Steel pipes for pressure piping and steel pipe materials

Abstract

Provided is a pressure piping steel pipe subjected to autofrettage, wherein the average hardness of an outer surface layer region is at least 1.20 times greater than the average hardness of an inner surface layer region, and, in the case in which it is assumed, regarding the steel pipe, that the outer diameter is D, the inner diameter is d, the actual measured value of the residual stress at the outer surface is σo1, the actual measured value of the residual stress at the outer surface after being halved is σo2, and the actual measured value of the residual stress at the inner surface after being halved is σi2, the estimated value σi1 of the residual stress at the inner surface of the steel pipe, determined from [σi1=(-σi2) / (A×(t / T)2-1)], [t / T=((σo2-σo1) / (A×(σo2-σo1)-C×σi2))1 / 2], [A=3.9829×exp(0.1071×(D / d)2)], and [C=-3.3966×exp(0.0452×(D / d)2)], is equal to or less than -150 MPa.

Description

[Technical field]

[0001] The present invention relates to a steel pipe for pressure piping and a steel pipe material. [Background technology]

[0002] Pressure piping such as hydraulic cylinders, airbag steel pipes, accumulators, and hydrogen piping requires not only high strength but also excellent internal pressure fatigue properties.

[0003] For example, Patent Document 1 discloses a method for producing a steel pipe for cylinder tubes having excellent internal pressure fatigue properties, characterized in that when producing the steel pipe for cylinder tubes by drawing, a heat treatment is performed at 300 to 350°C after drawing. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 4-183820 [Non-patent literature]

[0005] [Non-Patent Document 1] Eisuke Nakayama, Mitsuo Miyahara, Kazuo Okamura, Hiroki Fujimoto, Kiyoyuki Fukui, "Fatigue Strength Prediction of Automotive Thin Plate Spot Welded Joints Using Ultra-Small Test Pieces", Materials, October 2004, Vol. 53, No. 10, pp. 1136-1142 [Non-Patent Document 2] "Standards for X-Ray Stress Measurement (2002 Edition) - Iron and Steel Edition," edited by the Japan Society for Materials Science, March 2002 Summary of the Invention [Problem to be solved by the invention]

[0006] According to Patent Document 1, a steel pipe for cylinder tubes having excellent internal pressure fatigue properties can be obtained by improving the proportional limit strength. However, in recent years, there has been a demand for further improvement in internal pressure fatigue properties, and there remains room for improvement.

[0007] One method for improving the internal pressure fatigue properties is to carry out autofrettage, which is a process in which excessive internal pressure is applied to partially plastically deform the area near the inner surface, generating compressive residual stress.

[0008] Naturally, if the strength of the steel pipe material increases, the pressure applied to the inside of the steel pipe can be increased. However, the internal pressure at which fatigue failure occurs on the inner surface of the steel pipe when pressure is applied to the inside of the steel pipe (hereinafter referred to as the "critical internal pressure") does not depend only on the strength of the steel pipe material. By performing autofrettage treatment, it is possible to increase the critical internal pressure without increasing the tensile strength of the steel pipe.

[0009] However, in the autofrettage process, if the pressure applied to the inner surface of the steel pipe is too high, the risk of bursting increases, so for safety reasons, the pressure is generally set low. As a result, with conventional technology, even if the autofrettage process is performed, it is not possible to sufficiently increase the limit internal pressure, and there is still room for improvement.

[0010] In addition, in the case of thin-walled steel pipes, the stress difference between the inner and outer surfaces of the pipe becomes small, which may cause the entire steel pipe to yield, making it difficult to plastically deform only the area near the inner surface.

[0011] An object of the present invention is to solve the above problems and to provide a steel pipe for pressure piping and a steel pipe material having a high limit internal pressure. [Means for solving the problem]

[0012] The present invention has been made to solve the above problems, and is summarized as follows: a steel pipe for pressure piping and a steel pipe material.

[0013] (1) A steel pipe for pressure piping that has been subjected to autofrettage treatment, The steel pipe has an outer surface and an inner surface; The average hardness of an outer surface region from the outer surface to a depth position of 1 / 4 of the wall thickness is 1.20 times or more the average hardness of an inner surface region from the inner surface to a depth position of 1 / 4 of the wall thickness, The outer diameter of the steel pipe is D (mm) and the inner diameter is d (mm), The actual measured value of the residual stress on the outer surface of the steel pipe after autofrettage is σ o1 (MPa), and the actual measured value of the residual stress on the outer surface of the steel pipe after autofrettage and halving is σ o2 (MPa), and the actual measured value of the residual stress on the inner surface of the steel pipe after autofrettage and halving is σ i2 (MPa), The estimated residual stress σ of the inner surface of the steel pipe after autofrettage treatment is calculated by the following formulas (i) to (iv): i1 (MPa) is -150MPa or less, Steel pipe for pressure piping. σ i1 =(-σ i2 ) / (A×(t / T) 2 -1) (i) t / T=((σ o2 -σ o1 ) / (A×(σ o2 -σ o1 )-C×σ i2 )) 1 / 2 (ii) A = 3.9829 × exp(0.1071 × (D / d) 2 ) (iii) C = -3.3966 × exp(0.0452 × (D / d) 2 ) (iv)

[0014] (2) D / d is 2.0 or less. The steel pipe for pressure piping described in (1) above.

