufactured.

[0072] <Modification> The present disclosure is not limited to the above-described embodiment. For example, various modified examples described below may be adopted instead of the above-described embodiment.

[0073] In the above embodiment, the drawing process shown in Fig. 8(b) was performed following the ironing process shown in Fig. 7(c). However, instead of this drawing process, a cutting process may be performed in which the portions corresponding to the first region 11 and the second region 13 are cut and removed from the intermediate body W1 shown in Fig. 8(a), leaving only the circumferential hardness difference portion 12. In this case, a hollow member 10 can be manufactured that has a constant wall thickness and a circumferential hardness difference distribution over its entire length.

[0074] In the above embodiment, the maximum and minimum values ​​of Vickers hardness are positioned opposite each other across the central axis CL, as shown in Fig. 3. Instead of this configuration, as shown in Fig. 9, a configuration may be employed in which the maximum value of Vickers hardness is located at a position shifted from 180° (0.5 × Lr) to, for example, 270° (0.75 × Lr) relative to the measurement position of 0°, where the Vickers hardness is at its minimum.

[0075] In the above embodiment, as shown in Fig. 1, a configuration is adopted in which the circumferential hardness difference portion 12 is disposed between the first region 11 and the second region 13. Instead of this configuration, the hollow member 10 shown in Fig. 10(a) may be manufactured by carrying out the cutting process described above. This hollow member 10 has a circumferential hardness difference distribution over its entire length. Alternatively, in the above-mentioned cutting step, only one of the first region 11 and the second region 13 may be cut. For example, in Fig. 10(b), only the first region 11 is cut and removed, leaving the circumferential hardness difference region 12 and the second region 13. In this hollow member 10, half of its length has a distribution of Vickers hardness, while the remaining half has a uniform Vickers hardness in the circumferential direction.

[0076] The hollow member 10 of the embodiment shown in FIG. 1 will be compared with a hollow member 10 shown in FIGS. 10(a) and 10(b), which is a modified example. As described above, the hollow member 10 shown in Fig. 1 includes, in the longitudinal direction DL, a circumferential hardness difference portion 12, and a first region 11 and a second region 13 located at both ends of the circumferential hardness difference portion 12. By arranging the first region 11 and the second region 13, which have uniform hardness in the circumferential direction, at both ends of the circumferential hardness difference portion 12, which has a circumferential hardness distribution, when a load is applied along the axial direction to the hollow member 10, stress tends to concentrate in the circumferential hardness difference portion 12 over its entire length. As a result, the hollow member 10 can be reliably bent and deformed at the position of the circumferential hardness difference portion 12 in the longitudinal direction, resulting in even greater robustness than the hollow member 10 shown in Figs. 10(a) and 10(b). In addition, in the hollow member 10 shown in FIG. 1, stress tends to concentrate in the low strength area 12A, which is a weak part of the circumferential hardness difference part 12, when a load is input in the axial direction, compared to the hollow member 10 shown in FIG. 10(a), for example. Therefore, the HV max and HV min Even if the difference in hardness between the hollow members 10 is made smaller than that of the hollow member 10 shown in FIGS. 10(a) and 10(b), good robustness can be obtained.

[0077] When the first region 11 and the second region 13 are arranged at both ends of the circumferential hardness difference portion 12, as in the hollow member 10 shown in Fig. 1, the ratio of the length of the circumferential hardness difference portion 12 to the overall length is preferably 5% or more and 50% or less. The length of the circumferential hardness difference portion 12 in the longitudinal direction of the hollow member 10 is preferably 10 mm or more. Adjusting the length of the circumferential hardness difference portion 12 makes it easier to obtain good robustness.

[0078] As described above, the peak position P of the circumferential hardness difference portion 12 in the circumferential direction max For example, in the case of FIG. 11, the Vickers hardness is the maximum value HV max The peak position P max There are two positions P max Similarly, in the case of Figure 11, the Vickers hardness is the minimum value HV min Position P min There are also two positions P min The Vickers hardness at each point is also the same.

