Tubular parts

Abstract

The present invention provides a tubular component that can obtain sufficient compressive residual stress on the outer surface of the smallest diameter portion where a hardened layer has not been formed. [Solution] The tubular part (100) comprises a tubular section (10). The tubular section (10) is made of steel and includes a first parallel section (11), a second parallel section (12), and a constricted section (13) positioned between them. The first parallel section (11) has a hardened layer (1). The constricted section (13) has a minimum diameter section (6) where the outer diameter is smallest. No hardened layer is formed in the minimum diameter section (6). Based on d (mm) and h (mm) shown in the selected figure, the hardened layer depth parameter p calculated from formula (1) is 0.70 to 0.90. p = h / (2 × d) (1)

Description

[Technical Field]

[0001] This disclosure relates to tubular components. [Background technology]

[0002] Hollow shafts, which have a hollow interior, are widely used as power transmission shafts in automobiles and other vehicles. Compared to solid shafts, hollow shafts are lighter and can reduce inertial forces. Therefore, hollow shafts are advantageous for energy saving in automobiles and other vehicles.

[0003] Tubular parts such as hollow shafts are typically manufactured by applying a predetermined heat treatment to an intermediate product made from steel. In recent years, with the increasing power output of automobiles and other vehicles, tubular parts are required to have high static strength and high fatigue strength. For this reason, high-frequency induction hardening is sometimes used as a heat treatment when manufacturing tubular parts. In high-frequency induction hardening, the outer surface of the tubular part is heated locally and rapidly cooled, thereby forming a hardened layer with a certain depth starting from the outer surface. The hardened layer has higher hardness and greater residual stress in the compressive direction compared to the unhardened region where the hardened layer has not been formed. Therefore, by applying high-frequency induction hardening to form a hardened layer on a tubular part, the static strength and fatigue strength of the tubular part are improved. Such a technology is proposed, for example, in Japanese Patent Application Publication No. 2009-014203 (Patent Document 1). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2009-014203 [Overview of the project] [Problems that the invention aims to solve]

[0005] Incidentally, tubular parts may include a constricted section where the outer diameter is smaller than the surrounding part. In particular, stress concentrates on the outer surface of the smallest diameter section of the constricted part. Therefore, the smallest diameter section of the constricted part is prone to becoming the starting point for fatigue fracture. Furthermore, when high-frequency induction hardening is applied to the outer surface of a tubular part, the outer surface of the constricted part may not be hardened due to the design of the tubular part or limitations of the high-frequency induction hardening equipment. In such cases, a hardened layer is not formed on the smallest diameter section of the constricted part. The outer surface of the smallest diameter section where a hardened layer is not formed has lower hardness and does not receive sufficient compressive residual stress compared to the outer surface where a hardened layer is formed. Therefore, the occurrence of fatigue fracture starting from the outer surface of the smallest diameter section is further promoted. As a result, the fatigue strength of tubular parts including a constricted section where a hardened layer is not formed on the smallest diameter section is reduced.

[0006] To increase the fatigue strength of a tubular component that includes a constricted portion where a hardened layer is not formed at the minimum diameter, it is effective to increase the compressive residual stress on the outer surface of the minimum diameter portion as much as possible. However, Patent Document 1 does not consider the fatigue strength of a tubular component having a constricted portion where a hardened layer is not formed at the minimum diameter portion, and naturally, it does not consider the compressive residual stress on the outer surface of the minimum diameter portion where a hardened layer is not formed.

[0007] The object of this disclosure is to provide a tubular component that can obtain sufficient compressive residual stress on the outer surface of the smallest diameter portion where a hardened layer is not formed. [Means for solving the problem]

[0008] The tubular component of this disclosure comprises a tubular portion. The tubular portion is made of steel and is cylindrical in shape, having an outer circumferential surface and an inner circumferential surface. The tubular portion further includes a first parallel portion, a second parallel portion, and a constricted portion. The first parallel portion has a constant outer diameter. The first parallel portion further has a hardened layer. The hardened layer includes a portion of the outer circumferential surface of the first parallel portion and is formed in an annular manner along the circumferential direction of the outer circumferential surface. The second parallel portion is arranged coaxially with the first parallel portion and has a constant outer diameter. The constricted portion is arranged coaxially with the first and second parallel portions between the first and second parallel portions and is in contact with the end faces of the first and second parallel portions. The constricted portion further has a minimum diameter portion where the outer diameter is smallest. No hardened layer is formed in the minimum diameter portion. In a longitudinal section including the central axis of the tubular portion, the difference between the outer diameter at the minimum diameter portion and the outer diameter of the first parallel portion is d (mm), and the depth of the hardened layer at a position d (mm) from the outer circumference of the end face of the first parallel portion that contacts the constricted portion toward the axial center of the first parallel portion in the axial direction of the tubular portion is h (mm). The tubular component of this disclosure has a hardened layer depth parameter p represented by formula (1) of 0.70 to 0.90. p = h / (2 × d) (1) [Effects of the Invention]