[0015] (3) A steel pipe material for pressure piping used in applications where autofrettage is performed, The steel pipe blank has an outer surface and an inner surface, The average hardness of an outer surface region from the outer surface to a depth position of 1 / 4 of the wall thickness is 1.20 times or more of the average hardness of an inner surface region from the inner surface to a depth position of 1 / 4 of the wall thickness, When the auto-stressing process is performed, The outer diameter of the steel pipe after autofrettage treatment is D (mm) and the inner diameter is d (mm). The actual measured value of the residual stress on the outer surface of the steel pipe after autofrettage is σ o1 (MPa), and the actual measured value of the residual stress on the outer surface of the steel pipe after autofrettage and halving is σ o2 (MPa), and the actual measured value of the residual stress on the inner surface of the steel pipe after autofrettage and halving is σ i2 (MPa), The estimated residual stress σ of the inner surface of the steel pipe after autofrettage treatment is calculated by the following formulas (i) to (iv): i1 (MPa) is -150MPa or less, Steel pipe material for pressure piping. σ i1 =(-σ i2 ) / (A×(t / T) 2 -1) (i) t / T=((σ o2 -σ o1 ) / (A×(σ o2 -σ o1 )-C×σ i2 )) 1 / 2 (ii) A = 3.9829 × exp(0.1071 × (D / d) 2 ) (iii) C = -3.3966 × exp(0.0452 × (D / d) 2 ) (iv)

[0016] (4) D / d is 2.0 or less. A steel pipe material for pressure piping as described in (3) above. Effect of the Invention

[0017] According to the present invention, it is possible to stably obtain a steel pipe for pressure piping having a high limit internal pressure. [Brief description of the drawings]

[0018] [Figure 1] FIG. 2 is a diagram showing an example of a steel pipe after autofrettage treatment, the residual stress of which is estimated by an estimation device. [Diagram 2] FIG. 13 is a diagram for explaining a method of deriving a multivariate function. [Diagram 3] FIG. 13 is a diagram for explaining a method of deriving a multivariate function. [Figure 4] FIG. 13 is a diagram for explaining a method of deriving a multivariate function. [Diagram 5] 1 is a diagram for explaining a method for determining an appropriate range for the hardness ratio between the outer surface layer region and the inner surface layer region, and the ratio between the outer diameter and the inner diameter of a steel pipe. FIG. [Figure 6] 1 is a diagram for explaining a method for determining an appropriate range for the hardness ratio between the outer surface layer region and the inner surface layer region, and the ratio between the outer diameter and the inner diameter of a steel pipe. FIG. [Figure 7] 1 is a diagram for explaining a method for determining an appropriate range for the hardness ratio between the outer surface layer region and the inner surface layer region, and the ratio between the outer diameter and the inner diameter of a steel pipe. FIG. [Figure 8] 1 is a diagram for explaining a method for determining an appropriate range for the hardness ratio between the outer surface layer region and the inner surface layer region, and the ratio between the outer diameter and the inner diameter of a steel pipe. FIG. [Figure 9] 1 is a diagram for explaining a method for determining an appropriate range for the hardness ratio between the outer surface layer region and the inner surface layer region, and the ratio between the outer diameter and the inner diameter of a steel pipe. FIG. [Figure 10] FIG. 2 is a diagram for explaining the shape of an internal pressure fatigue test piece. [Figure 11] FIG. 13 is a diagram for explaining the collection position and shape of a dumbbell-shaped small test piece. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] In the prior art, one of the reasons why the autofrettage pressure for sufficiently increasing the critical internal pressure could not be optimized is that a method for determining the residual stress on the inner surface of a steel pipe had not been established. Note that in this specification, the residual stress means the residual stress in the circumferential direction of the steel pipe.

[0020] Conventionally, the inventors have relatively evaluated the residual stress on the inner surface of a steel pipe after autofrettage treatment by cutting the steel pipe in half and measuring the residual stress on the inner surface of the steel pipe after halving. Note that cutting in half means cutting the steel pipe so that the steel pipe is divided into two equal arc-shaped members when viewed from the axial direction.

[0021] However, in order to optimize the autofrettage pressure and sufficiently increase the critical internal pressure, it is necessary to quantitatively evaluate the residual stress on the inner surface of the steel pipe after the autofrettage treatment and before halving. Therefore, the present inventors have been researching a method for quantitatively evaluating the residual stress on the inner surface of the steel pipe. In the course of this research, the present inventors have considered evaluating the residual stress on the inner surface of the steel pipe after the autofrettage treatment and before halving by considering the residual stress on the outer surface of the steel pipe before and after halving in addition to the residual stress on the inner surface of the steel pipe after halving.

[0022] The inventors first performed numerical analysis (FEM analysis) under various conditions using an analytical model of the steel pipe to be evaluated, and determined the residual stress (calculated value) generated in each part of the steel pipe by the autofrettage treatment. Specifically, the inventors first used numerical analysis to determine the residual stress σ o1 , the residual stress σ on the inner surface of the steel pipe after autofrettage treatment and before splitting in half i1 , Residual stress σ on the outer surface of the steel pipe after autofrettage and halving o2 , and the residual stress σ on the inner surface of the steel pipe after autofrettage and halving. i2 asked for.

[0023] As a result of a detailed study of the residual stresses obtained as described above, the inventors have concluded that the residual stress σ i1 is the residual stress σ on the outer surface of the steel pipe before splitting o1 , Residual stress σ on the outer surface of the steel pipe after splitting in half o2 , and the residual stress σ on the inner surface of the steel pipe after splitting in half i2 We found that it can be estimated with high accuracy using

[0024] The estimated residual stress on the inner surface of the steel pipe before splitting is σ i1The inventors have now discovered that by adjusting the autofrettage treatment conditions so that the above-mentioned required conditions are satisfied, it is possible to stably obtain steel pipes having a high limit internal pressure.

[0025] In addition, as described above, in the case of a thin-walled steel pipe, there is a problem that it is difficult to partially plastically deform only the vicinity of the inner surface. As a result of further investigations to solve this problem, the present inventors have found that, by increasing the hardness of the outer surface side of the steel pipe in advance relative to the hardness of the inner surface side, it is possible to make only the vicinity of the inner surface yield and impart residual stress to even a thin-walled steel pipe.

