Steel pipe for pressure piping and steel pipe blank

Abstract

A steel pipe for pressure piping in which a self-tightening process is implemented, an average hardness of an outer-surface layer region is 1.20 times or more of an average hardness of an inner-surface layer region, when an outer diameter of the steel pipe is set to D, an inner diameter is set to d, a measured value of residual stress of the outer surface is set to σ o1 , a measured value of residual stress of the inner surface after half-cutting is set to σ o2 , and a measured value of residual stress of the outer surface after half-cutting is set to σ i2 , a calculated value σ i1 of residual stress of the inner surface of the steel pipe is -150 MPa or less, which is calculated by [σ i2 =(−σ 2 ) / (A×(t / T) o2 −1)], [t / T=((σ o1 −σ o2 ) / (A×(σ o1 −σ i2 )−C×σ 1 / 2 )) 2 ], [A=3.9829×exp(0.1071×(D / d) 2 )], and [C=−3.3966×exp(0.0452×(D / d) i1 )].

Description

Technical Field

[0001] This invention relates to steel pipes for pressure piping and steel pipe blanks. Background Technology

[0002] Pressure piping for hydraulic cylinders, airbag steel pipes, accumulators, hydrogen piping, etc., requires not only high strength but also excellent internal pressure fatigue characteristics.

[0003] For example, Patent Document 1 discloses a method for manufacturing a cylinder steel pipe with excellent internal pressure fatigue characteristics, characterized in that, when manufacturing a cylinder steel pipe by drawing the steel pipe, it is heat-treated at 300-350°C after drawing.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 4-183820

[0007] Non-patent literature

[0008] Non-Patent Literature 1: Eisuke Nakayama, Mitsuo Miyahara, Kazuo Okamura, Hiroki Fujimoto, Kiyoyuki Fukui, “Fatigue Strength Prediction of Spot-Welded Joints for Thin Plates in Automobiles Based on Ultra-Small Test Pieces”, Materials, October 2004, Vol. 53, No. 10, pp. 1136-1142

[0009] Non-patent document 2: Edited by the Japan Materials Society, "Standard for X-ray Stress Measurement (2002 Edition) - Steel Section", March 2002. Summary of the Invention

[0010] The problem the invention aims to solve

[0011] According to Patent Document 1, by increasing the proportional limit strength, it is possible to obtain cylinder steel tubes with excellent internal pressure fatigue characteristics. However, in recent years, there has been a demand for further improvement in internal pressure fatigue characteristics, indicating that there is still room for improvement.

[0012] One method to improve internal pressure fatigue characteristics is self-tightening treatment. Self-tightening treatment refers to the process of causing localized plastic deformation near the inner surface by applying excessive internal pressure, thereby generating compressive residual stress.

[0013] If the strength of the steel pipe material increases, the pressure applied to the inside of the steel pipe can naturally be increased. However, when pressure is applied to the inside of the steel pipe, the ultimate internal pressure (hereinafter referred to as "ultimate internal pressure"), which is the limit at which fatigue-induced failure will not occur on the inner surface of the steel pipe, depends not only on the strength of the steel pipe material. By performing autotightening treatment, the ultimate internal pressure can be increased even without increasing the tensile strength of the steel pipe.

[0014] However, during autotightening, if excessive pressure is applied to the inner surface of the steel pipe, the risk of bursting increases. Therefore, from a safety perspective, the pressure is usually set lower. As a result, in the prior art, even with autotightening, the ultimate internal pressure cannot be sufficiently increased, leaving room for improvement.

[0015] Moreover, if it is a thin-walled steel pipe, the stress difference between the inner and outer surfaces of the pipe becomes smaller, and the steel pipe as a whole is at risk of yielding. There is also the problem that it is difficult to cause plastic deformation only in a local area near the inner surface.

[0016] The purpose of this invention is to solve the above-mentioned problems and provide a pressure piping steel pipe and steel pipe blank with high ultimate internal pressure.

[0017] Solution for solving the problem

[0018] The present invention was made to solve the above-mentioned problems, and its main purpose is to provide the following pressure piping steel pipe and steel pipe blank.

[0019] (1) A pressure piping steel pipe that has undergone self-tightening treatment,

[0020] The steel pipe has an outer surface and an inner surface.

[0021] The average hardness of the outer surface region, extending from the outer surface to a depth of 1 / 4 of the wall thickness, is more than 1.20 times the average hardness of the inner surface region, extending from the inner surface to a depth of 1 / 4 of the wall thickness.

[0022] Let the outer diameter of the steel pipe be D (mm), the inner diameter be d (mm), and the measured value of the residual stress on the outer surface of the steel pipe after self-tightening treatment be σ. o1 (MPa), the measured value of the residual stress on the outer surface of the steel pipe after self-tightening treatment and after half-cutting is set as σ. o2 The measured value of the residual stress on the inner surface of the steel pipe after self-tightening treatment and after half-cutting is denoted as σ (MPa). i2 At (MPa),

[0023] The estimated value σ of the residual stress on the inner surface of the self-tightening steel pipe after the treatment is obtained from equations (i) to (iv) below. i1 (MPa) is below -150MPa

[0024] σ i1 =(-σ i2 ) / (A×(t / T) 2 -1)···(i)

[0025] t / T=((σ o2 -σ o1) / (A×(σ o2 -σ o1 )-C×σ i2 )) 1 / 2 ···(ii)

[0026] A = 3.9829 × exp(0.1071 × (D / d)) 2 (iii)

[0027] C=-3.3966×exp(0.0452×(D / d) 2 )···(iv).

[0028] (2) The pressure piping steel pipe according to (1) above, wherein D / d is 2.0 or less.

[0029] (3) A steel pipe blank for pressure piping intended for use in implementing self-tightening treatment.

[0030] The steel pipe blank has an outer surface and an inner surface.

[0031] The average hardness of the outer surface region, extending from the outer surface to a depth of 1 / 4 of the wall thickness, is more than 1.20 times the average hardness of the inner surface region, extending from the inner surface to a depth of 1 / 4 of the wall thickness.

