Tubular parts

Abstract

The present invention provides a tubular component that includes an unhardened surface having sufficient compressive residual stress. [Solution] The tubular part (100) comprises a tubular section (10). The tubular section (10) is made of steel and includes a first hardened layer (1), a second hardened layer (2), and an unhardened surface (3A). The first hardened layer (1) and the second hardened layer (2) are formed in an annular shape along the circumferential direction, and the unhardened surface (3A) is positioned between them. In the longitudinal section of the tubular section (10), the length along the outer surface of the unhardened surface (3A) is d (mm), the minimum distance from the unhardened surface (3A) to the inner surface is t (mm), the depth of the first hardened layer (1) at a position t / 2 (mm) from the end along the outer surface is h1 (mm), the depth of the second hardened layer (2) at a position t / 2 (mm) from the end along the outer surface is h2 (mm), and the hardened layer depth parameter p, expressed by formula (1), is 0.50 to 0.70. p = (h1 + h2) / 2d (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, when high-frequency induction hardening is applied to tubular parts, due to the design of the tubular part or limitations of the high-frequency induction hardening equipment, a portion of the outer surface of the tubular part may not be subjected to high-frequency induction hardening. A hardened layer will not be formed on the portion of the outer surface that is not subjected to high-frequency induction hardening. In this specification, the region on the outer surface of a tubular part in which a hardened layer has not been formed is referred to as the "unhardened surface." The unhardened surface has lower hardness and does not have sufficient compressive residual stress compared to the outer surface in which a hardened layer has been formed. Therefore, the unhardened surface becomes the starting point for fatigue fracture in the tubular part.

[0006] To increase the fatigue strength of tubular components with an uncured surface, it is effective to increase the compressive residual stress of the uncured surface as much as possible. However, Patent Document 1 does not consider the fatigue strength of tubular components with an uncured surface at all, and naturally, it does not consider the compressive residual stress of the uncured surface.

[0007] The object of this disclosure is to provide a tubular component that includes an uncured surface having sufficient compressive residual stress. [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 circular in shape, having an outer circumferential surface and an inner circumferential surface. The tubular portion further includes a first hardened layer, a second hardened layer, and an unhardened surface. The first hardened layer includes a portion of the outer circumferential surface and is formed in an annular shape along the circumferential direction of the outer circumferential surface. The portion of the outer circumferential surface on which the first hardened layer is formed is defined as the first hardened surface. The second hardened layer includes a portion of the outer circumferential surface and is formed in an annular shape along the circumferential direction of the outer circumferential surface. The portion of the outer circumferential surface on which the second hardened layer is formed is defined as the second hardened surface. The unhardened surface is a region of the outer circumferential surface located between the first hardened surface and the second hardened surface, and in contact with the first hardened surface and the second hardened surface. In a longitudinal section including the central axis of the tubular part, the length along the outer circumferential surface of the uncured surface is defined as d (mm), and the minimum distance from the uncured surface to the inner circumferential surface is defined as t (mm). The first reference position of the first cured surface is defined as t / 2 (mm) from the end in contact with the uncured surface along the outer circumferential surface in the opposite direction to the uncured surface, and the depth of the first cured layer at the first reference position in the direction perpendicular to the outer circumferential surface is defined as h1 (mm). The second reference position of the second cured surface is defined as t / 2 (mm) from the end in contact with the uncured surface along the outer circumferential surface in the opposite direction to the uncured surface, and the depth of the second cured layer at the second reference position in the direction perpendicular to the outer circumferential surface is defined as h2 (mm). The tubular part of this disclosure has a cured layer depth parameter p represented by formula (1) of 0.50 to 0.70. p = (h1 + h2) / 2d (1) [Effects of the Invention]

[0009] The tubular component according to this disclosure includes an uncured surface having sufficient compressive residual stress. [Brief explanation of the drawing]