[0026] The present invention has been made based on the above findings. Each feature of the present invention will be described in detail below.

[0027] 1. Steel pipes for pressure piping and steel pipe materials A steel pipe according to one embodiment of the present invention is a steel pipe for pressure piping that has been subjected to auto-frettage treatment. Pressure piping includes hydraulic cylinders, airbag steel pipes, accumulators, hydrogen piping, fuel injection pipes, etc. A steel pipe material according to another embodiment of the present invention is a material for the above-mentioned steel pipe for pressure piping, and is used for applications in which auto-frettage treatment is performed. In other words, a steel pipe for pressure piping is obtained by performing auto-frettage treatment on a steel pipe material.

[0028] The average hardness of the outer surface layer region of the steel pipe material is 1.20 times or more the average hardness of the inner surface layer region. The reason for this definition will be described later. Here, the outer surface layer region refers to the region from the outer surface of the steel pipe material to a depth position of 1 / 4 of the wall thickness, and the inner surface layer region refers to the region from the inner surface of the steel pipe material to a depth position of 1 / 4 of the wall thickness.

[0029] By making the average hardness of the outer surface layer region of the steel pipe material 1.20 times or more the average hardness of the inner surface layer region, when the steel pipe material is subjected to autofrettage treatment, it becomes possible to plastically deform only the inner surface layer region without causing the outer surface layer region to yield, and to impart compressive residual stress. The average hardness of the outer surface layer region of the steel pipe material is preferably 1.50 times or more, and more preferably 2.00 times or more, the average hardness of the inner surface layer region.

[0030] Here, the hardness does not change significantly due to the autofrettage treatment, so the same relationship as for the steel pipe material holds for the steel pipe after the autofrettage treatment. That is, the average hardness in the outer surface layer region of the steel pipe is 1.2 times or more, preferably 1.5 times or more, and more preferably 2.0 times or more, of the average hardness in the inner surface layer region. Here, the outer surface layer region refers to the region from the outer surface of the steel pipe to a depth position of 1 / 4 of the wall thickness, and the inner surface layer region refers to the region from the inner surface of the steel pipe to a depth position of 1 / 4 of the wall thickness.

[0031] The average hardness of the inner surface layer region and the outer surface layer region of the steel pipe material or steel pipe is measured as follows. First, the Vickers hardness distribution in the cross section of the steel pipe material or steel pipe is measured in accordance with JIS Z 2244:2009 (Vickers hardness test - Test method). The cross section may be perpendicular to the axial direction of the steel pipe material or steel pipe, or parallel to the axial direction and passing through the central axis.

[0032] A general-purpose micro Vickers hardness tester is used for the hardness test, and the test force is set to 1 to 10 N depending on the dimensions of the steel pipe. Measurement points are spaced radially from the inner surface to the outer surface at a distance of 1 / 10 to 1 / 20 of the wall thickness on the mirror-polished observation surface. If the indentations are close to each other, the measurement positions may be shifted in a direction perpendicular to the radial direction, and measurements may be taken at staggered positions. From the hardness distribution thus obtained, the hardness values ​​contained in the inner surface layer region and the outer surface layer region are averaged to obtain the average hardness in the inner surface layer region and the outer surface layer region.

[0033] There is no particular restriction on the method for increasing the average hardness in the outer surface layer region of the steel pipe material (hereinafter also referred to as "hardening treatment"). For example, there is a method of performing high-frequency hardening from the outer surface side of the steel pipe material. Specifically, it is possible to increase only the hardness of the outer surface layer region by instantaneously heating the outer surface to a temperature of 900°C or higher by high-frequency heating and immediately cooling with water. If necessary, the steel pipe material after high-frequency hardening may be subjected to a tempering treatment in which it is held at 100 to 300°C for 30 minutes or more and then air-cooled.

[0034] Other methods that may be adopted include a method of subjecting the outer surface of a steel pipe material to shot peening or rolling to work-harden only the outer surface region, or a method of subjecting the outer surface to carburizing, nitriding or carbonitriding.

[0035] In the present invention, there is no particular restriction on the dimensions of the steel pipe. In general, in order to withstand high internal pressure, it is desirable to increase the wall thickness of the steel pipe as the inner diameter increases. If the inner diameter of the steel pipe is constant, the outer diameter of the steel pipe increases as the wall thickness increases. That is, in order to withstand high internal pressure, it is desirable to increase the outer diameter of the steel pipe as the inner diameter increases. However, as described above, the effect of the present invention is more remarkable in thin-walled steel pipes. Therefore, in the present invention, when the outer diameter of the steel pipe is D (mm) and the inner diameter is d (mm), D / d may be 2.0 or less, 1.9 or less, or 1.8 or less.

[0036] Other dimensions may be selected according to the application, and no particular restrictions need to be imposed. For example, when the steel pipe is used as a hydraulic cylinder, it is preferable to select the inner diameter according to the operating pressure (internal pressure) from the range of 15 to 580 mm in order to ensure the output (load) of the piston. Also, it is preferable to select the wall thickness from the range of 5 to 60 mm and the outer diameter from the range of 30 to 700 mm so as to be able to withstand repeated internal pressure. The higher the internal pressure fatigue strength, the thinner the wall thickness can be, and the outer diameter is determined accordingly.

[0037] When the steel pipe is used as an airbag inflator, the outer diameter of the steel pipe is preferably 20 to 100 mm, more preferably 20 to 60 mm, and the wall thickness of the steel pipe is preferably 1 to 5 mm, more preferably 1 to 4 mm.

[0038] When the steel pipe is used as an accumulator, the outer diameter of the steel pipe is preferably 25 to 500 mm, more preferably 50 to 400 mm, and the wall thickness of the steel pipe is preferably 2 to 40 mm, more preferably 4 to 30 mm.