[0032] When self-tightening is implemented,

[0033] Let the outer diameter of the self-tightening steel pipe be D (mm), the inner diameter be d (mm), and the measured value of the residual stress on the outer surface of the self-tightening steel pipe be σ. o1 (MPa), the measured value of the residual stress on the outer surface of the steel pipe after self-tightening treatment and after half-cutting is set as σ. o2 The measured value of the residual stress on the inner surface of the steel pipe after self-tightening treatment and after half-cutting is denoted as σ (MPa). i2 At (MPa),

[0034] The estimated value σ of the residual stress on the inner surface of the self-tightening steel pipe after the treatment is obtained from equations (i) to (iv) below. i1 (MPa) is below -150MPa.

[0035] σ i1 =(-σ i2 ) / (A×(t / T) 2 -1)···(i)

[0036] t / T=((σ o2 -σ o1 ) / (A×(σ o2 -σ o1 )-C×σi2 )) 1 / 2 ···(ii)

[0037] A = 3.9829 × exp(0.1071 × (D / d)) 2 (iii)

[0038] C=-3.3966×exp(0.0452×(D / d) 2 (iv)

[0039] (4) The steel pipe blank for pressure piping as described in (3) above, wherein D / d is 2.0 or less.

[0040] The effects of the invention

[0041] According to the present invention, it is possible to reliably obtain pressure piping steel pipes with high ultimate internal pressure. Attached Figure Description

[0042] Figure 1 This is a diagram illustrating an example of a self-tightening steel pipe for which residual stress is estimated using an estimation device.

[0043] Figure 2 This is a diagram used to illustrate the method of deriving multivariable functions.

[0044] Figure 3 This is a diagram used to illustrate the method of deriving multivariable functions.

[0045] Figure 4 This is a diagram used to illustrate the method of deriving multivariable functions.

[0046] Figure 5 This diagram illustrates a suitable range for studying the hardness ratio between the outer and inner surface regions, and the ratio of the outer diameter to the inner diameter of a steel pipe.

[0047] Figure 6 This diagram illustrates a suitable range for studying the hardness ratio between the outer and inner surface regions, and the ratio of the outer diameter to the inner diameter of a steel pipe.

[0048] Figure 7 This diagram illustrates a suitable range for studying the hardness ratio between the outer and inner surface regions, and the ratio of the outer diameter to the inner diameter of a steel pipe.

[0049] Figure 8 This diagram illustrates a method for studying the appropriate range of the hardness ratio between the outer and inner surface regions, and the ratio of the outer diameter to the inner diameter of a steel pipe.

[0050] Figure 9This diagram illustrates a suitable range for studying the hardness ratio between the outer and inner surface regions, and the ratio of the outer diameter to the inner diameter of a steel pipe.

[0051] Figure 10 This is a diagram used to illustrate the shape of an internal pressure fatigue test piece.

[0052] Figure 11 It is a diagram used to illustrate the sampling location and shape of small dumbbell-shaped test pieces. Detailed Implementation

[0053] In the prior art, one reason why the self-tightening treatment pressure used to sufficiently increase the ultimate internal pressure cannot be optimized is that a method for calculating the residual stress on the inner surface of the steel pipe has not yet been established. It should be noted that, in this application specification, residual stress refers to the residual stress in the circumferential direction of the steel pipe.

[0054] Previously, the inventors evaluated the residual stress on the inner surface of a self-tightening steel pipe by cutting it in half and measuring the residual stress on the inner surface of the cut pipe. It should be noted that cutting in half refers to cutting the steel pipe in such a way that it is divided into two equal arc-shaped components when viewed from the axial direction.

[0055] However, in order to optimize the autotightening pressure to fully improve the ultimate internal pressure, it is necessary to quantitatively evaluate the residual stress on the inner surface of the steel pipe before half-cutting, after autotightening. Therefore, the inventors have researched a method for quantitatively evaluating the residual stress on the inner surface of the steel pipe. In this study, the inventors investigated how to evaluate the residual stress on the inner surface of the steel pipe before half-cutting by considering not only the residual stress on the inner surface of the steel pipe after half-cutting, but also the residual stress on the outer surface of the steel pipe before and after half-cutting.

[0056] The inventors first used an analytical model of the steel pipe being evaluated to perform numerical analysis (FEM analysis) under various conditions to determine the residual stress (calculated value) generated at various parts of the steel pipe due to the self-tightening process. Specifically, the inventors first determined the residual stress σ on the outer surface of the steel pipe before partial cutting after the self-tightening process using numerical analysis. o1 The residual stress σ on the inner surface of the steel pipe after self-tightening treatment and before partial cutting. i1 The residual stress σ on the outer surface of the steel pipe after self-tightening treatment and after partial cutting. o2 The residual stress σ on the inner surface of the steel pipe after self-tightening treatment and after partial cutting. i2 .

[0057] A detailed study was conducted on the residual stresses obtained from the above-described operation. As a result, the inventors discovered that the residual stress σ on the inner surface of the steel pipe before partial cutting... i1 The residual stress σ on the outer surface of the steel pipe before half-cutting can be used. o1 The residual stress σ on the outer surface of the steel pipe after it has been partially cut o2 And the residual stress σ on the inner surface of the half-cut steel pipe. i2 To make high-precision estimations.

[0058] The following insights were then obtained: Based on the estimated value σ of the residual stress on the inner surface of the steel pipe before it was cut in half. i1 By adjusting the self-tightening treatment conditions in a manner that meets the specified requirements, steel pipes with high ultimate internal pressure can be stably obtained.

[0059] Furthermore, as mentioned above, for thin-walled steel pipes, there is a problem that it is difficult to induce plastic deformation only in the vicinity of the inner surface. In order to solve this problem, the inventors conducted further research and found that by increasing the hardness of the outer surface of the steel pipe relative to the hardness of the inner surface beforehand, even for thin-walled steel pipes, it is possible to induce yielding only in the vicinity of the inner surface, thereby imparting residual stress.

[0060] This invention is based on the above-mentioned insights. The features of this invention will now be described in detail.

[0061] 1. Steel pipes and steel pipe blanks for pressure piping

[0062] One embodiment of the present invention relates to a pressure piping steel pipe that has undergone autotightening treatment. Pressure piping includes hydraulic cylinders, airbag steel pipes, accumulators, hydrogen piping, fuel injection pipes, etc. Furthermore, other embodiments of the present invention involve steel pipe blanks that are the materials used to form the blanks of the aforementioned pressure piping steel pipes, for applications requiring autotightening treatment. That is, by autotightening the steel pipe blank, a pressure piping steel pipe can be obtained.