[0009] The tubular component according to this disclosure can obtain sufficient compressive residual stress on the outer surface of the smallest diameter portion where a hardened layer is not formed. [Brief explanation of the drawing]

[0010] [Figure 1] Figure 1 is a perspective view of the tubular component according to this embodiment. [Figure 2] Figure 2 is a longitudinal cross-sectional view of the tubular component according to this embodiment. [Figure 3] Figure 3 is a schematic diagram of the two-dimensional axially symmetric model of the tubular section used in Analysis Example 1. [Figure 4] Figure 4 shows the heat pattern of the high-frequency induction hardening treatment in the heat treatment simulation of the analysis example. [Figure 5]FIG. 5 is a diagram showing the relationship between the hardened layer depth parameter p and the residual stress on the outer peripheral surface of the minimum diameter portion, obtained based on the results of Analysis Example 1. [Figure 6] FIG. 6 is a schematic diagram of a two-dimensional axisymmetric model of the tubular portion used in Analysis Example 2. [Figure 7] FIG. 7 is a diagram showing the relationship between the hardened layer depth parameter p and the residual stress on the outer peripheral surface of the minimum diameter portion, obtained based on the results of Analysis Example 2. [Figure 8] FIG. 8 is a schematic diagram of a two-dimensional axisymmetric model of the tubular portion used in Analysis Example 3. [Figure 9] FIG. 9 is a diagram showing the relationship between the hardened layer depth parameter p and the residual stress on the outer peripheral surface of the minimum diameter portion, obtained based on the results of Analysis Example 3. [Figure 10] FIG. 10 is a schematic diagram of a two-dimensional axisymmetric model of the tubular portion used in Analysis Example 4. [Figure 11] FIG. 11 is a diagram showing the relationship between the hardened layer depth parameter p and the residual stress on the outer peripheral surface of the minimum diameter portion, obtained based on the results of Analysis Example 4. MODE FOR CARRYING OUT THE INVENTION

[0011] The present inventors studied a means for increasing the residual stress on the outer peripheral surface of the minimum diameter portion in the compressive direction in a tubular component including a constricted portion in which no hardened layer is formed at the minimum diameter portion.

[0012] The hardened layer is mainly composed of a martensite structure. The martensite structure of the hardened layer expands in volume during the transformation process from structures such as ferrite in the unhardened region. As a result, the structure of the hardened layer whose volume is increased by induction hardening and the structure of the unhardened region around the hardened layer whose volume is relatively reduced constrain each other, whereby compressive residual stress is imparted to the hardened layer, and tensile residual stress is imparted to the unhardened region around the hardened layer. However, the present inventors considered that even for a tubular component including a constricted portion in which no hardened layer is formed at the minimum diameter portion, the residual stress imparted to the outer peripheral surface of the minimum diameter portion can be increased in the compressive direction by appropriately adjusting the depth of the hardened layer in the vicinity of the minimum diameter portion in accordance with the outer diameter of the minimum diameter portion.

[0013] Therefore, the present inventors analyzed residual stress by heat treatment simulation using, as models, tubular components having various shapes provided with constricted portions and including hardened layers with various depths in the vicinity of the minimum diameter portion. As a result, it was found that a tubular component having the following configuration includes an unhardened surface having sufficient compressive residual stress.

[0014] The tubular component of the first configuration comprises a tubular section. The tubular section is made of steel and is cylindrical, having an outer circumferential surface and an inner circumferential surface. The tubular section further includes a first parallel section, a second parallel section, and a constricted section. The first parallel section has a constant outer diameter. The first parallel section further has a hardened layer. The hardened layer includes a portion of the outer circumferential surface of the first parallel section and is formed in an annular shape along the circumferential direction of the outer circumferential surface. The second parallel section is arranged coaxially with the first parallel section and has a constant outer diameter. The constricted section is arranged coaxially with the first and second parallel sections between the first and second parallel sections and is in contact with the end faces of the first and second parallel sections. The constricted section further has a minimum diameter section where the outer diameter is smallest. No hardened layer is formed in the minimum diameter section. In a longitudinal section including the central axis of the tubular portion, the difference between the outer diameter at the minimum diameter portion and the outer diameter of the first parallel portion is d (mm), and the depth of the hardened layer at a position d (mm) from the outer circumference of the end face of the first parallel portion that contacts the constricted portion toward the axial center of the first parallel portion in the axial direction of the tubular portion is h (mm). The tubular component of this disclosure has a hardened layer depth parameter p represented by formula (1) of 0.70 to 0.90. p = h / (2 × d) (1)

[0015] The tubular component of the second configuration is the tubular component of the first configuration, wherein the outer diameter of the first parallel section is equal to the outer diameter of the second parallel section.

[0016] The tubular component of the third configuration is the tubular component of the first configuration, wherein the outer diameter of the first parallel section is larger than the outer diameter of the second parallel section.