[0039] Furthermore, when steel pipes are used as hydrogen gas piping or fuel injection pipes, a certain amount of 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, and more preferably 3.0 mm or more. Also, since it is necessary to withstand high internal pressure, the wall thickness of the steel pipe is preferably 1.5 mm or more, and 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.

[0040] The mechanical properties may also be selected according to the application, and no particular restrictions need to be imposed. However, for use as pressure piping, the tensile strength of the steel pipe material before hardening treatment, or the tensile strength of the inner surface layer region of the steel pipe after hardening treatment and the subsequent autofrettage 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.

[0041] The hardness of the steel pipe material before hardening treatment, or the hardness of the inner surface layer region of the steel pipe after hardening treatment and the subsequent autofrettage treatment, is preferably 150 HV or more, more preferably 180 HV or more, and even more preferably 220 HV or more in Vickers hardness. Although there is no need to set an upper limit to the above hardness, if the hardness is excessive, it becomes difficult to introduce compressive residual stress by autofrettage treatment, so that the Vickers hardness is preferably less than 500 HV.

[0042] The yield ratio is preferably 0.50 to 0.95, and in order to perform the autofrettage treatment at a higher pressure and obtain a large compressive residual stress, the yield ratio is more preferably 0.60 or more, and even more preferably 0.70 or more. In order to more efficiently introduce compressive residual stress by the autofrettage treatment at a low pressure, the yield ratio is more preferably 0.90 or less, and even more preferably 0.85 or less.

[0043] In the present invention, the mechanical properties of the inner surface layer region of a steel pipe can be measured by cutting a small dumbbell-shaped test piece with a thickness of about 0.2 mm by electrical discharge machining so as to be in contact with the inner surface of the steel pipe, and performing a tensile test, as shown in Non-Patent Document 1. Strain measurement in the tensile test is performed according to the method shown in Non-Patent Document 1, by converting the actuator displacement (stroke) of the tensile tester and the parallel part length of the test piece.

[0044] If a steel pipe material before hardening is available, its mechanical properties are determined by cutting out a straight pipe section of the steel pipe, chucking a certain length of the area (hereinafter referred to as the "gripped section") from both end faces, attaching an extensometer to the parallel section between the gripped sections, and conducting a tensile test. Chucking is performed by pressing the gripped section with a contact piece provided with a V-groove or R-groove shallower than the outer radius of the steel pipe, using hydraulic pressure, bolt tightening, or a wedge jig. The length of the gripped section may be determined in consideration of the pressing pressure and the test load so that the test steel pipe does not slip during the test. The length of the parallel section may be secured to a length that allows an extensometer to be attached and that does not affect the necking deformation immediately before breakage by the chuck. If the steel pipe does not have a straight pipe section of sufficient length, a tensile test may be performed by cutting out a small test piece in a thin-walled dumbbell shape as shown in Non-Patent Document 1.

[0045] Moreover, the steel pipe for pressure piping according to the present invention preferably has a limit internal pressure which satisfies the following formula (I). IP≧0.44×TS×α (I) α = [(D / d) 2 -1] / [0.776×(D / d) 2 ] ···(II) In the above formula (I), IP is the limit internal pressure of the steel pipe (MPa), TS is the tensile strength (MPa) of the inner surface layer region of the steel pipe or the steel pipe material, and α is the value expressed by the above formula (II). In addition, D in the above formula (II) is the outer diameter (mm) of the steel pipe, and d is the inner diameter (mm). α is a coefficient that corrects for the change in the relationship between the internal pressure and the stress generated on the inner surface of the pipe depending on the ratio of the outer diameter to the inner diameter of the pipe.

[0046] In the present invention, the limit internal pressure is defined as the minimum internal pressure of 18 MPa in an internal pressure fatigue test, and the internal pressure is repeatedly changed to a sinusoidal wave with respect to time. The number of cycles is 10 7 It means the maximum internal pressure (MPa) at which no breakage (leakage) occurs even after 10 times. Specifically, on an SN diagram with the maximum internal pressure on the vertical axis and the number of breakage repetitions on the horizontal axis, the minimum value of the maximum internal pressure at which breakage occurs and the number of breakage repetitions on the horizontal axis are calculated. 7 The median value of the maximum values ​​at which the specimen did not break even after 1000 times is set as the critical internal pressure.

[0047] 2. Residual stress estimation model Residual stress σ on the inner surface of a steel pipe before splitting in half i1 A model for estimating the residual stress will be described in detail. FIG. 1 is a diagram showing an example of a steel pipe after autofrettage treatment, in which the residual stress is estimated by this model. In FIG. 1, (a) is a left side view of a steel pipe 20 that has been subjected to autofrettage treatment, (b) is a front view of the steel pipe 20 shown in (a), (c) is a left side view of a half-split sample 22 obtained by cutting the steel pipe 20 shown in (a) in half, and (d) is a front view of the half-split sample 22 shown in (c). In this specification, the steel pipe after halving means a half-split sample obtained by cutting a steel pipe after autofrettage treatment in half.

[0048] In this model, the residual stress σ o1 , the residual stress σ of the outer surface 22a of the half specimen 22 o2 , and the residual stress σ i2 As described above, the residual stress means the residual stress in the circumferential direction of the steel pipe 20.

[0049] Referring to FIG. 1, the length L of the steel pipe 20 when measuring the residual stress is preferably three times or more the outer diameter D of the steel pipe 20, and can be, for example, about 30 mm. If the heat generated during cutting the steel pipe 20 in half is excessive, it will affect the residual stress on the inner surface. For this reason, it is necessary to adopt a cutting method that generates as little heat as possible, and it is preferable to cut the steel pipe in half by wire-cut electric discharge machining. In this case, in a side view of the half-split sample 22, the distance X between the cut surface 22c of the half-split sample 22 and the center of the outer surface 22a (the distance in the direction perpendicular to the cut surface 22c) is controlled to be within ±5% of the radius r of the steel pipe 20.