[0063] Furthermore, the average hardness of the outer surface region of the steel pipe billet is more than 1.20 times that of the average hardness of the inner surface region. The reasons for this stipulation will be explained later. Here, the outer surface region refers to the area from the outer surface of the steel pipe billet to a depth of 1 / 4 of the wall thickness, and the inner surface region refers to the area from the inner surface of the steel pipe billet to a depth of 1 / 4 of the wall thickness.

[0064] By making the average hardness of the outer surface region of the steel pipe billet at least 1.20 times that of the average hardness of the inner surface region, during the self-tightening treatment of the steel pipe billet, the outer surface region can be prevented from yielding, while only the inner surface region undergoes plastic deformation, thus imparting compressive residual stress. The average hardness of the outer surface region of the steel pipe billet is preferably at least 1.50 times that of the inner surface region, and more preferably at least 2.00 times.

[0065] Here, since the hardness does not change significantly due to the self-tightening treatment, the same relationship applies even to the steel pipe after self-tightening treatment, as it does to the steel pipe blank. That is, the average hardness of the outer surface region of the steel pipe is at least 1.2 times, preferably at least 1.5 times, and more preferably at least 2.0 times, the average hardness of the inner surface region. Here, the outer surface region refers to the region extending from the outer surface of the steel pipe to a depth of 1 / 4 of the wall thickness, and the inner surface region refers to the region extending from the inner surface of the steel pipe to a depth of 1 / 4 of the wall thickness.

[0066] It should be noted that the average hardness of the inner and outer surface regions of the steel pipe blank or steel pipe is determined as follows. First, the Vickers hardness distribution in the cross-section of the steel pipe blank or steel pipe is determined according to JIS Z 2244:2009 (Vickers Hardness Test - Test Method). The aforementioned cross-section can be a section perpendicular to the axial direction of the steel pipe blank or steel pipe, or a section parallel to the axial direction and passing through the central axis.

[0067] The hardness testing machine uses a general-purpose micro Vickers hardness tester, and the test force is set from 1 to 10 N depending on the size of the steel pipe. For the testing area, within the mirror-polished observation surface, the indentations are spaced radially from the inner surface to the outer surface at distances of 1 / 10 to 1 / 20 of the wall thickness. Even when indentations are adjacent, the testing positions can be staggered in a direction perpendicular to the radial direction, resulting in alternating measurements. Based on the resulting hardness distribution, the average hardness of the inner and outer surface regions is obtained by averaging the hardness values ​​contained in the inner and outer surface regions, respectively.

[0068] Furthermore, there are no particular limitations on the methods for increasing the average hardness of the outer surface region of the steel pipe billet (hereinafter also referred to as "hardening treatment"). For example, a method of high-frequency quenching from the outer surface of the steel pipe billet can be cited. Specifically, by high-frequency heating, the outer surface temperature is instantly heated to above 900°C, and then immediately water-cooled, thereby increasing the hardness only in the outer surface region. For the steel pipe billet after high-frequency quenching, a tempering treatment can also be performed as needed, holding it at 100–300°C for more than 30 minutes followed by natural cooling.

[0069] In addition, methods such as shot peening or rolling the outer surface of the steel pipe blank to harden only the outer surface area can also be used; or methods such as carburizing, nitriding or carburizing and nitriding can be applied to the outer surface.

[0070] In this invention, the size of the steel pipe is not particularly limited. Generally speaking, to withstand high internal pressure, it is ideal to have a larger inner diameter and a correspondingly larger wall thickness. If the inner diameter of the steel pipe is constant, the outer diameter of the steel pipe also increases with the increase in wall thickness. That is, to withstand high internal pressure, it is ideal to have a larger inner diameter and a larger outer diameter. However, as mentioned above, the effects of this invention are more pronounced in thin-walled steel pipes. Therefore, in this invention, when the outer diameter of the steel pipe is set as D (mm) and the inner diameter as d (mm), D / d can be 2.0 or less, 1.9 or less, or 1.8 or less.

[0071] Regarding other dimensions, selection is based solely on the intended application, without any particular restrictions. For example, when using steel pipes as hydraulic cylinders, to ensure piston output (load), the inner diameter, matching the operating pressure (internal pressure), is typically selected within the range of 15–580 mm. Furthermore, to withstand repeated internal pressure, a wall thickness of 5–60 mm and an outer diameter of 30–700 mm are preferred. Higher internal pressure fatigue strength allows for thinner wall thicknesses, and the outer diameter can be determined accordingly.

[0072] When a steel pipe is used as an airbag inflator, the outer diameter of the steel pipe is preferably 20–100 mm, more preferably 20–60 mm. The wall thickness of the steel pipe is preferably 1–5 mm, more preferably 1–4 mm.

[0073] When a steel pipe is used as an accumulator, the outer diameter of the steel pipe is preferably 25–500 mm, more preferably 50–400 mm. The wall thickness of the steel pipe is preferably 2–40 mm, more preferably 4–30 mm.

[0074] Furthermore, when using steel pipes as hydrogen piping or fuel injection pipes, a certain capacity is required to reduce internal pressure fluctuations during use. Therefore, the inner diameter of the steel pipe is preferably 2.5 mm or more, more preferably 3.0 mm or more. Additionally, since it needs to withstand high internal pressure, the wall thickness of the steel pipe is preferably 1.5 mm or more, more preferably 2.0 mm or more. On the other hand, the outer diameter of the steel pipe is preferably 20 mm or less, more preferably 15 mm or less, and even more preferably 10 mm or less.

[0075] Regarding mechanical properties, selection is based solely on the intended use, and no particular restrictions are necessary. However, for use in pressure piping, the tensile strength of the steel pipe blank before hardening treatment, or the tensile strength of the inner surface region of the steel pipe after hardening treatment and subsequent autocrowding treatment, is preferably 500 MPa or more, more preferably 600 MPa or more, and even more preferably 700 MPa or more. The yield stress is preferably 300 MPa or more, more preferably 360 MPa or more, and even more preferably 420 MPa or more.