[0017] The tubular component of the fourth configuration is the tubular component of the first configuration, wherein the outer diameter of the first parallel section is smaller than the outer diameter of the second parallel section.

[0018] Embodiments of this disclosure will be described below with reference to the drawings. In each drawing, the same or equivalent components will be denoted by the same reference numerals, and redundant descriptions will not be repeated.

[0019] Figure 1 is a perspective view of the tubular component 100 according to this embodiment. Figure 2 is a longitudinal cross-sectional view of the tubular portion 10. The longitudinal cross-section of the tubular portion 10 is the cross-section obtained when the tubular portion is cut by a plane containing the central axis X. Since the longitudinal cross-section of the tubular portion 10 is symmetrical with respect to the central axis X, Figure 2 shows only one side of the longitudinal cross-section of the tubular portion 10 that straddles the central axis X. Figure 2 further shows an enlarged view of a portion of the longitudinal cross-section of the tubular portion 10 that straddles the central axis X.

[0020] Referring to Figures 1 and 2, the tubular component 100 comprises a tubular portion 10. The tubular portion 10 is made of steel. The chemical composition of the steel used to make the tubular portion 10 is not specified and may consist of a well-known chemical composition. For example, the chemical composition of the steel used to make the tubular portion 10 may satisfy any one selected from the group consisting of S10C, S12C, S15C, S17C, S20C, S22C, S25C, S28C, S30C, S33C, S35C, S38C, S40C, S43C, S45C, S48C, S50C, S53C, S55C, and S58C as specified in JIS G 4051 (2016). The chemical composition of the steel may also be other than those described above.

[0021] The tubular portion 10 is circular in shape. That is, the cross-section of the tubular portion 10 is circular with respect to the central axis X. The cross-section of the tubular portion 10 is the cross-section obtained when the tubular portion is cut by a plane perpendicular to the central axis X of the tubular portion 10. In this specification, the direction in which the central axis of the tubular portion 10 is simply referred to as the "axial direction," and the direction in which the radial direction of the tubular portion 10 is simply referred to as the "radial direction."

[0022] The tubular portion 10 includes an outer circumferential surface 10A and an inner circumferential surface 10B. The diameter of the outer circumferential surface 10A in a cross-section at any position of the tubular portion 10 is defined as the outer diameter at that position. Furthermore, the radius of the outer circumferential surface 10A in a cross-section at any position of the tubular portion 10 is defined as the outer radius at that position. The diameter of the inner circumferential surface 10B in a cross-section at any position of the tubular portion 10 is defined as the inner diameter at that position. The inner diameter of the tubular portion 10 may be constant, or it may change continuously or stepwise along the central axis. Preferably, the inner diameter of the tubular portion 10 is constant. The radius of the inner circumferential surface 10B in a cross-section at any position of the tubular portion 10 is defined as the inner radius at that position. The shortest distance between the outer circumferential surface 10A and the inner circumferential surface 10B at any position is defined as the wall thickness of the tubular portion 10 at that position.

[0023] Referring to Figures 1 and 2, the tubular portion 10 includes a first parallel portion 11, a second parallel portion 12, and a constricted portion 13. The first parallel portion 11, the second parallel portion 12, and the constricted portion 13 are each a part of the tubular portion 10 and are regions demarcated by the cross-section of the tubular portion 10. That is, the first parallel portion 11, the second parallel portion 12, and the constricted portion 13 are all arranged coaxially with respect to the central axis X. Here, when an arbitrary region that is a part of the tubular portion 10 is demarcated by the cross-section of the tubular portion 10, the cross-section of the tubular portion 10 that demarcates that region is defined as the end face of that region. For example, the cross-section of the tubular portion 10 that demarcates the first parallel portion 11 is defined as the end face of the first parallel portion 11. The constricted portion 13 is positioned between the first parallel portion 11 and the second parallel portion 12, and is in contact with the end faces of the first parallel portion 11 and the second parallel portion. In other words, in the tubular portion 10, the first parallel portion 11, the constricted portion 13, and the second parallel portion 12 are arranged coaxially in the axial direction in this order.

[0024] The first parallel section 11 is cylindrical and has a constant outer diameter. In other words, in the longitudinal cross-section of the tubular section 10, the outer circumferential surface 10A of the first parallel section 11 is parallel to the central axis X. Of the end faces of the first parallel section 11, the end face that is in contact with the constricted section 13 is defined as end face 4.

[0025] The second parallel section 12 is cylindrical and has a constant outer diameter. In other words, in the longitudinal cross-section of the tubular section 10, the outer circumferential surface 10A of the second parallel section 12 is parallel to the central axis X. The outer diameter of the second parallel section 12 may be equal to the outer diameter of the first parallel section 11, or it may be smaller than the outer diameter of the first parallel section 11, or it may be larger than the outer diameter of the first parallel section 11. Of the end faces of the second parallel section 12, the end face that is in contact with the constricted section 13 is defined as the end face 5.