[0050] The residual stress is measured after removing the surface layer of the outer surface 20a of the steel pipe 20 and the inner surface 22b of the half-split sample 22 by electrolytic polishing to a range of 10 μm or less. 2 The ψ method can be used, and can be carried out in accordance with Non-Patent Document 2.

[0051] Residual stress σ measured by the above method o1 ,σ o2 ,σ i2 , and the residual stress σ i1 Calculate the estimate of .

[0052] Specifically, as shown in Fig. 2(a), a circular arc-shaped analytical model 40 (1 / 4 model) is created by modeling the cross section (cross section perpendicular to the pipe axis direction) of the steel pipe 20 using two-dimensional plane strain elements. Although not shown in the figure, the analytical model 40 is divided into multiple elements (meshes). The physical properties of the analytical model 40 are assumed to be elastic.

[0053] First, in order to simulate the steel pipe 20, as shown in Fig. 2(b), constraint conditions are set so as to restrict the circumferential movement of both ends 40a, 40b of the analysis model 40. Then, as the initial state, a volume force simulating the state of the steel pipe 20 during autofrettage is set. Specifically, in the initial state, a circumferential compressive residual stress (-100 MPa) is applied to the inner surface 40c of the analysis model 40.

[0054] In addition, in the initial state, no stress is generated in the region between the outer surface 40d and a position P (position indicated by a dashed arc-shaped line) radially away from the inner surface 40c of the analysis model 40. Furthermore, in the initial state, the stress distribution in the region between the inner surface 40c and the position P is linearly distributed so that the compressive stress gradually decreases from the inner surface 40c toward the position P. Note that FIG. 2(b) and FIGS. 3 and 4 described later show the stress state at the end 40b. In the following, the radial distance between the point P1 on the end 40b where the compressive stress becomes 0 and the inner surface 40c is referred to as the distance t, and the thickness of the analysis model 40 is referred to as the thickness T. Note that, when there are multiple points on the end 40b where the compressive stress becomes 0, the point closer to the inner surface 40c is referred to as P1.

[0055] After the volume force is set as described above, elastic analysis is performed to redistribute the stress. This changes the stress state of the analysis model 40, for example, as shown in FIG. 3. In FIG. 3, the positions where the stress is zero are indicated by dashed lines. In the region inside the dashed lines, circumferential compressive stress occurs, and in the region outside the dashed lines, circumferential tensile stress occurs. In the state shown in FIG. 3, the integral value of the entire stress distribution of the analysis model 40 is zero. The stress state shown in FIG. 3 corresponds to the stress state of the steel pipe 20 after autofrettage treatment. In the state shown in FIG. 3, the stress at the intersection between the inner surface 40c and the end portion 40b is expressed as the residual stress σ of the inner surface 20b of the steel pipe 20 after autofrettage treatment. i1 The stress at the intersection of the outer surface 40d and the end 40b is calculated as the residual stress σ o1 Obtain as.

[0056] Next, in order to simulate the halved sample 22 (steel pipe 20 after halving), as shown in FIG. 4, the end portion 40a is released from constraint and elastic analysis is performed. This causes a further change in the stress state of the analysis model 40. Note that in FIG. 4, positions where the stress is zero are indicated by dashed lines. In the example shown in FIG. 4, in the analysis model 40, circumferential tensile stress is generated in the radial center, 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.

[0057] 4, the end 40a corresponds to the cut surface 22c (see FIG. 1) of the half sample 22, and the end 40b corresponds to the central portion 22d (see FIG. 1) in the circumferential direction of the half sample 22. In the state shown in FIG. 4, the stress at the intersection of the inner surface 40c and the end 40b is defined as the residual stress σ i2 The stress at the intersection of the outer surface 40d and the end portion 40b is taken as the residual stress σ o2 Obtain as.

[0058] For a steel pipe 20 of any dimensions, the above analysis described in Figures 2(b), 3 and 4 is performed by changing the distance t in the initial state (i.e., by changing the position of point P1 where the compressive stress becomes 0 in Figure 2(b)).

[0059] As a result of various investigations by the present inventors, it has become possible to obtain a satisfactory result by determining the thickness T of the steel pipe 20, the distance t in the steel pipe 20 after the autofrettage treatment obtained as described above (see FIG. 3), and the residual stress σ o1 (See FIG. 3), the residual stress σ i1 (See FIG. 3 ), the residual stress σ of the outer surface 22a of the half-split specimen 22 o2 (see FIG. 4), and the residual stress σ i2 (See Figure 4) It was found that there is a certain relationship between them.

[0060] Specifically, the inventors have determined that the (t / T) of the steel pipe 20 after the autofrettage treatment 2 and the value of (σ i2 / -σ i1 ) and (t / T) 2 and the value of (σ i2 / -σ i1 The relationship between the value of and the value of were linearly approximated by the least squares method to obtain the following equation (1). In the following equation (1), A and B are coefficients. σ i2 / (-σ i1 )=A×(t / T) 2 -B (1)

[0061] The above formula (1) is σ i1 By rearranging the above, the following formula (i) is obtained. Note that, in the present invention, A is a value represented by the following formula (iii), and B is 1. σ i1 =(-σ i2 ) / (A×(t / T) 2 -1) (i) A = 3.9829 × exp(0.1071 × (D / d) 2 ) (iii)

[0062] In addition, the inventors have determined that the (t / T) of the steel pipe 20 after the autofrettage treatment 2 and the value of ((σ o2 -σ o1 ) / (-σ i1 We also found that there is a certain correlation between the value of (t / T) 2 and the value of ((σ o2 -σ o1 ) / (-σ i1 The relationship between the value of and the value of were linearly approximated by the least squares method to obtain the following equation (2): In the following equation (2), C and E are coefficients. (σ o2 -σ o1 ) / (-σ i1 )=-C×(t / T) 2 -E (2)