[0076] Furthermore, the hardness of the steel pipe billet before hardening treatment, or the hardness of the inner surface region of the steel pipe after hardening treatment and subsequent autoclavation treatment, is preferably 150 HV or higher using a Vickers hardness tester, more preferably 180 HV or higher, and even more preferably 220 HV or higher. It should be noted that there is no need to set an upper limit for the above hardness, but especially if the hardness of the inner surface region is too high, it will become difficult to introduce compressive residual stress through autoclavation treatment. Therefore, a Vickers hardness of less than 500 HV is preferred.

[0077] The yield ratio is preferably 0.50 to 0.95. In order to obtain a large compressive residual stress by performing self-tightening treatment under higher pressure, the yield ratio is more preferably 0.60 or more, and even more preferably 0.70 or more. In addition, in order to more efficiently introduce compressive residual stress in self-tightening treatment based on low pressure, the yield ratio is more preferably 0.90 or less, and even more preferably 0.85 or less.

[0078] In this invention, the mechanical properties of the inner surface region of the steel pipe can be determined by cutting out a small dumbbell-shaped test piece with a thickness of approximately 0.2 mm, as shown in Non-Patent Document 1, through electrical discharge machining in contact with the inner surface of the steel pipe, and then measuring it through a tensile test. For strain measurement in the tensile test, it can be calculated by converting the displacement (stroke) of the tensile testing machine actuator and the length of the parallel portion of the test piece, as shown in Non-Patent Document 1.

[0079] It should be noted that if there is a steel pipe blank before hardening treatment, its mechanical properties can be determined by cutting out a straight section of the steel pipe, clamping a region of a certain length from its two ends (hereinafter referred to as the "clamping section"), installing an elongation meter in the parallel section between the clamping sections, and performing a tensile test. Clamping is performed by pressing contact pieces with V-grooves or R-grooves smaller than the outer radius of the steel pipe into the clamping section using hydraulic pressure, bolt connections, or wedge tools. The length of the clamping section should be determined by considering the pressing pressure and test load in a way that prevents the test steel pipe from slipping during the test. Furthermore, the length of the parallel section should be sufficient to allow the installation of the elongation meter and to ensure that the diameter reduction deformation before fracture is not affected by the clamp. It should be noted that if the steel pipe does not have a sufficiently long straight section, a small test piece in a thin-walled dumbbell shape, not as shown in Patent Document 1, can be cut out for the tensile test.

[0080] In addition, the steel pipe for pressure piping involved in this invention preferably has a limiting internal pressure that satisfies the following formula (I).

[0081] IP≥0.44×TS×α···(I)

[0082] α=[(D / d) 2 -1] / [0.776×(D / d) 2 ]···(II)

[0083] In equation (I) above, IP refers to the ultimate internal pressure of the steel pipe (MPa), TS refers to the tensile strength (MPa) of the inner surface region of the steel pipe or the steel pipe blank, and α is the value shown in equation (II) above. Furthermore, in equation (II) above, D is the outer diameter (mm) of the steel pipe, and d is the inner diameter (mm). α is a coefficient used to correct for changes in the relationship between internal pressure and stress generated on the inner surface of the pipe due to the ratio of the outer diameter to the inner diameter.

[0084] It should be noted that, in this invention, the ultimate internal pressure refers to: in the internal pressure fatigue test, setting the minimum internal pressure to 18 MPa, applying repeated internal pressure variations that exhibit a sinusoidal wave pattern relative to time, even if the number of repetitions reaches 10... 7 The highest internal pressure (MPa) at which failure (leakage) will not occur even after 10 failures. Specifically, the minimum value of the maximum internal pressure at which failure occurs will be compared with the minimum value of the maximum internal pressure at which failure occurs on the SN line graph with the vertical axis representing the maximum internal pressure and the horizontal axis representing the number of failure repetitions. 7 The median of the maximum values ​​that do not break even after a certain number of cycles is taken as the ultimate internal pressure.

[0085] 2. Residual stress estimation model

[0086] The residual stress σ on the inner surface of the steel pipe before half-cutting is used to estimate the stress. i1 The model will be explained in detail. Figure 1This diagram illustrates an example of using this model to estimate the residual stress in a self-tightening steel pipe. Figure 1 In the diagram, (a) is a left-side view of the self-tightening steel pipe 20, (b) is a front view of the steel pipe 20 shown in (a), (c) is a left-side view of the halved specimen 22 obtained by cutting the steel pipe 20 shown in (a) in half, and (d) is a front view of the halved specimen 22 shown in (c). It should be noted that, in this specification, the halved steel pipe refers to the halved specimen obtained by cutting the self-tightening steel pipe in half.

[0087] In this model, the residual stress σ on the outer surface 20a of the self-tightening steel pipe 20 is used. o1 The residual stress σ on the outer surface 22a of the half-cut specimen 22 o2 The residual stress σ on the inner surface 22b of the halved specimen 22. i2 The measured value. It should be noted that, as mentioned above, residual stress refers to the residual stress in the circumferential direction of the steel pipe 20.

[0088] Reference Figure 1 When measuring residual stress, the length L of the steel pipe 20 is preferably set to more than three times the outer diameter D of the steel pipe 20, for example, about 30 mm. If the heat generated during the cutting of the steel pipe 20 is excessive, it will affect the residual stress on the inner surface. Therefore, a cutting method that generates as little heat as possible is required, preferably by electrical discharge machining of a metal wire. At this time, control is performed so that, in the side view of the half-cut specimen 22, the distance X (the distance in the direction perpendicular to the cutting surface 22c) between the cut surface 22c of the half-cut specimen 22 and the center of the outer surface 22a is within ±5% of the radius r of the steel pipe 20.

[0089] The residual stress was measured after the outer surface 20a of the steel pipe 20 and the inner surface 22b of the halved specimen 22 were removed by electrolytic grinding to a depth of less than 10 μm. As a measurement method, X-ray diffraction-based sin... 2 The ψ method can be performed according to non-patent literature 2.

[0090] The residual stress σ was measured using the method described above. o1 σ o2 σ i2 The residual stress σ is calculated by using a multivariable function with the outer diameter D and inner diameter d as variables. i1 The estimated value.