[0079] When the circumferential hardness distribution shown in FIG. 11 is viewed from the measurement position 0° of the circumferential hardness difference portion 12 along the circumferential direction, the position of 0° is the minimum Vickers hardness HV min Position P min Next, position P min The Vickers hardness increases as you move in the circumferential direction from the 90° position P max The maximum Vickers hardness is HV max Next, at the 90° position Pmin The Vickers hardness decreases as you move in the circumferential direction from the 180° position P min The minimum Vickers hardness is HV min Next, at the 180° position P min The Vickers hardness increases as you move in the circumferential direction from the 270° position P max The minimum Vickers hardness is HV max And finally, at the 270° position P max The Vickers hardness decreases as you move from the circumferential direction, and the hardness at the 360° position P min , that is, the Vickers hardness is at its minimum value HV when returned to the measurement position 0°. min This becomes: As described above, in the circumferential hardness difference portion 12 in the case of Fig. 11, the Vickers hardness periodically increases and decreases multiple times along the circumferential direction. With such a circumferential distribution, weak portions with low Vickers hardness can be formed in two locations along the circumferential direction, making it possible to intentionally increase the number of folding directions of the hollow member 10 in two predetermined directions. The increase and decrease in Vickers hardness may be curved or linear.

[0080] The circumferential distribution of Vickers hardness shown in FIG. 11 can be imparted by a plug 20 having another head portion 120H shown in FIG. 12 includes a tapered portion (tip portion) 120a having a tip surface 121, and a parallel portion (main body portion) 120b. The parallel portion 120b has an outer dimension that is larger than the inner dimension of the mother tube and smaller than the inner dimension of the first structural portion 40a of the die 40.

[0081] The connection line between the tapered portion 120a and the parallel portion 120b includes a plurality of (two in this modified example) first connection points p1 that are closest to the tip surface 121 of the head portion 120H in a side view, and a plurality of (two in this modified example) second connection points p2 that are located between each of the first connection points p1 in a front view and that are located farther from the tip surface 121 than each of the first connection points p1 in a side view.

[0082] That is, when the connection line is viewed along the circumferential direction of the head portion 120H, it starts from the first of the two first connection points p1 closest to the tip surface 121, moves away from the tip surface 121 as it progresses along the circumferential direction, and reaches the first second connection point p2 that is located farthest from the tip surface 121. Next, it moves closer to the tip surface 121 as it progresses along the circumferential direction from the first second connection point p2, and reaches the second first connection point p1 that is located closest to the tip surface 121. Next, it moves away from the second first connection point p1 as it progresses along the circumferential direction, and reaches the second second connection point p2 that is located farthest from the tip surface 121. Finally, it moves closer to the tip surface 121 as it progresses along the circumferential direction from the second second connection point p2, and returns to the first first connection point p1 that is located closest to the tip surface 121. In this way, the connection line may be employed in the head portion 120H in which the connection line repeatedly approaches and moves away from the tip surface 121 as it advances in the circumferential direction. In this case, the number of first connection points p1 and second connection points p2 is not limited to two each, and may be three or more each.

[0083] When the ironing process shown in FIG. 7(c) is performed using a plug 20 having a head portion 120H shown in FIG. 12, a flow of meat can be formed along the surface of the tapered portion 120a in the direction shown by the arrow in FIG. 12. The reason for this flow of material during ironing will be explained. Because the head portion 120H has the above-described connection lines, the timing at which each first connection point p1 presses the inner wall of the blank tube 30 can be made earlier than the timing at which each second connection point p2 presses the inner wall of the blank tube 30 in the pushing direction of the head portion 120H. That is, among the points on the connection lines, each first connection point p1 located at the leading end of the pushing direction presses the inner wall earlier, while each second connection point p2 located at the trailing end of the pushing direction presses the inner wall later. As a result, the material that is ironed earlier moves along the circumferential direction toward the area that is ironed later. In this way, two high-strength regions 12B, where the material is concentrated and has a high Vickers hardness, and two low-strength regions 12A, where the material has flowed out and has a relatively low Vickers hardness, are formed on the inner wall after ironing. As a result, the circumferential distribution of Vickers hardness shown in FIG. 11 is formed.