[0063] From the above equations (1) and (2), (t / T) can be expressed by the following equation (3). t / T=((B×(σ o2 -σ o1 )-E×σ i2 ) / (A×(σ o2 -σ o1 )-C×σ i2 )) 1 / 2 (3)

[0064] Furthermore, in the present invention, B is 1, E is 0, and the following formula (ii) is obtained: C is a value represented by the following formula (iv). t / T=((σ o2 -σ o1 ) / (A×(σ o2 -σ o1 )-C×σ i2 )) 1 / 2 (ii) C = -3.3966 × exp(0.0452 × (D / d) 2 ) (iv)

[0065] From the formulas (i) to (iv) obtained from the above estimation model, the estimated residual stress σ of the inner surface 20b of the steel pipe 20 after the autofrettage treatment is calculated. i1 can be calculated.

[0066] And, the steel pipe according to the present invention has σ i1 If the compressive residual stress exceeds -150 MPa, that is, if the absolute value of the residual stress is less than 150 MPa, the effect of improving the critical internal pressure cannot be obtained, as shown in the examples described later. i1 By setting the internal pressure at -150 MPa or less, a high limit internal pressure can be obtained.

[0067] 3. Consideration of the appropriate range of the hardness ratio of the outer surface layer region / inner surface layer region and the outer diameter / inner diameter ratio of the steel pipe As described above, in the present invention, the appropriate ranges are set to 1.20 or more for the ratio Hvo / Hvi of the average hardness Hvo in the outer surface layer region of the steel pipe material to the average hardness Hvi in ​​the inner surface layer region, and 2.0 or less for the ratio D / d of the outer diameter D and the inner diameter d of the steel pipe material. The basis for this is the numerical calculation based on the FEM analysis shown below.

[0068] The analysis target was the steel pipe shape shown in Fig. 1, and the analytical model of a quarter cylinder shape using three-dimensional hexahedral secondary elements shown in Fig. 5(a) was used for the FEM analysis. Four types of shapes with D / d of 1.2, 1.5, 1.8, and 2.0 were used for the analytical model.

[0069] The physical properties of the model were assumed to be elastoplastic. The Young's modulus in the elastic region was 205.8 GPa, and the Poisson's ratio was 0.3. In the elastoplastic region, a stress-strain curve was used based on the true stress and true plastic strain, an example of which is shown in Figure 6. There are multiple stress-strain curves in the figure depending on the Vickers hardness. This is an approximation of the hardness dependency of the stress-strain curve based on the results of preliminary tests using carbon steel with different hardness levels. The kinematic hardening law was used as the hardening law for the elastoplastic region. In the analysis model, different stress-strain curves were set according to the hardness distribution depending on the radial position, as shown in Figure 7. There are three types of Hvo / Hvi: 1.00, 1.20, and 1.75.

[0070] To analyze the state where internal pressure is applied, a radial stress σr equivalent to the internal pressure P is applied to the inner surface of the model as shown in Fig. 5(b), and an axial stress σax is simultaneously applied to the cross section of the model. σr and σax are given by the following equation using the inner diameter d. σr=-P (4) σax=P×d 2 / (D 2 -d 2 ) ···(5) Here, the above formula (5) is a basic formula for a cylindrical vessel, and is based on the following idea. If the analysis target is assumed to be a closed cross-section member, the axial load P×πd 2 Since this axial load acts on the cross section of the model, the axial stress σax is calculated by multiplying the axial load by the cross-sectional area π(D 2 -d 2 ) / 4.

[0071] First, an analysis was conducted to reproduce the burst test. First, σr and σax in equations (4) and (5) above were applied to the model, gradually increasing linearly from zero. The analysis was then terminated when the entire model underwent plastic deformation and reached a limit where the analysis did not converge. The internal pressure at this limit was taken as the burst strength obtained by the analysis. It was confirmed in a preliminary test that the burst strength obtained from the experiment corresponded to that obtained from the analysis.

[0072] Next, to reproduce the autofrettage process, σr and σax corresponding to internal pressures of 0.60, 0.70, and 0.85 times the burst strength were applied to the model, and the load was removed. The circumferential residual stress on the inner surface after removal of the load was output, and the minimum value (basically a compressive residual stress, so the absolute value is the maximum value) was taken as the representative value of the residual stress introduced by the autofrettage process.

[0073] Regarding the residual stress obtained by the autofrettage treatment, the relationship between Hvo / Hvi and D / d is shown in Fig. 8 and Fig. 9, respectively. In Fig. 8, when D / d is 1.2, the compressive residual stress on the inner surface is -150MPa or more when Hvo / Hvi is 1.20 or more. It can also be seen that when D / d is 1.8, the compressive residual stress does not change when Hvo / Hvi is 1.20 or more.

[0074] In FIG. 9, when Hvo / Hvi is 1.20, the compressive residual stress on the inner surface is -150MPa or more when D / d is 1.2 or more. The compressive residual stress increases with increasing D / d, but when Hvo / Hvi is 1.75, it does not increase when D / d is 1.5 or more and is saturated at a constant value. When Hvo / Hvi is 1.20, when D / d is 1.8 or more, it becomes equal to the saturated value of the compressive residual stress when Hvo / Hvi is 1.75. When D / d is further increased from 1.8, the difference between the compressive residual stress when Hvo / Hvi is 1.00 and that when Hvo / Hvi is 1.20 or more becomes small. And when D / d is 2.0, the difference between the compressive residual stress when Hvo / Hvi is 1.00 and that when Hvo / Hvi is 1.20 or more becomes small, and it can be said that the effect of reducing the compressive residual stress after the autofrettage treatment by the hardening treatment is small in the region where D / d exceeds 2.0.