[0091] Specifically, such as Figure 2As shown in (a), an arc-shaped analysis model 40 (1 / 4 model) is created by modeling the cross-section (the section perpendicular to the tube axis) of the steel tube 20 using two-dimensional plane strain elements. Although the illustration is omitted, the analysis model 40 is divided into multiple meshes. The physical properties of the analysis model 40 are set as those of an elastic body.

[0092] First, in order to simulate steel pipe 20, such as Figure 2 As shown in (b), constraints are set to limit the circumferential movement of the two ends 40a and 40b of the analysis model 40. Then, as an initial state, volume forces are set to simulate the state of the steel pipe 20 during self-tightening. Specifically, in the initial state, a circumferential compressive residual stress (-100 MPa) is applied to the inner surface 40c of the analysis model 40.

[0093] Furthermore, in the initial state, no stress is generated in the region between the inner surface 40c and the outer surface 40d at a position P (represented by a dashed arc) that radially deviates from the inner surface 40c towards the analysis model 40. Moreover, in the initial state, the stress distribution in the region between the inner surface 40c and position P is set to a linear distribution in which the compressive stress gradually decreases from the inner surface 40c towards position P. It should be noted that... Figure 2 (b) and the following Figure 3 and Figure 4 The stress state at end 40b is shown. Hereinafter, the radial distance between point P1, where the compressive stress at end 40b is zero, and the inner surface 40c will be denoted as distance t, and the thickness of the analysis model 40 will be denoted as thickness T. It should be noted that when there are multiple points where the compressive stress at end 40b is zero, the point closest to the inner surface 40c will be designated as P1.

[0094] After setting the volumetric forces as described above, an elastic analysis is performed to redistribute the stress. Thus, for example... Figure 3 As shown, the stress state of analysis model 40 changes. It should be noted that... Figure 3 In the diagram, dashed lines indicate the locations where stress is zero. In the region closer to the dashed lines, circumferential compressive stress is generated, while in the region further outwards, circumferential tensile stress is generated. Figure 3 Under the conditions shown, the integral value of the stress distribution of the entire analysis model 40 becomes 0. Figure 3 The stress state shown corresponds to the stress state of the steel pipe 20 after self-tightening treatment. Furthermore, in Figure 3 In the state shown, the stress at the intersection of the inner surface 40c and the end 40b is obtained as the residual stress σ on the inner surface 20b of the self-tightening steel pipe 20. i1The stress at the intersection of the outer surface 40d and the end 40b is obtained as the residual stress σ on the outer surface 20a of the self-tightening steel pipe 20. o1 .

[0095] Next, in order to simulate the halved specimen 22 (the steel pipe 20 after being cut in half), as... Figure 4 As shown, the constraint on end 40a is removed, and an elastic analysis is performed. Consequently, the stress state of the analysis model 40 changes further. It should be noted that... Figure 4 In the diagram, dashed lines indicate the locations where the stress is zero. Figure 4 In the example shown, for the analysis model 40, circumferential tensile stress is generated in the radial central part, and circumferential compressive stress is generated in the arc-shaped region along the inner surface 40c and the arc-shaped region along the outer surface 40d.

[0096] In addition, Figure 4 In the analytical model 40 shown, end 40a corresponds to the cutting surface 22c of the halved specimen 22 (refer to...). Figure 1 End 40b corresponds to the center portion 22d in the circumferential direction of the half-cut specimen 22 (refer to...). Figure 1 ). And, in Figure 4 In the state shown, the stress at the intersection of the inner surface 40c and the end 40b is obtained as the residual stress σ of the inner surface 22b of the half-cut specimen 22. i2 The stress at the intersection of the outer surface 40d and the end 40b is obtained as the residual stress σ on the outer surface 22a of the half-cut specimen 22. o2 .

[0097] For a steel pipe 20 of arbitrary size, the distance t in the initial state is varied in various ways (i.e., changed). Figure 2 (b) The location of point P1 where the compressive stress becomes 0) Figure 2 (b) Figure 3 and Figure 4 The above analysis is explained in the text.

[0098] The inventors have conducted various studies and have determined that the thickness T of the steel pipe 20, and the distance t in the self-tightening steel pipe 20 obtained by the above-described operation (refer to...) Figure 3 The residual stress σ on the outer surface 20a of the steel pipe 20 o1 (Refer to Figure 3 The residual stress σ on the inner surface 20b of the steel pipe 20 i1 (Refer to Figure 3 The residual stress σ on the outer surface 22a of the half-cut specimen 22 o2 (Refer to Figure 4), and the residual stress σ on the inner surface 22b of the half-cut specimen 22. i2 (Refer to Figure 4 There is a certain relationship between them.

[0099] Specifically, the inventors discovered that the self-tightening treatment of the steel pipe 20 (t / T) 2 The value of (σ) i2 / -σ i1 There is a certain relationship between the values ​​of (t / T). Therefore, by using the least squares method... 2 The value of (σ) i2 / -σ i1 The relationship between the values ​​of ) is linearly approximated to obtain the following equation (1). It should be noted that in the following equation (1), A and B are coefficients.

[0100] σ i2 / (-σ i1 )=A×(t / T) 2 -B···(1)

[0101] For σ i1 Rearranging equation (1) above, we obtain equation (i) below. It should be noted that in this invention, A is set to the value shown in equation (iii) below, and B is set to 1.

[0102] σ i1 =(-σ i2 ) / (A×(t / T) 2 -1)···(i)

[0103] A = 3.9829 × exp(0.1071 × (D / d)) 2 (iii)

[0104] Furthermore, the inventors discovered that the (t / T) in the self-tightening steel pipe 20 2 The value of ((σ) o2 -σ o1 ) / (-σ i1 There is also a certain relationship between the values ​​of (t / T). Therefore, by using the least squares method... 2 The value of ((σ) o2 -σ o1 ) / (-σ i1 The relationship between the values ​​of )) is linearly approximated to obtain the following equation (2). It should be noted that in the following equation (2), C and E are coefficients.

[0105] (σ o2 -σ o1 ) / (-σ i1 )=-C×(t / T) 2-E···(2)

[0106] Based on equations (1) and (2) above, (t / T) can be expressed by equation (3) below.