[0084] The hollow member 10 of the present disclosure may be used in, but is not limited to, automobile parts, for example. Examples of automobile parts include frame members such as cross members, suspension members, suspension arms, front side members, and rear side members, collision-resistant parts such as perimeter bars and side impact bars, and drivetrain pipe parts such as drive shafts. [Example]

[0085] [First Example] The robustness of deformation under impact conditions was evaluated by performing finite element analysis (FEM) on various hollow members. Specifically, Examples T2, T4, and T5 and Comparative Examples T1 and T3 shown in Figure 13 were evaluated. Each steel pipe shown in Figure 13 is a circular steel pipe having a central axis CL, and all have the same outer diameter. In addition, in Examples T2, 4, and 5, which have a circumferential distribution of Vickers hardness corresponding to the circumferential hardness difference portion 12, the Vickers hardness was set to be minimum at measurement position 0° and maximum at measurement position 180°. The Vickers hardness level is indicated by shading. On the other hand, in Comparative Examples T1 and T3, the circumferential Vickers hardness was set to a constant value.

[0086] In Example T2, the entire longitudinal length is the circumferential hardness difference portion 12. That is, the Vickers hardness increases and decreases in the circumferential direction within a range of 245 to 277 HV. The weak portion (low strength range 12A) where the Vickers hardness is the minimum is located at the bottom of the paper, and the strong portion (high strength range 12B) where the Vickers hardness is the maximum is located at the top of the paper. The thickness is 1.5 mm uniformly throughout the entire length and at all positions around the circumference.

[0087] Similar to the hollow member 10 shown in FIG. 1, Example T4 has a first region 11, a circumferential hardness difference region 12, and a second region 13 aligned along its longitudinal direction. The first region 11 and the second region 13 each have a constant Vickers hardness of 213 HV along the circumferential direction and a constant wall thickness of 3.0 mm along the circumferential direction. Meanwhile, the circumferential hardness difference region 12 has a Vickers hardness that varies between 245 and 277 HV along the circumferential direction and a constant wall thickness of 1.5 mm along the circumferential direction. The circumferential hardness difference region 12 is located at the center in the longitudinal direction, and its length is 10% of the total length of the hollow member 10. In the circumferential hardness difference region 12, a weak region (low strength region 12A) where the Vickers hardness is minimum is located at the bottom of the drawing, and a strong region (high strength region 12B) where the Vickers hardness is maximum is located at the top of the drawing.

[0088] Similar to the hollow member 10 shown in FIG. 1, Example T5 has a first region 11, a circumferential hardness difference region 12, and a second region 13 aligned along its longitudinal direction. The first region 11 and the second region 13 each have a constant Vickers hardness of 277 HV along the circumferential direction and a constant wall thickness of 1.5 mm along the circumferential direction. On the other hand, the circumferential hardness difference region 12 has a Vickers hardness that varies circumferentially within a range of 245 to 277 HV and a constant wall thickness of 1.5 mm along the circumferential direction. The circumferential hardness difference region 12 is located at the center in the longitudinal direction, and its length is 10% of the total length of the hollow member 10. In the circumferential hardness difference region 12, a weak region (low strength region 12A) where the Vickers hardness is minimum is located at the bottom of the drawing, and a strong region (high strength region 12B) where the Vickers hardness is maximum is located at the top of the drawing.

[0089] In the comparative example T1, the entire length in the longitudinal direction is the first region 11. That is, the Vickers hardness is a constant 245 HV along the circumferential direction, and the thickness is also a constant 1.5 mm along the circumferential direction. Comparative Example T3 has three regions aligned along its longitudinal direction. That is, at each end, the Vickers hardness is a constant 213 HV along the circumferential direction, and the wall thickness is also a constant 3.0 mm along the circumferential direction. Meanwhile, the central region has a constant Vickers hardness of 245 HV along the circumferential direction, and the wall thickness is also a constant 1.5 mm along the circumferential direction. This central region is located at the center in the longitudinal direction, and its length is 10% of the total length of the hollow member 10.

[0090] The analysis software used was Abaqus / Explicit, and the analysis conditions were set for Model A and Model B shown in Figure 14(a) and (b). Specifically, Model A had a fixed end with no inclination relative to the support surface (the central axis CL of the hollow member was perpendicular to the support surface). On the other hand, Model B had a fixed end with the central axis CL of the hollow member inclined at 15° relative to the support surface. If the folding deformation occurred so that the bottom of the paper was the concave side and the top of the paper was the convex side, it was evaluated as OK. On the other hand, if axial collapse occurred or if folding occurred in the reaction force direction, it was evaluated as NG. The results are shown in Table 1.