[0010] [Figure 1] Figure 1 is a perspective view of a tubular component according to the first embodiment. [Figure 2] Figure 2 is a longitudinal cross-sectional view of a tubular component according to the first 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 and Analysis Example 2. [Figure 4]FIG. 4 is a diagram showing a heat pattern of induction hardening in a heat treatment simulation of an 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 unhardened surface, obtained based on the results of Analysis Example 1. [Figure 6] FIG. 6 is a diagram showing the relationship between the hardened layer depth parameter p and the residual stress on the unhardened surface, obtained based on the results of Analysis Example 2. [Figure 7] FIG. 7 is a schematic diagram of a two-dimensional axisymmetric model of a tubular portion used in Analysis Example 3. [Figure 8] FIG. 8 is a diagram showing the relationship between the hardened layer depth parameter p and the residual stress on the unhardened surface, obtained based on the results of Analysis Example 3. [Figure 9] FIG. 9 is a longitudinal cross-sectional view of a tubular component according to a second embodiment. [Figure 10] FIG. 10 is a schematic diagram of a two-dimensional axisymmetric model of a 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 unhardened surface, obtained based on the results of Analysis Example 4. DESCRIPTION OF EMBODIMENTS

[0011] The inventors of the present invention studied a means for increasing the residual stress of an unhardened surface in the compression direction in a tubular component including the unhardened surface.

[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 decreased, constrain each other. 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 an unhardened surface, the residual stress imparted to the unhardened surface can be increased in the compressive direction by appropriately adjusting the size of the unhardened surface and the depth of the hardened layer in the vicinity of the unhardened surface.

[0013] Accordingly, the present inventors analyzed residual stress by heat treatment simulation using, as a model, a tubular component including unhardened surfaces of various sizes and hardened layers having various depths. 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 portion. The tubular portion is made of steel and is circular in shape, having an outer circumferential surface and an inner circumferential surface. The tubular portion further includes a first hardened layer, a second hardened layer, and an unhardened surface. The first hardened layer includes a portion of the outer circumferential surface and is formed in an annular shape along the circumferential direction of the outer circumferential surface. The portion of the outer circumferential surface on which the first hardened layer is formed is defined as the first hardened surface. The second hardened layer includes a portion of the outer circumferential surface and is formed in an annular shape along the circumferential direction of the outer circumferential surface. The portion of the outer circumferential surface on which the second hardened layer is formed is defined as the second hardened surface. The unhardened surface is a region of the outer circumferential surface located between the first hardened surface and the second hardened surface, and in contact with the first hardened surface and the second hardened surface. In a longitudinal section including the central axis of the tubular part, the length along the outer surface of the uncured surface is defined as d (mm), and the minimum distance from the uncured surface to the inner surface is defined as t (mm). The first reference position of the first cured surface is defined as t / 2 (mm) from the end in contact with the uncured surface along the outer surface in the opposite direction to the uncured surface, and the depth of the first cured layer perpendicular to the outer surface at the first reference position is defined as h1 (mm). The second reference position of the second cured surface is defined as t / 2 (mm) from the end in contact with the uncured surface along the outer surface in the opposite direction to the uncured surface, and the depth of the second cured layer perpendicular to the outer surface at the second reference position is defined as h2 (mm). The tubular part of the first configuration has a cured layer depth parameter p represented by equation (1) of 0.50 to 0.70. p = (h1 + h2) / 2d (1)

[0015] The tubular component of the second configuration is the tubular component of the first configuration, wherein the region of the outer surface from the first reference position to the second reference position has a constant outer diameter.

[0016] The third tubular component is the same as the first tubular component, and the tubular portion includes a thin-walled portion, a thick-walled portion, and a tapered portion. The thick-walled portion is arranged coaxially with the thin-walled portion and has a greater wall thickness than the thin-walled portion. The tapered portion is arranged coaxially with the thin-walled portion and the thick-walled portion between them, is in contact with the thin-walled portion and the thick-walled portion, and has a tapered shape. The thin-walled portion and the tapered portion have uncured surfaces.

[0017] The tubular component of the fourth configuration is the tubular component of the third configuration, wherein the thick-walled portion has an uncured surface.

[0018] The tubular component of the fifth configuration is a tubular component of any one of the first to fourth configurations, where h1 (mm) and h2 (mm) are equal.

[0019] 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.