[0107] t / T=((B×(σ o2 -σ o1 )-E×σ i2 ) / (A×(σ o2 -σ o1 )-C×σ i2 )) 1 / 2 ···(3)

[0108] Furthermore, in this invention, B is set to 1 and E is set to 0, thus obtaining equation (ii) below. Additionally, C is set to the value shown in equation (iv) below.

[0109] t / T=((σ o2 -σ o1 ) / (A×(σ o2 -σ o1 )-C×σ i2 )) 1 / 2 ···(ii)

[0110] C=-3.3966×exp(0.0452×(D / d) 2 (iv)

[0111] Using equations (i) to (iv) obtained from the above-mentioned presumed model, the estimated value σ of the residual stress on the inner surface 20b of the self-tightening steel pipe 20 can be calculated. i1 .

[0112] Furthermore, the σ of the steel pipe involved in this invention i1 The value is below -150 MPa. When the compressive residual stress is greater than -150 MPa, that is, when the absolute value of the residual stress is less than 150 MPa, as shown in the examples described later, the effect of increasing the ultimate internal pressure cannot be obtained. Through self-tightening treatment, σ i1 Setting it to below -150MPa allows for the attainment of high ultimate internal pressure.

[0113] 3. Research on the appropriate range of hardness ratio of outer surface area / inner surface area and outer diameter / inner diameter ratio of steel pipe.

[0114] As described above, in this invention, a suitable range is defined as the ratio of the average hardness Hvo of the outer surface region to the average hardness Hvi of the inner surface region of the steel pipe billet, Hvo / Hvi, being 1.20 or higher, and the ratio of the outer diameter D to the inner diameter d of the steel pipe billet, D / d, being 2.0 or lower. This is based on numerical calculations using FEM analysis as shown below.

[0115] The object of analysis is Figure 1 The shape of the steel pipe shown will be based on Figure 5 The analytical model of a quarter-cylindrical shape of a three-dimensional hexahedral second-order element shown in (a) was used in the FEM analysis. The analytical model used four shapes with D / d set to 1.2, 1.5, 1.8 and 2.0.

[0116] The model's physical properties are set as those of an elasto-plastic body. The Young's modulus in the elastic region is set to 205.8 GPa and the Poisson's ratio to 0.3. In the elasto-plastic region, [the model is used in...]. Figure 6 The figure shows an example of stress-strain curves based on true stress and true plastic strain. Multiple stress-strain curves exist depending on Vickers hardness. This is based on preliminary tests conducted with carbon steel at varying hardness levels, approximating the hardness dependence of the stress-strain curves. The hardening law for the elastoplastic region is the moving hardening law. Within the analytical model, such as... Figure 7 As shown, different stress-strain curves corresponding to the hardness distribution were set according to the radial position. Three types were available for Hvo / Hvi: 1.00, 1.20, and 1.75.

[0117] To analyze the state that causes internal pressure, such as Figure 5 As shown in (b), a radial stress σr, equivalent to the internal pressure P, is applied to the inner surface of the model, while an axial stress σax is applied to the cross-section of the model. σr and σax are calculated using the inner diameter d by the following formula.

[0118] σr=-P···(4)

[0119] σax=P×d 2 / (D 2 -d 2 (5)

[0120] Here, equation (5) above is the basic equation for cylindrical containers, based on the following concept. Assuming the object of analysis is a closed section member, an axial load P×πd is generated by the internal pressure acting on the inner section of the model. 2 / 4. Since this axial load acts on the cross-section of the model, the axial stress σax can be calculated by dividing the axial load by the cross-sectional area π(D) of the model. 2 -d 2 ) / 4 is obtained.

[0121] An analysis of the reproducible fracture test was initially performed. First, the model was linearly and gradually increased from 0, and σr and σax were assigned according to equations (4) and (5) above. Then, the analysis was terminated after the model underwent plastic deformation and the analysis reached the non-convergence limit. The internal pressure at this limit was taken as the fracture strength obtained through the analysis. It was confirmed that the fracture strength obtained experimentally in the preliminary test corresponded to the fracture strength obtained through the analysis.

[0122] Next, to reproduce the self-tightening treatment, σr and σax, corresponding to internal pressures of 0.60, 0.70, and 0.85 times the fracture strength, were applied to the model and then unloaded. The circumferential residual stress on the inner surface after unloading was output, and its minimum value (since it is essentially compressive residual stress, the absolute value is the maximum value) was used as the representative value of the residual stress introduced by the self-tightening treatment.

[0123] The relationship between the residual stress obtained from the self-tightening treatment and Hvo / Hvi is shown in the figure. Figure 8 In the middle, its relationship with D / d is shown in Figure 9 middle. Figure 8 In the case where D / d is 1.2, Hvo / Hvi is greater than 1.20, and the compressive residual stress on the inner surface is greater than -150 MPa. Furthermore, it is known that when D / d is 1.8, Hvo / Hvi is greater than 1.20, and the compressive residual stress does not change.

[0124] Figure 9 In the case where Hvo / Hvi is 1.20, D / d is 1.2 or higher and the compressive residual stress on the inner surface is -150 MPa or higher. Although the compressive residual stress increases with the increase of D / d, it does not increase further when D / d is 1.5 or higher when Hvo / Hvi is 1.75, reaching a saturation value. When D / d reaches 1.8 or higher when Hvo / Hvi is 1.20, the saturation value of compressive residual stress is the same as when Hvo / Hvi is 1.75. When D / d increases further from 1.8, the difference between the compressive residual stress at Hvo / Hvi of 1.00 and the compressive residual stress at Hvo / Hvi of 1.20 or higher decreases. Furthermore, when D / d is 2.0, the difference between the compressive residual stress at Hvo / Hvi of 1.00 and the compressive residual stress at Hvo / Hvi of 1.20 or higher becomes negligible. It can be considered that in the region where D / d is greater than 2.0, the effect of self-tightening treatment based on hardening treatment on reducing compressive residual stress is small.

[0125] In summary, a suitable range is defined as a ratio of Hvo / Hvi, where the average hardness of the outer surface region of the hardened steel pipe billet is greater than 1.20, and the ratio of the outer diameter D to the inner diameter d of the steel pipe billet is less than 2.0.