[0026] The constricted portion 13 is cylindrical and has a minimum diameter portion 6 at a position other than the end face where the outer diameter is smallest. The constricted portion 13 has a constricted shape in which the outer diameter decreases from both end faces toward the minimum diameter portion 6. The constricted shape of the constricted portion 13 is not particularly limited as long as the outer diameter decreases from both end faces toward the minimum diameter portion 6. The position of the outer circumferential surface 10A corresponding to the minimum diameter portion 6 is defined as the minimum diameter outer circumferential portion 6A.

[0027] Of the two end faces of the constricted portion 13, the outer diameter of the end face that contacts the end face 4 of the first parallel portion 11 may be equal to the outer diameter of end face 4, or it may be smaller than the outer diameter of end face 4. Of the two end faces of the constricted portion 13, the outer diameter of the end face that contacts the end face 5 of the second parallel portion 12 may be equal to the outer diameter of end face 5, or it may be smaller than the outer diameter of end face 5.

[0028] Referring to Figure 2, the constricted portion 13 may have, for example, a curved portion 16 including the minimum diameter portion 6. The curved portion 16 is a part of the constricted portion 13 and is a region demarcated in the cross-section of the tubular portion 10. Of the two end faces of the curved portion 16, the end face closer to the first parallel portion is defined as end face 7, and the end face closer to the second parallel portion is defined as end face 8. In the longitudinal section of the tubular portion 10, the outer circumferential surface 10A of the curved portion 16 is curved. In the longitudinal section of the tubular portion 10, the outer circumferential surface 10A of the curved portion 16 may consist of a single radius of curvature or multiple radii of curvature.

[0029] The constricted portion 13 having the curved portion 16 may further have, for example, a straight portion 14. The straight portion 14 is part of the constricted portion 13 and is a region demarcated in the cross-section of the tubular portion 10. The straight portion 14 is positioned between the first parallel portion 11 and the curved portion 16 and is in contact with the end face 4 of the first parallel portion 11 and the end face 7 of the curved portion 16. The straight portion 14 has a taper of a constant taper angle from the end face in contact with end face 4 toward the end face in contact with end face 7. In other words, in the longitudinal cross-section of the tubular portion 10, the outer circumferential surface of the straight portion 14 is straight.

[0030] The constricted portion 13 having the curved portion 16 may further have, for example, a straight portion 15. The straight portion 15 is part of the constricted portion 13 and is a region demarcated in the cross-section of the tubular portion 10. The straight portion 15 is positioned between the second parallel portion 12 and the curved portion 16 and is in contact with the end face 5 of the second parallel portion 12 and the end face 8 of the curved portion 16. The straight portion 15 has a taper of a constant taper angle from the end face in contact with end face 5 toward the end face in contact with end face 8. In other words, in the longitudinal cross-section of the tubular portion 10, the outer circumferential surface of the straight portion 15 is straight.

[0031] Of the outer surface 10A of the tubular portion 10, which includes a constricted portion 13 having a curved portion 16, the position corresponding to the outer circumference of end face 4 is defined as the outer circumference 4A, the position corresponding to the outer circumference of end face 5 is defined as the outer circumference 5A, the position corresponding to the outer circumference of end face 7 is defined as the outer circumference 7A, and the position corresponding to the outer circumference of end face 8 is defined as the outer circumference 8A. In the longitudinal cross-section of the tubular portion 10, when the outer circumference 4A and the outer circumference 7A coincide, the curved portion 16 is in contact with the first parallel portion 11. In this case, the constricted portion 13 does not have a straight portion 14. Also, the end face 7 of the curved portion 16 coincides with the end face of the constricted portion 13, and its outer diameter is equal to the outer diameter of the end face 4 of the first parallel portion 11. In the longitudinal cross-section of the tubular portion 10, when the outer circumference 4A and the outer circumference 7A do not coincide, the angle that the straight line passing through the outer circumference 4A and the outer circumference 7A makes with the outer diameter of the first parallel portion 11 is defined as θ1(°). The angle θ1 in the tubular portion 10 is greater than 0 and less than 90°. When θ1 is greater than 0 and less than 90°, and the constricted portion 13 having the curved portion 16 further has a straight portion 14, the taper angle of the straight portion 14 is θ1(°). When θ1 is 90°, the curved portion 16 is in contact with the first parallel portion 11. In other words, the constricted portion 13 does not have a straight portion 14. Also, the end face 7 of the curved portion 16 coincides with the end face of the constricted portion 13, and its outer diameter is smaller than the outer diameter of the end face 4 of the first parallel portion 11.