[0075] Based on the above, it has been determined that the appropriate range is that the ratio Hvo / Hvi of the average hardness Hvo in the outer surface layer region of the hardened steel pipe material to the average hardness Hvi in ​​the inner surface layer region should be 1.20 or more, and that the ratio D / d of the outer diameter D to the inner diameter d of the steel pipe material should be 2.0 or less.

[0076] 3. Manufacturing method There is no particular limitation on the method for producing a steel pipe for pressure piping according to the present invention. For example, a plurality of steel pipe materials in which the average hardness in the outer surface layer region is 1.20 times or more the average hardness in the inner surface layer region are subjected to autofrettage treatment under various conditions, and the obtained steel pipes are subjected to σ i1 The steel pipes can be manufactured by carrying out a process of selecting steel pipes with a compressive strength of -150 MPa or less.

[0077] Regarding the autofrettage treatment conditions, for example, the autofrettage treatment pressure and / or the autofrettage treatment time can be controlled to reduce σ i1 As described above, by accurately estimating the residual stress on the inner surface of the steel pipe after autofrettage treatment and before halving, it is possible to optimize the autofrettage treatment conditions and stably obtain steel pipes with a high limit internal pressure.

[0078] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. EXAMPLES

[0079] Steel having 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 subjected to normalizing treatment by heating to 880°C to prepare a test piece. The yield stress of the test piece was 382 MPa and the tensile strength was 621 MPa.

[0080] [Table 1]

[0081] The above test piece material was then subjected to rough machining, heat treatment, and finish machining to obtain multiple internal pressure fatigue test pieces having the shape shown in Fig. 10. Here, the shape after rough machining was 1 mm larger in outer diameter and 1 mm smaller in inner diameter than the shape after finish machining in Fig. 10. The unit of length shown in Fig. 10 is mm.

[0082] Of the obtained internal pressure fatigue test pieces (Test Nos. 1 to 7), Test Nos. 1 to 3 were used as they were (normalized products), while Test Nos. 4 to 7 were further subjected to induction hardening and tempering treatment in which the outer surface was instantaneously heated to 1000°C by high-frequency heating, immediately quenched, then heated to 150°C, held for 1 hour, and air-cooled (hardened products). Then, final finishing was performed, and the inner and outer surfaces of the test parts were polished. In the test piece shape shown in Figure 10, the outer diameter of Test Nos. 1, 2, and 4 to 6 was 9.0 mm (D / d: 1.5), and the outer diameter of Test Nos. 3 and 7 was 13.2 mm (D / d: 2.2).

[0083] Furthermore, autofrettage was performed on test pieces No. 2, 3, and 5 to 7 at the pressures shown in Table 2. Autofrettage was performed by sealing one end face of the internal pressure fatigue test piece in Fig. 10, sealing the inside of the piece from the other end face with hydraulic oil as a pressure medium, and controlling the internal pressure of the enclosed part. Autofrettage is performed by increasing the internal pressure of the enclosed part to the autofrettage pressure and then removing the load.

[0084] [Table 2]

[0085] Six test pieces were prepared for each test number, and one of them was used to measure the average hardness of the outer surface layer region and the inner surface layer region, and to perform a tensile test on the inner surface layer region. In the hardness measurement, a cross section parallel to the axial direction of the test piece and passing through the central axis was cut out from the test piece test part, and this cross section was embedded in resin so that it would be the observation surface, and was mirror-polished with emery paper and then buffed. A general-purpose micro Vickers hardness tester was used as the hardness tester, and the test force was 3N. The measurement was performed at a pitch of 0.1 mm in the radial direction from the inner surface to the outer surface on the above observation surface. That is, the measurement points were 14 points in total for test Nos. 1, 2, and 4 to 6, and 35 points in total for test Nos. 3 and 7, and Vickers indentations were introduced at the corresponding positions. From the hardness distribution thus obtained, the hardness data of three points in the inner surface layer region and three points in the outer surface layer region were averaged to obtain the average hardness of the inner surface layer region and the outer surface layer region.

[0086] As shown in Table 2, the hardness of the outer and inner surface regions of Test Nos. 1 to 3 and the hardness of the inner surface region of Test Nos. 4 to 7 were in the range of 196 to 205 HV, and no change was observed due to the presence or absence of hardening treatment or the presence or absence of autofrettage treatment. The hardness of the outer surface region of Test Nos. 4 to 7 was 433 to 445 HV, and Hvo / Hvi was 2.17 to 2.24.

[0087] As shown in FIG. 11, two small dumbbell-shaped test pieces with a thickness of about 0.2 mm were cut out from the inner surface layer region of the internal pressure fatigue test piece test part by electric discharge machining so as to be in contact with the inner surface of the internal pressure fatigue test piece test part, and the tensile test was performed using these. The tensile tester used was a Tytron 250 manufactured by MTS. Strain was measured by converting the tensile tester actuator displacement (stroke) and the parallel part length of the test piece according to the method shown in Non-Patent Document 1. In the stress-strain curve thus obtained, the 0.2% proof stress was defined as the yield stress, the maximum stress was defined as the tensile strength, and the average value of the two small test pieces was defined as the measured value.

[0088] As shown in Table 2, in the inner surface layer regions of Test Nos. 1 to 7, the tensile strength was 621 to 632 MPa, the yield stress was 382 to 391 MPa, and the yield ratio was 0.61 to 0.62, and no changes due to the hardening treatment and autofrettage treatment were observed.

[0089] The other test piece was used for measuring residual stress. First, the surface layer of the outer surface at the longitudinal center of the test piece was removed by electrolytic polishing to a depth of 10 μm or less, and then the circumferential residual stress σ o1 The measurement method is sin by X-ray diffraction. 2 The ψ method was used and was performed in accordance with Non-Patent Document 2. Detailed measurement conditions are as follows.