[0126] 3. Manufacturing method

[0127] The method for manufacturing pressure piping steel pipes involved in this invention is not particularly limited. For example, multiple steel pipe blanks with an average hardness of 1.20 times or more that of the inner surface region can be subjected to autotightening treatment under various conditions. For each steel pipe obtained, σ can be calculated using the above method. i1 The process involves screening steel pipes with a strength below -150MPa, which are then used to manufacture them.

[0128] It should be noted that regarding the self-tightening conditions, for example, by controlling the self-tightening pressure and / or the self-tightening time, it is possible to achieve σ i1 Adjustments are made for pressures below -150 MPa. As described above, by accurately estimating the residual stress on the inner surface of the steel pipe after autotightening and before semi-cutting, the autotightening conditions can be optimized, and steel pipes with high ultimate internal pressure can be stably obtained.

[0129] The present invention will be described in more detail below through embodiments, but the present invention is not limited to these embodiments.

[0130] Example

[0131] The steel with the chemical composition shown in Table 1 was melted and then hot-forged to obtain a round bar with a diameter of 50 mm. The round bar was then normalized to 880 °C to obtain a billet for the test piece. The yield stress of the test piece billet was 382 MPa, and the tensile strength was 621 MPa.

[0132] [Table 1]

[0133]

[0134] Next, the above-mentioned test piece blanks were subjected to rough machining, heat treatment, and finish machining, and multiple samples were collected. Figure 10 The internal pressure fatigue test specimen is shown in the diagram. Here, for the shape after rough machining, it is... Figure 10 The finished shape is 1mm larger in outer diameter and 1mm smaller in inner diameter compared to a shape with the same surface finish. It should be noted that... Figure 10 The unit of length recorded in the text is mm.

[0135] Of the obtained internal pressure fatigue test pieces (Tests No. 1–7), Tests No. 1–3 were used directly (normalized), while Tests No. 4–7 underwent further high-frequency quenching and tempering treatment as follows (quenched): the outer surface was instantaneously heated to 1000°C by high-frequency heating and then immediately quenched, followed by heating to 150°C and holding for 1 hour, then allowing natural cooling. Finally, a final finishing process was performed, with grinding and finishing on both the inner and outer surfaces of the test section. It should be noted that in… Figure 10 Among the test pieces, the outer diameter of tests No.1, 2 and 4-6 is 9.0 mm (D / d: 1.5), and the outer diameter of tests No.3 and 7 is 13.2 mm (D / d: 2.2).

[0136] Furthermore, for test pieces No. 2, 3, and 5–7, a self-tightening treatment was performed under the pressures shown in Table 2. The self-tightening treatment was carried out as follows: [The text abruptly ends here, so the translation stops as well.] Figure 10 One end face of the internal pressure fatigue test piece is sealed, and working oil, which serves as the pressure medium, is sealed into the test piece from the other end face to control the internal pressure of the sealed part. Self-tightening treatment is performed by raising the internal pressure of the sealed part to the self-tightening treatment pressure and then unloading.

[0137] [Table 2]

[0138]

[0139] For each test No., six test pieces were prepared. One of these test pieces was used to determine the average hardness of the outer and inner surface regions, and to perform a tensile test on the inner surface region. In the hardness measurement, a parallel section passing through the central axis relative to the test section was cut from the test portion of the test piece. This section was then embedded in resin as the observation surface, and mirror polishing was performed using sandpaper and a polishing wheel. A general-purpose micro Vickers hardness tester was used, with the test force set to 3N. Measurements were taken at 0.1mm intervals in the radial direction from the inner surface to the outer surface within the observation surface. Specifically, there were 14 measurement locations in tests No. 1, 2, and 4-6, and 35 locations in tests No. 3 and 7. Vickers indentations were then introduced at these locations. Based on the resulting hardness distribution, the average hardness of the inner and outer surface regions was obtained by averaging the hardness data from three locations in the inner surface region and three locations in the outer surface region.

[0140] As shown in Table 2, the hardness of the outer and inner surface regions of tests No. 1–3, and the hardness of the inner surface region of tests No. 4–7, were in the range of 196–205 HV. No variation due to the presence or absence of hardening treatment or self-tightening treatment was identified. The hardness of the outer surface region of tests No. 4–7 was 433–445 HV, and the Hvo / Hvi ratio was 2.17–2.24.

[0141] Regarding tensile testing, such as Figure 11 As shown, two small dumbbell-shaped test pieces, approximately 0.2 mm thick, were cut from the inner surface region of the internal pressure fatigue test piece by electrical discharge machining in contact with the inner surface of the test section. These pieces were then used for testing. A Tytron 250 tensile testing machine manufactured by MTS was used. Regarding strain measurement, the strain was calculated based on the displacement (stroke) of the tensile testing machine actuator and the length of the parallel portion of the test piece, according to the method shown in Non-Patent Document 1. In the resulting stress-strain curve, the 0.2% yield strength was taken as the yield stress, the maximum stress as the tensile strength, and the average value of the two small test pieces as the measured value.

[0142] As shown in Table 2, in the inner surface region of tests No.1–7, the tensile strength was 621–632 MPa, the yield stress was 382–391 MPa, and the yield ratio was 0.61–0.62. No changes caused by hardening and autotightening treatments were identified.

[0143] In addition, another test piece was used for residual stress measurement. First, the surface layer of the outer surface at the center of the test piece along its length was removed by electrolytic polishing, removing an area of ​​less than 10 μm. Then, the residual stress σ in the circumferential direction was measured. o1 As the measurement method, sin(s) based on X-ray diffraction was used. 2 The ψ method was performed according to non-patent literature 2. Detailed measurement conditions are shown below.

[0144] • Scanning methods: tilt method, constant η method (PSPC method)

[0145] • X-ray stress measurement device: PSPC-RSF manufactured by Rigaku Corporation

[0146] • Characteristic X-ray: Crkα

[0147] • Determination of diffraction plane: α-Fe211

[0148] • Entrance slit: Single collimator, 0.3mm in diameter

[0149] • Angle of incidence (ψ): 0°, 12.9°, 18.5°, 22.8°, 26.6°, 30.0°, 33.3°, 36.3°, 39.3°

[0150] • Angle of incidence (ψ): ψ P Shaft oscillation ±3°

[0151] • Method for determining diffraction angle: half-value width method

[0152] • Stress constant (K): -318 MPa / °

[0153] It should be noted that the residual stress measurement conditions shown below are all as described above.