[0032] In the longitudinal cross-section of the tubular portion 10, if the outer circumference 5A and the outer circumference 8A coincide, the curved portion 16 is in contact with the second parallel portion 12. In this case, the constricted portion 13 does not have a straight portion 15. Also, the end face 8 of the curved portion 16 coincides with the end face of the constricted portion 13, and its outer diameter is equal to the outer diameter of the end face 5 of the second parallel portion 12. In the longitudinal cross-section of the tubular portion 10, if the outer circumference 5A and the outer circumference 8A do not coincide, the angle that the straight line passing through the outer circumference 5A and the outer circumference 8A makes with the outer diameter of the second parallel portion 12 is defined as θ2(°). θ2 in the tubular portion 10 is greater than 0 and less than 90°. When θ2 is greater than 0 and less than 90°, and the constricted portion 13 having the curved portion 16 further has a straight portion 15, the taper angle of the straight portion 15 is θ2(°). When θ2 is 90°, the curved portion 16 is in contact with the second parallel portion 12. In other words, the constricted portion 13 does not have a straight portion 15. Also, the end face 8 of the curved portion 16 coincides with the end face of the constricted portion 13, and its outer diameter is smaller than the outer diameter of the end face 5 of the second parallel portion 12.

[0033] The first parallel section 11 has a hardened layer 1. The hardened layer is a region that is harder than the unhardened region, which is the region of the tubular section 10 other than the hardened layer. The microstructure of the hardened layer mainly consists of martensite. The hardened layer and the unhardened region in the tubular section 10 can be distinguished based on the method described in JIS G 0559 (2019). Specifically, in the tubular section 10, the region with a hardness equal to or greater than the limiting hardness described in JIS G 0559 (2019) is considered the hardened layer, and the region with a hardness less than the limiting hardness is considered the unhardened region. The hardened layer is formed, for example, by applying a well-known high-frequency induction hardening treatment to the outer circumferential surface 10A of the tubular section 10. In this case, the hardened layer is formed starting from any region of the outer circumferential surface 10A that has been subjected to high-frequency induction hardening treatment. Furthermore, a person skilled in the art can adjust the position and depth of the hardened layer in the tubular portion 10 by appropriately changing the area on the outer surface 10A to be subjected to high-frequency induction hardening, as well as the heating temperature and holding time during the high-frequency induction hardening process.

[0034] The hardened layer 1 includes a portion of the outer circumferential surface 10A of the first parallel portion 11 and is formed in an annular shape along the circumferential direction of the outer circumferential surface 10A. The hardened layer 1 may or may not be formed on the constricted portion 13, as long as it is formed on at least the first parallel portion 11. The portion of the outer circumferential surface 10A on which the hardened layer 1 is formed is defined as the hardened surface 1A. The hardened layer 1 is formed from the hardened surface 1A to any position in the radial direction. The radial length of the hardened layer 1 starting from any position on the hardened surface 1A is defined as the depth of the hardened layer 1 at that position. Referring to Figure 2, near the end of the hardened surface 1A that is in contact with the unhardened region, the depth of the hardened layer 1 increases as you move from that end toward the axial center of the hardened surface 1A.

[0035] The second parallel portion 12 may have a separate hardened layer spaced apart from the hardened layer 1, or it may be an unhardened region without a hardened layer. Preferably, the second parallel portion 12 does not have a hardened layer.

[0036] In the narrowest diameter portion 6 of the constricted portion 13, no hardened layer, including the hardened layer 1, is formed. On the other hand, if no hardened layer is formed in the narrowest diameter portion 6, the constricted portion 13 may have a hardened layer or it may be an unhardened region without a hardened layer.

[0037] Referring to Figure 2, let d (mm) be the difference between the outer diameter at the minimum diameter portion 6 and the outer diameter of the first parallel portion 11. In this embodiment, the lower limit of the ratio of d to the wall thickness at the end face 4 of the first parallel portion 11 is, for example, 0.10, for example, 0.20, for example, 0.30. The upper limit of the ratio of d to the wall thickness at the end face 4 of the first parallel portion 11 is, for example, 0.80, for example, 0.70, for example, 0.60. In the longitudinal cross-section of the tubular portion 10, the reference position 1C is defined as the position d (mm) from the outer peripheral portion 4A toward the axial center position of the first parallel portion 11 in the outer peripheral surface 10A. The reference position 1C is included in the hardened surface 1A. Let h (mm) be the depth of the hardened layer 1 at the reference position 1C.

[0038] The hardened layer depth parameter p is defined as shown in equation (1). p = h / (2 × d) (1) In the tubular component 100, the hardened layer depth parameter p is 0.70 to 0.90. As shown in the analysis example described later, when the hardened layer depth parameter p is 0.70 to 0.90, the residual stress in the smallest diameter outer circumference 6A where the hardened layer is not formed increases in the compressive direction.

[0039] [Analysis example 1] Using the tubular component 100 of this embodiment as a model, the residual stress at the smallest diameter outer circumference 6A when hardened layers 1 of various shapes are formed by high-frequency induction hardening was analyzed by heat treatment simulation using the finite element method.