[0020] <First Embodiment> Figure 1 is a perspective view of a tubular component 100 according to the first embodiment. Referring to Figure 1, the tubular component 100 comprises a tubular portion 10. The tubular portion 10 is made of steel. The chemical composition of the steel used as the material for the tubular portion 10 is not particularly specified and may consist of a well-known chemical composition. For example, the chemical composition of the steel used as the material for 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 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. The tubular portion 10 includes an outer circumferential surface 10A and an inner circumferential surface 10B. The diameter of the outer circumferential surface 10A at any position in the cross-section of the tubular portion 10 is defined as the outer diameter at that position. Furthermore, the radius of the outer circumferential surface 10A at any position in the cross-section of the tubular portion 10 is defined as the outer radius at that position. The outer diameter of the tubular portion 10 may be constant, or it may change continuously or stepwise along the central axis. The diameter of the inner circumferential surface 10B at any position in the cross-section 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 shortest distance between any position on the outer circumferential surface 10A and the inner circumferential surface 10B is defined as the wall thickness of the tubular portion 10 at that position.

[0022] Referring to Figure 1, the tubular portion 10 includes a first hardened layer 1, a second hardened layer 2, and an unhardened surface 3A.

[0023] The hardened layer is a region of the tubular portion 10 that is harder than the unhardened region, which is the region other than the hardened layer. The microstructure of the hardened layer mainly consists of martensite. The hardened layer and the unhardened region of the tubular portion 10 can be distinguished based on the method described in JIS G 0559 (2019). Specifically, in the tubular portion 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 portion 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. 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 region on the outer circumferential surface 10A to which high-frequency induction hardening treatment is applied, as well as the heating temperature and holding time during the high-frequency induction hardening treatment.

[0024] 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, straddling the central axis X. Furthermore, Figure 2 shows a magnified view of a portion of the longitudinal cross-section of the tubular portion 10, straddling the central axis X.

[0025] Referring to Figure 2, the direction perpendicular to the outer surface 10A at any position on the outer surface 10A and toward the interior of the tubular portion 10 is defined as the depth direction D of the tubular portion 10 at that position. The direction tangential to the outer surface 10A at any position on the outer surface 10A and coplanetized with the central axis X is defined as the longitudinal direction L of the outer surface. In other words, the longitudinal direction L of the outer surface is the direction along the outer surface 10A in the longitudinal cross-section of the tubular portion 10.

[0026] The first hardened layer 1 includes a portion of the outer circumferential surface 10A and is formed in an annular shape along the circumferential direction of the outer circumferential surface 10A. The portion of the outer circumferential surface 10A on which the first hardened layer 1 is formed is defined as the first hardened surface 1A. The first hardened layer 1 is formed from the first hardened surface 1A to an arbitrary position in the depth direction D. The maximum length in the depth direction D of the first hardened layer 1 at any position on the first hardened surface 1A is defined as the depth of the first hardened layer 1 at that position.

[0027] The second hardened layer 2 is formed coaxially with the first hardened layer 1. The second hardened layer 2 includes a portion of the outer peripheral surface 10A and is formed in an annular shape along the circumferential direction of the outer peripheral surface 10A. Of the outer peripheral surface 10A, the outer peripheral surface on which the second hardened layer 2 is formed is defined as the second hardened surface 2A. The second hardened layer 2 is formed from the second hardened surface 2A to an arbitrary position in the depth direction D. The maximum length in the depth direction D of the second hardened layer 2 at any position on the second hardened surface 2A is defined as the depth of the second hardened layer 2 at that position.

[0028] The uncured surface 3A is a part of the outer peripheral surface 10A and consists of an uncured region where a hardened layer has not been formed. In the tubular portion 10, no hardened layer is formed in the region from the uncured surface 3A to the inner peripheral surface 10B in the depth direction D. The uncured surface 3A is located on the outer peripheral surface 10A between the first hardened surface 1A and the second hardened surface 2A, and is in contact with the first hardened surface 1A and the second hardened surface 2A. The first hardened surface 1A is in contact with the uncured surface 3A at the end 1E of the first hardened surface. The second hardened surface 2A is in contact with the uncured surface 3A at the end 2E of the second hardened surface. The first hardened surface 1A, the uncured surface 3A, and the second hardened surface 2A are arranged in this order along the longitudinal direction L of the outer peripheral surface.