[0091] [Table 1]

[0092] As shown in Table 1, in Examples T2, T4, and T5, which are provided with a circumferential hardness difference portion 12 having a Vickers hardness distribution, it was confirmed that folding occurred in the weak portion (low strength range 12A) in both Model A and Model B. This suggests that the robustness of the folding deformation against the collision conditions is high. In contrast, in Comparative Examples T1 and T3, which are not provided with a circumferential hardness difference portion 12, axial crushing or folding in the reaction force direction occurred, suggesting that the robustness of the folding deformation against the collision conditions is low.

[0093] [Second Example] A hollow member 10 was manufactured by the manufacturing method shown in FIG. 7 using a plug 20 having a head portion 20H shown in FIG. 5 and a blank pipe (steel pipe) 30. The Vickers hardness of the circumferential hardness difference portion 12 of the obtained hollow member 10 was measured at 5 mm intervals in the circumferential direction with an indentation load of 1 kgf. The results of linear function approximation of the Vickers hardness by the least squares method in the range of θ=0° to θ=180° are shown in FIG. 15. As shown in FIG. 15, although there was some variation, it was confirmed that the Vickers hardness monotonically increased from θ=0° to θ=180°. In addition, HV max From HV min The difference after subtracting was about 25HV or more. max From T min The difference after subtracting this was less than 0.1 mm. From the above, it was confirmed by actual measurements that the circumferential hardness difference portion 12 can be formed by the plug 20.

[0094] [Third Example] The circumferential Vickers hardness difference required to cause bending deformation was calculated numerically and compared for a hollow member 10 in which the circumferential hardness difference portion 12 is formed over the entire longitudinal length and a hollow member 10 in which the circumferential hardness difference portion 12 is formed only at a midpoint in the longitudinal direction. That is, first, Example T6 shown in Figure 16(a) was prepared as a model of a hollow member 10 in which a circumferential hardness difference portion 12 was formed over the entire length in the longitudinal direction. In Example T6, the Vickers hardness was set to be lowest at the lower part of the page and highest at the upper part of the page over the entire length. Example T6 has the same configuration as the hollow member 10 shown in Figure 10(a).

[0095] Furthermore, Example T7 shown in Figure 17(a) was prepared as a model of a hollow member 10 in which the circumferential hardness difference portion 12 was formed only at the center position in the longitudinal direction. In Example T7, the circumferential hardness difference portion 12 was set so that the Vickers hardness was lowest in the lower part of the page and highest in the upper part of the page. Example T7 has the same configuration as the hollow member 10 shown in Figure 1.

[0096] In Examples T6 and T7, the plate thickness and dimensions were the same. Meanwhile, the Vickers hardness distribution was calculated by varying the difference (circumferential hardness difference) obtained by subtracting the minimum value from the maximum value of the Vickers hardness. Based on these calculation results, the boundary value at which the bending deformation switches from an unstable state to a stable state was determined when the circumferential hardness difference was gradually increased. The analysis software used was Abaqus / Explicit, as in Example 1.

[0097] First, in Example T6, the bending deformation was unstable when the circumferential hardness difference was 22 HV (Fig. 16(c)), but the bending deformation was stable when the circumferential hardness difference was 26 HV (Fig. 16(b)). Therefore, it was found that Example T6 required a circumferential hardness difference of 26 HV. Next, in Example T7, the bending deformation was unstable when the circumferential hardness difference was 10 HV (Fig. 17(c)), but the bending deformation was stable when the circumferential hardness difference was 13 HV (Fig. 17(b)). Therefore, it was found that Example T7 required a circumferential hardness difference of 13 HV. From the results of Examples T6 and T7, it was confirmed that forming the circumferential hardness difference portion 12 only at the central position in the longitudinal direction results in stable bending deformation with a lower circumferential hardness difference than when it is formed along the entire longitudinal length. [Industrial Applicability]