[0040] In Analysis Example 1, the two-dimensional axially symmetric model shown in Figure 3 was used as the model for the tubular section 10 provided in the tubular component 100. In the model shown in Figure 3, the inner radius rI of the tubular section 10 was constant at 7.5 mm. In this model, the outer radius r1 of the first parallel section 11 was 17.5 mm, and the outer radius r2 of the second parallel section 12 was 15.0 mm. The difference d between the outer diameter at the minimum diameter section 6 and the outer diameter of the first parallel section 11 was 4.5 mm. In addition, the constricted section 13 had a curved section 16. In the longitudinal section of the tubular section 10, the radius of curvature of the outer circumferential surface 10A of the curved section 16 was constant at 1.0 mm. In this model, the outer circumferential sections 4A and 7A did not coincide, and the outer circumferential sections 5A and 8A did not coincide. The angle θ1 between the straight line passing through the outer circumference 4A and the outer circumference 7A and the outer diameter of the first parallel section 11 was 60°, and the constricted section 13 had a straight section 14. The angle θ2 between the straight line passing through the outer circumference 5A and the outer circumference 8A and the outer diameter of the second parallel section 12 was 45°, and the constricted section 13 had a straight section 15. The tubular section 10 of this model had a heat input region 1F on a part of the outer circumference 10A that included the end face on the side of the first parallel section 11 that was not the end face 4.

[0041] The analysis software used was Abaqus (product name) manufactured by Dassault Systèmes K.K. The chemical composition of the steel used as the material for the tubular section 10 in the model corresponds to S38C as specified in JIS G 4051 (2016). In the analysis, experimental values ​​for steel corresponding to S38C were used for the mechanical properties, and calculated values ​​derived from a chemical composition consisting of C:0.40%, Si:0.20%, Mn:0.75%, P:0.015%, S:0.015%, Cr:0.10%, and the remainder being Fe were used for the thermal properties.

[0042] The analysis simulated a heat treatment that mimicked high-frequency induction hardening on a portion of the outer surface 10A. The heat pattern of the heat treatment was as shown in Figure 4. Specifically, a constant heat input Q (W / mm²) was applied to the heat input region 1F of the outer surface 10A. 2 The heat input region 1F was heated from room temperature (20°C) to T1°C by applying a heat source () for t1 seconds. Then, the heat input region 1F was air-cooled in room temperature air for 0.5 seconds and water-cooled with 20°C cooling water. The heat transfer coefficient of the air during air cooling was 10 W / (m²). 2 The heat transfer coefficient of the cooling water during water cooling was set to 10,000 W / (m²). 2 The heat treatment was performed as follows: A hardened layer 1 was formed starting from the heat input region 1F.

[0043] In Analysis Example 1, the heat input Q to the heat input region 1F was set to 3.2 to 16.0 W / mm² so that the heating temperature T1 of the heat input region 1F was 1000 to 1200°C. 2 The heat input time t1 was varied from 0.5 to 11.0 seconds. As a result, the depth h of the hardened layer 1 at the reference position 1C varied from 0.60 to 8.83 mm. In all heat treatment conditions, the hardened layer 1 was also formed in the constricted portion 13. On the other hand, in all heat treatment conditions, the hardened layer 1 was not formed in the minimum diameter outer circumference 6A. Based on equation (1), the hardened layer depth parameter p for each heat treatment condition was calculated. Furthermore, from the analysis results, the circumferential and axial residual stress σ (MPa) in the minimum diameter outer circumference 6A was calculated. Whether or not the minimum diameter outer circumference 6A has sufficient compressive residual stress was determined by the maximum value of the residual stress σ in the analyzed range. max (MPa) is used, and the minimum value of residual stress σ within the analyzed range is σmin When the unit is MPa, evaluation was performed using the dimensionless parameter pσ defined by formula (A). pσ=(σ−σ min ) / (σ max −σ min ) (A)

[0044] Residual stress is represented by a positive value in the tensile direction and a negative value in the compressive direction. In other words, the smaller the value of residual stress (the larger it is in the negative direction), the greater the residual stress in the compressive direction. Referring to formula (A), the closer pσ is to 0, the greater compressive residual stress is imparted to the outer peripheral portion 6A with the minimum diameter. When pσ is 0.20 or less in both the circumferential direction and the axial direction, it is determined that the outer peripheral portion 6A with the minimum diameter has sufficient compressive residual stress.

[0045] Based on the analysis results, a graph was prepared with the hardened layer depth parameter p as the horizontal axis and pσ as the vertical axis. The obtained graph is shown in Figure 5. Referring to Figure 5, when the hardened layer depth parameter p is in the range of 0.70 to 0.90, pσ was always 0.20 or less. Accordingly, sufficient compressive residual stress was obtained in the outer peripheral portion 6A with the minimum diameter.