[0029] Referring to Figure 2, in a longitudinal section containing the central axis X of the tubular portion 10, the length along the outer surface of the uncured surface 3A is defined as d (mm). In a longitudinal section containing the central axis X of the tubular portion 10, the minimum distance from the uncured surface 3A to the inner surface 10B is defined as t (mm). In other words, the minimum wall thickness at any position on the uncured surface 3A is defined as t (mm).

[0030] In a longitudinal section of the tubular portion 10 including the central axis X, the first reference position 1C is defined as the position t / 2 (mm) away from the end 1E of the first hardened surface 1A along the outer circumferential surface 10A in the opposite direction to the unhardened surface 3A. The depth of the first hardened layer 1 from the end 1E of the first hardened surface to the first reference position 1C increases as it moves from the end 1E of the first hardened surface towards the first reference position 1C along the outer circumferential surface 10A. Alternatively, the depth of the first hardened layer 1 from the end 1E of the first hardened surface to the first reference position 1C increases as it moves from the end 1E of the first hardened surface towards the first reference position 1C along the outer circumferential surface 10A, and becomes constant once the distance from the end 1E of the first hardened surface exceeds a certain point. The depth of the first hardened layer 1 at the first reference position 1C is defined as h1 (mm). In a longitudinal section of the tubular portion 10 including the central axis X, the second reference position 2C is defined as the position t / 2 (mm) away from the end 2E of the second hardened surface 2A along the outer circumferential surface 10A in the opposite direction to the unhardened surface 3A. The depth of the second hardened layer 2 from the end 2E of the second hardened surface to the second reference position 2C increases as it moves from the end 2E of the second hardened surface towards the second reference position 2C along the outer circumferential surface 10A. Alternatively, the depth of the second hardened layer 2 from the end 2E of the second hardened surface to the second reference position 2C increases as it moves from the end 2E of the second hardened surface towards the second reference position 2C along the outer circumferential surface 10A, and becomes constant once the distance from the end 2E of the second hardened surface exceeds a certain point. The depth of the second hardened layer 2 at the second reference position 2C is defined as h2 (mm). Note that h1 and h2 may or may not be equal.

[0031] In the tubular component 100 according to the first embodiment, the region of the outer circumferential surface 10A from the first reference position 1C to the second reference position 2C has a constant outer diameter. In other words, the region from the first reference position 1C to the end of the first hardened surface 1E, the unhardened surface 3A, and the region from the second reference position 2C to the end of the second hardened surface 2E have a constant and identical outer diameter. In this case, in the region from the first reference position 1C to the second reference position 2C, the longitudinal direction L of the outer circumferential surface is parallel to the central axis X.

[0032] The hardened layer depth parameter p is defined as shown in equation (1). p = (h1 + h2) / 2d (1) In the tubular component 100, the hardened layer depth parameter p is 0.50 to 0.70. As shown in the analysis example described later, when the hardened layer depth parameter p is 0.50 to 0.70, the residual stress of the unhardened surface 3A increases in the compressive direction. In other words, the tubular component 100 with a hardened layer depth parameter p of 0.50 to 0.70 contains an unhardened surface 3A with sufficient compressive residual stress.

[0033] [Analysis example 1] Using the tubular component 100 of the first embodiment as a model, the residual stress on the unhardened surface 3A when a first hardened layer 1 and a second hardened layer 2 of various shapes are formed by high-frequency induction hardening was analyzed by heat treatment simulation using the finite element method.