[0154] Next, the test piece, whose residual stress on the outer surface was measured, was cut in half along the tube axis using electrical discharge machining (EDM) with wire cutting. With the residual stress measurement position set at 0° in the circumferential direction, the cutting position was set approximately ±90°. The thickness t of the outer surface at the cut surface and the center of the length direction of each halved sample was set to the range of D / 2 ± 0.2 mm.

[0155] Next, in the halved specimen, the circumferential residual stress σ was measured again at the location where the residual stress was measured on the outer surface before the halving. o2 Furthermore, after removing a layer of less than 10 μm from the inner surface of the test piece at its center along the length direction by electrolytic polishing after half-cutting, the residual circumferential stress σ at the center of the inner surface of the tube was measured. i2 .

[0156] The residual stress value σ obtained in this way o1 σ o2 σ i2 The values ​​are shown in Table 2. Substituting them into equations (i) to (iv), we obtain the estimated value σ of the residual stress on the inner surface before the half-cut after the self-tightening treatment. i1 .

[0157] Furthermore, internal pressure fatigue tests were conducted on the remaining test pieces to determine the ultimate internal pressure. In the internal pressure fatigue test, the pressure was repeatedly varied within a range from the maximum to the minimum of 18 MPa, exhibiting a sinusoidal wave pattern relative to time. The frequency of the internal pressure variation was set to 8 Hz. As a result of the internal pressure fatigue test, even if the number of repetitions reached 10... 7 The maximum internal pressure that does not cause damage (leakage) is evaluated as the ultimate internal pressure. The results are shown in Table 2.

[0158] The results in Table 2 clearly show that when comparing Test No. 1 and Test No. 2 of the normalized products, in Test No. 2, since no hardening treatment was performed and the hardness ratio was not greater than 1.20, even though self-tightening treatment was performed, sufficient compressive stress was not applied, and the ultimate internal pressure was not improved compared to Test No. 1, which did not undergo self-tightening treatment.

[0159] In contrast, when comparing the quenched products in Test No. 4 with Tests No. 5 and 6, the increased hardness of the outer surface layer resulted in sufficient residual stress imparted through the self-tightening treatment, leading to an increase in the ultimate internal pressure.

[0160] It should be noted that although Test No. 3 is a comparative example whose hardness ratio does not meet the requirements of this invention, its high D / d ratio of 2.2 ensured that residual stress was adequately imparted through self-tightening treatment, resulting in an increased ultimate internal pressure. Furthermore, in Test No. 7, the D / d ratio was the same as in Test No. 3, increasing the hardness of the outer surface region and adequately imparting residual stress through self-tightening treatment resulted in the same value as in Test No. 3. Therefore, for the ultimate internal pressure, Test No. 7 and Test No. 3 are also identical.

[0161] Industrial utilization potential

[0162] According to the present invention, pressure piping steel pipes with high ultimate internal pressure can be stably obtained. Therefore, the pressure piping steel pipes of the present invention are particularly suitable for use as hydraulic cylinders, airbag pipes, accumulators, hydrogen piping, fuel injection pipes, etc.

[0163] Explanation of reference numerals in the attached figures

[0164] 20 steel pipes

[0165] 20a Outer surface

[0166] 20b Inner Surface

[0167] 22 pairs of half-cut specimens

[0168] 22a Outer surface

[0169] 22b Inner Surface

[0170] 22c cut facet

[0171] 22d central part

[0172] 40 Analysis Model

[0173] 40a, 40b ends

[0174] 40c inner surface

[0175] 40d outer surface

Claims

1. A steel pipe for pressure piping which has been subjected to a self-tightening treatment, said steel pipe having an outer surface and an inner surface, an average hardness of an outer skin layer region at a depth position of 1 / 4 of a wall thickness from the outer surface is 1.20 times or more of an average hardness of an inner skin layer region at a depth position of 1 / 4 of the wall thickness from the inner surface, The outer diameter of the steel pipe is set as D, the inner diameter is set as d, the measured value of the residual stress of the outer surface of the steel pipe after the shrinkage fitting treatment is set as σ o1 , the measured value of the residual stress of the outer surface of the steel pipe after the shrinkage fitting treatment and half cutting is set as σ o2 , the measured value of the residual stress of the inner surface of the steel pipe after the shrinkage fitting treatment and half cutting is set as σ i2 , the measured value of the residual stress of the inner surface of the steel pipe after the shrinkage fitting treatment and half cutting is set as σ A residual stress estimation value σ of the inner surface of the steel pipe after the self-tightening treatment, which is calculated from the following (i) to (iv) i1 is -150 MPa or less, where D and d are in mm, and σ o1 , σ o2 , σ i2 , σ i1 is in MPa, σ i1 = (-σ i2 ) / (A x (t / T 2 - 1))... (i) t / T = ((σ o2 -σ o1 ) / (A x (σ o2 -σ o1 )-C x σ i2 )) 1 / 2 ···(ii) A = 3.9829 x exp(0.1071 x (D / d) 2 ) ··· (iii) C = -3.3966 x exp(0.0452 x (D / d)) 2 ) ··· (iv).

2. The steel pipe for pressure piping according to claim 1, wherein the average hardness of the outer skin layer region is 2.00 times or more of the average hardness of the inner skin layer region.

3. The steel pipe for pressure piping according to claim 1, wherein D / d is 2.0 or less.

4. The steel pipe for pressure piping according to claim 3, wherein D / d is 1.5 or more.

5. The steel pipe for pressure piping according to any one of claims 1 to 4, which has an ultimate internal pressure satisfying the following (I) formula, IP > 0.44 x TS x a ••• (I) a = [(D / d) 2 -1] / [0.776 x (D / d) 2 ]... (II) wherein, IP in the above (I) formula is an ultimate internal pressure of the steel pipe, TS is a tensile strength of the inner skin layer region of the steel pipe, and a is a value shown in the above (II) formula, D in the above (II) formula is an outer diameter of the steel pipe, and d is an inner diameter, wherein IP and TS have units of MPa, and D and d have units of mm.

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