[0046] [Analysis Example 2] In analysis example 2, the two-dimensional axially symmetric model shown in Figure 6 was used as the model for the tubular portion 10 provided in the tubular component 100. In the model shown in Figure 6, the inner radius rI of the tubular portion 10 was constant at 7.5 mm. In this model, the outer radius r1 of the first parallel portion 11 was 15.0 mm, and the outer radius r2 of the second parallel portion 12 was 17.5 mm. The difference d between the outer diameter at the minimum diameter portion 6 and the outer diameter of the first parallel portion 11 was 2.0 mm. In addition, the constricted portion 13 had a curved portion 16. In the longitudinal section of the tubular portion 10, the radius of curvature of the outer circumferential surface 10A of the curved portion 16 was constant at 1.0 mm. In this model, the outer circumferential portion 4A and the outer circumferential portion 7A did not coincide, and the outer circumferential portion 5A and the outer circumferential portion 8A did not coincide. The angle θ1 between the straight line passing through the outer circumference 4A and the outer circumference 7A and the outer diameter of the first parallel section 11 is 45°, and the constricted section 13 had a straight section 14. The angle θ2 between the straight line passing through the outer circumference 5A and the outer circumference 8A and the outer diameter of the second parallel section 12 is 60°, and the constricted section 13 had a straight section 15. The tubular section 10 of this model had a heat input region 1F on a part of the outer circumference 10A that included the end face on the side of the first parallel section 11 that is not the end face 4. In analysis example 2, the heat input Q to the heat input region 1F was set to 5.4 to 16.0 W / mm² so that the heating temperature T1 of the heat input region 1F was 1000 to 1200°C. 2 The heat input time t1 was varied within the range of 0.5 to 6.0 seconds. As a result, the depth h of the hardened layer 1 at the reference position 1C varied within the range of 0.64 to 4.43 mm. The other analysis conditions were the same as in Analysis Example 1. In addition, under heat treatment conditions where the heat input time t1 was somewhat long, the hardened layer 1 was also formed in the constricted portion 13. On the other hand, under all heat treatment conditions, the hardened layer 1 was not formed in the outer circumference 6A with the smallest diameter.

[0047] Based on the analysis results, a graph was created with the hardened layer depth parameter p on the horizontal axis and pσ on the vertical axis. The obtained graph is shown in Figure 7. Referring to Figure 7, when the hardened layer depth parameter p was in the range of 0.70 to 0.90, pσ was always 0.20 or less. Therefore, sufficient compressive residual stress was obtained in the smallest diameter outer circumference 6A.

[0048] [Analysis example 3] In analysis example 3, the two-dimensional axially symmetric model shown in Figure 8 was used as the model for the tubular portion 10 provided in the tubular component 100. In the model shown in Figure 8, the inner radius rI of the tubular portion 10 was constant at 5.0 mm. In this model, the outer radius r1 of the first parallel portion 11 was 15.0 mm, and the outer radius r2 of the second parallel portion 12 was 20.0 mm. The difference d between the outer diameter at the minimum diameter portion 6 and the outer diameter of the first parallel portion 11 was 1.0 mm. In addition, the constricted portion 13 had a curved portion 16. In the longitudinal section of the tubular portion 10, the radius of curvature of the outer circumferential surface 10A of the curved portion 16 was constant at 2.0 mm. In this model, the outer circumferential portion 4A and the outer circumferential portion 7A did not coincide, and the outer circumferential portion 5A and the outer circumferential portion 8A did not coincide. The angle θ1 between the straight line passing through the outer circumference 4A and the outer circumference 7A and the outer diameter of the first parallel section 11 was 30°, and the constricted section 13 had a straight section 14. The angle θ2 between the straight line passing through the outer circumference 5A and the outer circumference 8A and the outer diameter of the second parallel section 12 was 90°, and the constricted section 13 did not have a straight section 15. The tubular section 10 of this model had a heat input region 1F on a part of the outer circumference 10A that included the end face on the side of the first parallel section 11 that was not the end face 4. In analysis example 3, the heat input Q to the heat input region 1F was set to 5.1 to 16.0 W / mm² so that the heating temperature T1 of the heat input region 1F was 1000 to 1200°C. 2 The heat input time t1 was varied within the range of 0.5 to 6.0 seconds. As a result, the depth h of the hardened layer 1 at the reference position 1C varied within the range of 0.42 to 2.10 mm. All other analysis conditions were the same as in Analysis Example 1. In all heat treatment conditions, the hardened layer 1 was not formed in the constricted portion 13. Also, in all heat treatment conditions, the hardened layer 1 was not formed in the outer circumference 6A with the smallest diameter.

[0049] Based on the analysis results, a graph was created with the hardened layer depth parameter p on the horizontal axis and pσ on the vertical axis. The obtained graph is shown in Figure 9. Referring to Figure 9, when the hardened layer depth parameter p was in the range of 0.70 to 0.90, pσ was always 0.20 or less. Therefore, sufficient compressive residual stress was obtained in the smallest diameter outer circumference 6A.