[0034] In Analysis Example 1, the two-dimensional axially symmetric model shown in Figure 3 was used as the model for the tubular portion 10 provided on the tubular component 100. The model shown in Figure 3 was a 1 / 2 model in which the plane Y perpendicular to the central axis X was the plane of symmetry, and the central position 3O in the longitudinal direction L of the outer peripheral surface of the uncured surface 3A was included. In other words, in this model, the depth h1 of the first cured layer 1 at the first reference position 1C was equal to the depth h2 of the second cured layer 2 at the second reference position 2C. In addition, in this model, the outer radius of the tubular portion 10 was constant at r (mm), and the wall thickness of the tubular portion 10 was also constant at t (mm). Here, in the model used in Analysis Example 1, the outer radius r of the tubular portion 10 was 10 mm, and the wall thickness t was 5 mm. In other words, in this model, the region from the first reference position 1C to the second reference position 2C had a constant outer diameter. In this model, the tubular portion 10 had a heat input region 1F that included one end of the tubular portion 10 and a heat input region 2F that included the other end of the tubular portion 10, both located on a part of the outer surface 10A. The heat input regions 1F and 2F included the central position 3O in the longitudinal direction L of the outer surface of the uncured surface 3A and were symmetrical with respect to a plane Y perpendicular to the central axis X. In other words, in the model shown in Figure 3, which is a 1 / 2 model with plane Y as the plane of symmetry, the heat input region 1F and the heat input region 2F were identical.

[0035] 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.

[0036] 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 was applied to heat input regions 1F and 2F of the outer surface 10A for t1 seconds, heating heat input regions 1F and 2F from room temperature (20°C) to T1°C. Subsequently, heat input regions 1F and 2F were air-cooled in room temperature air for 0.5 seconds and then 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 first hardened layer 1 was formed starting from the heat input region 1F. A second hardened layer 2 was formed starting from the heat input region 2F.

[0037] In Analysis Example 1, the heat input to heat input regions 1F and 2F was set to 9.0 to 16.0 W / mm² so that the heating temperature T1 of heat input regions 1F and 2F would be 1000 to 1200°C. 2The heat input time t1 was varied from 0.5 to 20.0 seconds. As a result, the length d along the outer circumferential surface 10A of the uncured surface 3A varied from 3.0 to 4.4 mm, and the depth h1 of the first cured layer 1 at the first reference position 1C, and the depth h2 of the second cured layer 2 at the second reference position 2C, varied from 0.65 to 2.91 mm. Based on equation (1), the cured layer depth parameter p was calculated for each heat treatment condition. Furthermore, from the analysis results, the residual stress σ (MPa) in the circumferential and central axis directions (hereinafter simply referred to as "axial direction") at the central position 3O in the longitudinal direction L of the outer circumferential surface of the uncured surface 3A was calculated. Whether or not the uncured surface 3A has a large compressive residual stress was determined by the maximum value of the residual stress in the analyzed range σ max (MPa) is used, and the minimum residual stress in the analyzed range is σ min When given (MPa), the evaluation was performed using the dimensionless parameter pσ defined by equation (A). pσ=(σ-σ min ) / (σ max -σ min ) (A)

[0038] Residual stress is expressed as a positive value in the tensile direction and a negative value in the compressive direction. In other words, the smaller the residual stress value (the larger the negative value), the greater the residual stress in the compressive direction. Referring to equation (A), the closer pσ is to 0, the greater the compressive residual stress at the central position 3O of the uncured surface 3A. If pσ is 0.20 or less in both the circumferential and axial directions, it was determined that the uncured surface 3A has sufficient compressive residual stress.

[0039] 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 resulting graph is shown in Figure 5. Referring to Figure 5, when the hardened layer depth parameter p was in the range of 0.50 to 0.70, pσ was always 0.20 or less. Therefore, a large compressive residual stress was obtained even in the unhardened surface 3A.

[0040] [Analysis example 2] In Analysis Example 2, as a model for the tubular portion 10 provided in the tubular component 100, the two-dimensional axisymmetric model shown in FIG. 3 was used in the same manner as in Analysis Example 1. In the model used in Analysis Example 2, the outer radius r of the tubular portion 10 was 20 mm, and the wall thickness t thereof was 10 mm. In Analysis Example 2, the amount of heat input to the heat input region 1F and the heat input region 2F was adjusted to 4.0 to 16.0 W / mm so that the heating temperature T1 of the heat input region 1F and the heat input region 2F was 1000 to 1200°C 2 , and the heat input time t1 was varied within a range of 0.5 to 60.0 seconds. As a result, the length d of the unhardened surface 3A along the outer peripheral surface 10A varied within a range of 3.8 to 5.1 mm, and the depth h1 of the first hardened layer 1 at the first reference position 1C and the depth h2 of the second hardened layer 2 at the second reference position 2C varied within a range of 0.63 to 2.83 mm. Analysis conditions other than the above were the same as those in Analysis Example 1.