[0098] According to the present disclosure, it is possible to provide a hollow member having improved robustness without degrading component performance during normal operation, and a method for manufacturing the hollow member, and thus the present disclosure has great industrial applicability. [Explanation of symbols]

[0099] 10 Hollow member 12 Circumferential hardness difference area 12A low intensity range 12B high intensity range 20 plugs 20a Tapered part (tip) 20b Parallel section (main body) 30 Raw pipe 40 dice CL center axis HV av Hardness Threshold Lr Total outer circumference length P min Minimum hardness position P max Maximum hardness position p1 First connection point p2 Second connection point VS Virtual plane (plane perpendicular to the central axis of the plug)

Claims

1. a circumferential hardness difference portion in at least a portion of the longitudinal direction along the central axis; When the circumferential hardness difference portion is viewed in a cross section perpendicular to the central axis, a thickness difference obtained by subtracting the minimum value of the wall thickness from the maximum value of the wall thickness in the circumferential direction of the cross section is 20% or less of the average value of the wall thickness over the entire circumference of the cross section, The cross section includes a low strength range in which the Vickers hardness along the circumferential direction is equal to or less than the hardness threshold, and a high strength range in which the Vickers hardness along the circumferential direction is greater than the hardness threshold, where the average of the integral of the Vickers hardness around the entire circumference of the cross section is the hardness threshold; the difference between the maximum value of the Vickers hardness and the minimum value of the Vickers hardness in the cross section is 25 HV or more; A hollow member characterized by:

2. The ratio of the perimeter of the low strength area to the total perimeter of the cross section is within the range of 20% to 80%.

2. The hollow member according to claim 1.

3. The total perimeter length of the cross section is Lr (mm); a maximum hardness position where the Vickers hardness is maximum is located within a range of 0.3×Lr (mm) to 0.7×Lr (mm) in the circumferential direction, based on a minimum hardness position where the Vickers hardness is minimum; 3. The hollow member according to claim 1 or 2.

4. 3. The hollow member according to claim 1, wherein the difference in thickness in the cross section is 0.10 mm or less.

5. 3. The hollow member according to claim 1, wherein the circumferential hardness difference portion is formed only in a portion of the longitudinal direction.

6. 3. The hollow member according to claim 1, wherein the circumferential hardness difference portion is formed over the entire length in the longitudinal direction.

7. A method for manufacturing a hollow member from a hollow cylindrical mother tube, comprising the steps of: a blank tube placement step of placing the blank tube in a die; an ironing process step of forcing a plug into the mother tube to expand the inner wall of the mother tube while ironing the inner wall so as to feed the wall in a circumferential direction of the inner wall when viewed along the central axis of the mother tube; A method for manufacturing a hollow member, comprising the steps of:

8. the plug has a tip portion tapered in the pushing direction, and a main body portion connected to the rear end of the tip portion and having a maximum outer dimension in a cross section perpendicular to the pushing direction; a plane including a connecting line between the tip portion and the body portion is inclined with respect to a plane perpendicular to a central axis of the plug; 8. The method for manufacturing a hollow member according to claim 7.

9. the plug has a tip portion tapered in the pushing direction, and a main body portion connected to the rear end of the tip portion and having a maximum outer dimension in a cross section perpendicular to the pushing direction; A connecting line between the tip portion and the main body portion, a plurality of first connection points closest to the tip end surface of the plug in a side view; a plurality of second connection points located between the first connection points in a front view and located farther from the tip end surface than the first connection points in a side view; Includes; 8. The method for manufacturing a hollow member according to claim 7.

10. a locking step, which is performed after the mother tube arrangement step and before the ironing step, of pushing the plug into the end of the mother tube to enlarge the outer shape of the end, thereby locking the enlarged portion in the die; a drawing process step in which the expanded portion is passed through another die to reduce the shape after the ironing process step; 10. The method for manufacturing a hollow member according to claim 7, further comprising:

11. 11. The method for manufacturing a hollow member according to claim 10, further comprising, after the drawing step, a press forming step of press-forming the hollow member so that the cross section perpendicular to the central axis has a rectangular shape.

12. The method for manufacturing a hollow member according to any one of claims 7 to 9, further comprising, after the drawing process, a press forming process of press-forming the hollow member so that the cross-sectional shape perpendicular to the central axis is rectangular.

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