[0050] [Analysis example 4] In analysis example 4, the two-dimensional axially symmetric model shown in Figure 10 was used as the model for the tubular section 10 provided in the tubular component 100. In the model shown in Figure 10, the inner radius rI was constant at 5.0 mm. In this model, the outer radius r1 of the first parallel section 11 was 20.0 mm, and the outer radius r2 of the second parallel section 12 was 15.0 mm. The difference d between the outer diameter at the minimum diameter section 6 and the outer diameter of the first parallel section 11 was 6.0 mm. In addition, the constricted section 13 had a curved section 16. In the longitudinal section of the tubular section 10, the radius of curvature of the outer circumferential surface 10A of the curved section 16 was constant at 2.0 mm. In this model, the outer circumferential sections 4A and 7A did not coincide, and the outer circumferential sections 5A and 8A did not coincide. The angle θ1 between the straight line passing through the outer circumference 4A and the outer circumference 7A and the outer diameter of the first parallel section 11 was 90°, and the constricted section 13 did not have a straight section 14. The angle θ2 between the straight line passing through the outer circumference 5A and the outer circumference 8A and the outer diameter of the second parallel section 12 was 30°, and the constricted section 13 had a straight section 15. The tubular section 10 of this model had a heat input region 1F on a part of the outer circumference 10A that included the end face on the side of the first parallel section 11 that was not the end face 4. In analysis example 4, the heat input Q to the heat input region 1F was set to 2.8 to 16.0 W / mm² so that the heating temperature T1 of the heat input region 1F was 1000 to 1200°C. 2 The heat input time t1 was varied within the range of 0.5 to 20.0 seconds. As a result, the depth h of the hardened layer 1 at the reference position 1C varied within the range of 0.55 to 11.91 mm. All other analysis conditions were the same as in Analysis Example 1. In all heat treatment conditions, the hardened layer 1 was also formed in the constricted portion 13. On the other hand, in all heat treatment conditions, the hardened layer 1 was not formed in the outer circumference 6A with the smallest diameter.

[0051] Based on the analysis results, a graph was created with the hardened layer depth parameter p on the horizontal axis and pσ on the vertical axis. The obtained graph is shown in Figure 11. Referring to Figure 11, when the hardened layer depth parameter p was in the range of 0.70 to 0.90, pσ was always 0.20 or less. Therefore, sufficient compressive residual stress was obtained in the smallest diameter outer circumference 6A.

[0052] As described above, the tubular component 100 according to this embodiment has a hardened layer depth parameter p represented by formula (1) of 0.70 to 0.90, so sufficient compressive residual stress can be obtained on the outer surface of the smallest diameter outer portion 6A, i.e., the smallest diameter portion where the hardened layer is not formed.

[0053] Embodiments of the present disclosure have been described above. However, the embodiments described above are merely examples for carrying out the present disclosure. Therefore, the present disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit thereof. For example, the tubular component of the present disclosure may include, in addition to the tubular portion, one or more selected from the group consisting of a joint portion, a threaded portion, a hole portion, a keyway portion, a flange portion, a gear portion, and a spline portion. [Explanation of Symbols]

[0054] 100: Tubular parts 10: Tubular part 10A: Outer surface 10B: Inner surface 11: First parallel section 12: Second parallel section 13: Waist area 4: End face 5: End face 6:Minimum diameter part 1: Hardened layer X: Central axis

Claims

1. It is made of steel and has a cylindrical tubular section having an outer surface and an inner surface, The tubular portion is, A first parallel section having a constant outer diameter and a hardened layer, A second parallel section is arranged coaxially with the first parallel section and has a constant outer diameter, Between the first parallel portion and the second parallel portion is a constricted portion that is arranged coaxially with the first parallel portion and the second parallel portion, contacts the end face of the first parallel portion and the end face of the second parallel portion, and has a minimum diameter portion where the outer diameter is smallest, The aforementioned hardened layer is The first parallel portion includes a part of the outer circumferential surface and is formed in an annular shape along the circumferential direction of the outer circumferential surface, In the minimum diameter portion, no hardened layer is formed. In a longitudinal section including the central axis of the tubular portion, Let d (mm) be the difference between the outer diameter at the minimum diameter portion and the outer diameter of the first parallel portion. Of the outer circumferential surface, the depth of the hardened layer at a position d (mm) from the outer circumference of the end face of the first parallel portion that is in contact with the constricted portion toward the axial center of the first parallel portion in the axial direction of the tubular portion is defined as h (mm). The hardened layer depth parameter p, represented by equation (1), is 0.70 to 0.

90. Tubular component. p=h / (2×d) (1)

2. A tubular component according to claim 1, The outer diameter of the first parallel section is equal to the outer diameter of the second parallel section. Tubular component.

3. A tubular component according to claim 1, The outer diameter of the first parallel portion is larger than the outer diameter of the second parallel portion. Tubular component.

4. A tubular component according to claim 1, The outer diameter of the first parallel portion is smaller than the outer diameter of the second parallel portion. Tubular component.

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