[0041] Based on the analysis results, a graph was created with the horizontal axis representing the hardened layer depth parameter p and the vertical axis representing pσ. The obtained graph is shown in FIG. 6. Referring to FIG. 6, when the hardened layer depth parameter p is in the range of 0.50 to 0.70, pσ was always 0.20 or less. Accordingly, sufficient compressive residual stress was obtained even on the unhardened surface 3A.

[0042] [Analysis Example 3] In Analysis Example 3, as a model for the tubular portion 10 provided in the tubular component 100, the two-dimensional axisymmetric model shown in FIG. 7 was used. In the model shown in FIG. 7, the outer radius r of the tubular portion 10 was constant at 15 mm, and the wall thickness t of the tubular portion 10 was also constant at 7.5 mm. The tubular portion 10 of this model had, on a part of the outer peripheral surface 10A thereof, a heat input region 1F including one end of the tubular portion 10 and a heat input region 2F including the other end of the tubular portion 10. In Analysis Example 3, the amount of heat input to the heat input region 1F was set to 12.0 W / mm 2 , and the heat input time t1 was fixed at 1.0 second. Then, the amount of heat input to the heat input region 2F was adjusted to 5.0 to 16.0 W / mm so that the heating temperature T1 of the heat input region 1F and the heat input region 2F was 1000 to 1200°C 2The heat input time t1 was varied within the range of 0.5 to 50.0 seconds. As a result, the length d along the outer peripheral surface 10A of the uncured surface 3A varied within the range of 3.3 to 6.4 mm, the depth h1 of the first cured layer 1 at the first reference position 1C varied within the range of 0.33 to 1.69 mm, and the depth h2 of the second cured layer 2 at the second reference position 2C varied within the range of 0.68 to 3.80 mm. All other analysis conditions were the same as in Analysis Example 1.

[0043] 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 resulting graph is shown in Figure 8. Referring to Figure 8, when the hardened layer depth parameter p was in the range of 0.50 to 0.70, pσ was always 0.20 or less. Therefore, sufficient compressive residual stress was obtained even in the unhardened surface 3A.

[0044] As described above, the tubular component 100 according to the first embodiment has an uncured surface 3A with sufficient compressive residual stress because the cured layer depth parameter p, represented by formula (1), is 0.50 to 0.70.

[0045] In addition, the tubular part 100 according to the first embodiment may include other hardened layers besides the first hardened layer 1 and the second hardened layer 2, or it may include unhardened surfaces other than the unhardened surface 3A, as long as it has the above-described configuration.

[0046] <Second Embodiment> In the tubular part 100 according to the first embodiment, the region of the outer circumferential surface 10A from the first reference position 1C to the second reference position 2C had a constant outer diameter. However, in the tubular part 200 according to this embodiment, the region of the outer circumferential surface 10A from the first reference position 1C to the second reference position 2C does not have a constant outer diameter, and may include a thick-walled portion and a thin-walled portion. The tubular part 200 according to the second embodiment will be described below.

[0047] Figure 9 is a longitudinal cross-sectional view of a tubular portion 10 provided in a tubular component 200 according to the second embodiment. Referring to Figure 9, the tubular portion 10 provided in the tubular component 200 includes a thin-walled portion 11, a thick-walled portion 12, and a tapered portion 13. The thick-walled portion 12 is arranged coaxially with the thin-walled portion 11 and has a greater wall thickness than the thin-walled portion 11. The tapered portion 13 is arranged coaxially with the thin-walled portion 11 and the thick-walled portion 12 between them. The tapered portion 13 is in contact with the thin-walled portion 11 at its end 13E. The tapered portion 13 is in contact with the thick-walled portion 12 at its end 13F. In the tubular portion 10, the thin-walled portion 11, the tapered portion 13, and the thick-walled portion 12 are arranged coaxially in this order in the axial direction.

[0048] The outer diameter of the end 13E of the tapered portion 13 that contacts the thin-walled portion 11 is equal to the outer diameter of the end of the thin-walled portion 11 that contacts the tapered portion 13. The outer diameter of the end 13F of the tapered portion 13 that contacts the thick-walled portion 12 is equal to the outer diameter of the end of the thick-walled portion 12 that contacts the tapered portion 13. In the tapered portion 13, a taper is formed from the end 13E that contacts the thin-walled portion 11 toward the end 13F that contacts the thick-walled portion 12. In other words, in the tapered portion 13, the outer diameter increases from the end 13E that contacts the thin-walled portion 11 toward the end 13F that contacts the thick-walled portion 12. Therefore, in the tapered portion 13, the longitudinal direction L of the outer circumferential surface is inclined with respect to the central axis X. The taper angle of the tapered portion 13 may or may not be constant. If the taper angle of the tapered portion 13 is constant at θ(°), then θ is, for example, greater than 0 and ~45°.

[0049] The thin-walled portion 11 and the tapered portion 13 have an uncured surface 3A. In other words, the uncured surface 3A is formed on at least the outer circumferential surface 10A of the thin-walled portion 11 and the tapered portion 13. Therefore, the outer diameter of the uncured surface 3A is not constant. In this respect, the tubular part 200 according to the second embodiment differs from the tubular part 100 according to the first embodiment.

[0050] If the thin-walled portion 11 and the tapered portion 13 have an uncured surface 3A, the thin-walled portion further has either the first cured layer 1 or the second cured layer 2. The thick-walled portion 12 may or may not have an uncured surface 3A. Hereinafter, referring to Figure 9, we will assume that the thin-walled portion 11 has the first cured layer 1. If the thick-walled portion 12 has an uncured surface 3A, the thick-walled portion 12 further has the second cured layer 2, and the tapered portion 13 does not have the second cured layer 2. If the thick-walled portion 12 does not have an uncured surface 3A, the thick-walled portion 12 has the second cured layer 2, and the tapered portion 13 has the second cured layer 2 in addition to the uncured surface 3A.

[0051] Referring to Figure 9, in a longitudinal section of the tubular portion 10 including the central axis X, the length along the outer circumferential surface 10A from the first hardened surface end 1E to the end 13E is defined as d1 (mm), and the length along the outer circumferential surface 10A from the end 13E to the second hardened surface end 2E is defined as d2 (mm). In the tubular component 200 according to the second embodiment, the length d (mm) along the outer circumferential surface of the unhardened surface 3A is expressed as the sum of d1 and d2.

[0052] In the tubular component 200 according to the second embodiment, similar to the tubular component 100 according to the first embodiment, the hardened layer depth parameter p, defined by equation (1) above, is 0.50 to 0.70. As shown in the analysis example described later, if the hardened layer depth parameter p is 0.50 to 0.70, the residual stress of the unhardened surface 3A increases in the compressive direction. In other words, the tubular component 200 with a hardened layer depth parameter p of 0.50 to 0.70 includes an unhardened surface 3A with sufficient compressive residual stress.

[0053] [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 portion 10 provided in the tubular component 200. In the model shown in Figure 10, the inner diameter of the tubular portion 10 was constant. The outer radius r of the thin-walled portion 11 was constant at 15 mm, and the wall thickness was constant at 7.5 mm. That is, the minimum distance t from the uncured surface 3A to the inner circumferential surface was 7.5 mm. The outer radius of the thick-walled portion 12 was constant at 20 mm. The taper angle θ of the tapered portion 13 was constant at 30°. The central position 3O in the longitudinal direction L of the outer circumferential surface on the uncured surface 3A coincided with the end 13E of the tapered portion 13 that was in contact with the thin-walled portion 11. In this model, the tubular portion 10 had a heat input region 1F including the end of the thin-walled portion 11 and a heat input region 2F including the end of the thick-walled portion 12 in a part of the outer circumferential surface 10A. In analysis example 4, the heat input to heat input regions 1F and 2F was set to 5.0 to 16.0 W / mm² so that the heating temperature T1 of heat input regions 1F and 2F was 1000 to 1200°C. 2 The heat input time t1 was varied within the range of 0.5 to 50.0 seconds. As a result, the length d along the outer peripheral surface 10A of the uncured surface 3A varied within the range of 3.3 to 4.7 mm, the depth h1 of the first cured layer 1 at the first reference position 1C varied within the range of 0.59 to 2.91 mm, and the depth h2 of the second cured layer 2 at the second reference position 2C varied within the range of 0.60 to 2.73 mm. All other analysis conditions were the same as in Analysis Example 1.

[0054] 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 resulting graph is shown in Figure 11. Referring to Figure 11, when the hardened layer depth parameter p was in the range of 0.50 to 0.70, pσ was always 0.20 or less. Therefore, sufficient compressive residual stress was obtained even in the unhardened surface 3A.

[0055] As described above, the tubular component 200 according to the second embodiment has an uncured surface 3A with sufficient compressive residual stress because the cured layer depth parameter p, represented by formula (1), is 0.50 to 0.70.

[0056] Furthermore, if the tubular component 200 according to the second embodiment has the above-described configuration, the tubular portion 10 may include a region with a wall thickness greater than the thick portion 12 on the opposite side of the tapered portion 13, sandwiching the thin portion 11, or it may include a region with a wall thickness less than the thin portion 11 on the opposite side of the tapered portion 13, sandwiching the thick portion 12.

[0057] 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]

[0058] 100: Tubular parts 10:Tubular part 10A: Outer surface 10B: Inner surface 1: First hardened layer 1 1A: First hardened surface 1A 2: Second hardened layer 2 2A: Second hardened surface 2A 3A: Uncured surface 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 hardened layer, which includes a portion of the outer surface and is formed in an annular shape along the circumferential direction of the outer surface, A second hardened layer, which includes a portion of the outer surface and is formed in an annular shape along the circumferential direction of the outer surface, The outer surface includes an uncured surface which is a region in contact with the first cured surface and the second cured surface, and is positioned between the first cured surface, which is a part of the outer surface on which the first cured layer is formed, and the second cured surface, which is a part of the outer surface on which the second cured layer is formed. In a longitudinal section including the central axis of the tubular portion, Let d (mm) be the length of the uncured surface along the outer circumferential surface. Let t (mm) be the minimum value of the distance from the uncured surface to the inner circumferential surface. Of the first cured surface, the first reference position is defined as a position t / 2 (mm) away from the end of the surface in contact with the uncured surface, along the outer circumferential surface in the direction opposite to the uncured surface. Let h1 (mm) be the depth of the first hardened layer in the direction perpendicular to the outer surface at the first reference position. Of the second cured surface, the second reference position is defined as a position t / 2 (mm) away from the end of the surface in contact with the uncured surface, along the outer peripheral surface in the direction opposite to the uncured surface. The depth of the second hardened layer in the direction perpendicular to the outer surface at the second reference position is defined as h2 (mm). The hardened layer depth parameter p, represented by equation (1), is 0.50 to 0.

70. Tubular component. p=(h1+h2) / 2d (1)

2. A tubular component according to claim 1, Of the outer circumferential surface, the region from the first reference position to the second reference position has a constant outer diameter. Tubular component.

3. A tubular component according to claim 1, The tubular portion is, Thin-walled section, A thick-walled portion is arranged coaxially with the thin-walled portion and has a wall thickness greater than that of the thin-walled portion, Between the thin-walled portion and the thick-walled portion, there is a tapered portion that is arranged coaxially with the thin-walled portion and the thick-walled portion, is in contact with the thin-walled portion and the thick-walled portion, and has a tapered shape. The thin-walled portion and the tapered portion have the uncured surface. Tubular component.

4. A tubular component according to claim 3, The thickened portion has the uncured surface, Tubular component.

5. A tubular component according to any one of claims 1 to 4, h1 (mm) and h2 (mm) are equal. Tubular component.

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