[0090] ·Scanning method: Lateral tilt method, constant η method (PSPC method) X-ray stress measurement device: Rigaku PSPC-RSF ·Characteristic X-ray: Crkα Measurement diffraction surface: α-Fe211 Entrance slit: Single collimator, diameter 0.3mm ·Incidence angle (ψ): 0°, 12.9°, 18.5°, 22.8°, 26.6°, 30.0°, 33.3°, 36.3°, 39.3° ·Incidence angle (ψ):ψ P Axis swing ±3° Diffraction angle determination method: Half-value width method Stress constant (K): -318MPa / °

[0091] The residual stress measurement conditions described below were all the same as those described above.

[0092] Next, the test pieces for which the residual stress on the outer surface was measured were cut in half in the axial direction of the tube by wire-cut electric discharge machining. The cutting position was set at approximately ±90° with the outer surface residual stress measurement position set at 0° in the circumferential direction. The thickness t of each sample from the cut surface to the outer surface at the center position in the longitudinal direction after halving was in the range of D / 2±0.2 mm.

[0093] Then, in the specimen after halving, the circumferential residual stress σ o2 After splitting the specimen in half, the surface layer of the inner surface at the center of the longitudinal direction of the specimen was removed by electrolytic polishing to a range of 10 μm or less, and the circumferential residual stress σ i2 was measured.

[0094] The residual stress measurement value thus obtained is σ o1 , σ o2 , σ i2 The values ​​are shown in Table 2. By substituting these values ​​into equations (i) to (iv), the estimated inner surface residual stress before the second half of the autofrettage treatment, σ i1 was obtained.

[0095] Furthermore, an internal pressure fatigue test was conducted on the remaining test pieces to clarify the limit internal pressure. In the internal pressure fatigue test, the internal pressure was repeatedly changed in a range from the maximum internal pressure to a minimum of 18 MPa, so as to form a sine wave with respect to time. The frequency of the internal pressure change was 8 Hz. As a result of the internal pressure fatigue test, the number of cycles was 10 7The maximum internal pressure at which damage (leakage) does not occur even after 1000 times the pressure was evaluated as the limit internal pressure. The results are also shown in Table 2.

[0096] As is clear from the results in Table 2, when the normalized products of Test No. 1 and Test No. 2 are compared, in Test No. 2, the hardness ratio was not made 1.20 or more because the hardening treatment was not performed, and therefore sufficient compressive stress was not imparted to Test No. 2, even though the autofrettage treatment was performed, and the critical internal pressure could not be improved compared to Test No. 1, which was not subjected to the autofrettage treatment.

[0097] In contrast, when comparing the hardened product test No. 4 with test Nos. 5 and 6, it was found that the hardness of the outer surface layer region was increased, and therefore sufficient residual stress was imparted by the autofrettage treatment, resulting in an improvement in the limiting internal pressure.

[0098] Test No. 3 is a comparative example in which the hardness ratio does not satisfy the specification of the present invention, but since D / d is high at 2.2, sufficient residual stress is imparted by the autofrettage treatment, resulting in an improvement in the critical internal pressure. Test No. 7 has the same D / d as Test No. 3, and the hardness of the outer surface layer region is increased, and sufficient residual stress is imparted by the autofrettage treatment, but the value is the same as that of Test No. 3. Therefore, the critical internal pressure was also the same in Test No. 7 and Test No. 3. [Industrial Applicability]

[0099] According to the present invention, it is possible to stably obtain a steel pipe for pressure piping having a high limit internal pressure. Therefore, the steel pipe for pressure piping according to the present invention can be suitably used particularly as a hydraulic cylinder, an airbag steel pipe, an accumulator, a hydrogen pipe, a fuel injection pipe, etc. [Explanation of symbols]

[0100] 20 Steel pipe 20a Exterior 20b Inner surface 22 Half-split sample 22a Exterior 22b Inside 22c cutting surface 22d center 40 Analysis Model 40a,40b end 40c Inner surface 40d exterior

Claims

1. A steel pipe for pressure piping that has been subjected to autofrettage treatment, The steel pipe has an outer surface and an inner surface; The average hardness of an outer surface layer region from the outer surface to a depth position of 1 / 4 of the wall thickness is 1.20 times or more of the average hardness of an inner surface layer region from the inner surface to a depth position of 1 / 4 of the wall thickness, The outer diameter of the steel pipe is D (mm) and the inner diameter is d (mm), The actual measured value of the residual stress on the outer surface of the steel pipe after autofrettage is σ ｏ１ (MPa), the actual measured value of the residual stress on the outer surface of the steel pipe after autofrettage treatment and halving is σ ｏ２ (MPa), the measured value of the residual stress on the inner surface of the steel pipe after autofrettage treatment and halving is σ ｉ２ When (MPa), The estimated residual stress value σ of the inner surface of the steel pipe after autofrettage treatment is calculated by the following formulas (i) to (iv): ｉ１ (MPa) is -150 MPa or less, Steel pipe for pressure piping. s ｉ１ =(-s ｉ２ ) / (A×(t / T) ２ -1) ・・・(i) t / T=((σ ｏ２ -s ｏ１ ) / (A×(σ ｏ２ -s ｏ１ )-C×σ ｉ２ )) １／２ ・・・(ii) __________×_________________ ２ ) ・・・(iii) C=-3.3966×exp(0.0452×(D / d) ２ ) ・・・(iv)

2. D / d is 2.0 or less; The steel pipe for pressure piping according to claim 1.

3. D / d is 1.5 or more, The steel pipe for pressure piping according to claim 2.

4. Having a limit internal pressure that satisfies the following formula (I): The steel pipe for pressure piping according to claim 1 or 2. IP≧0.44×TS×α...(I) α=[(D / d) 2 -1] / [0.776×(D / d) 2]...(II) In the above formula (I), IP means the limit internal pressure (MPa) of the steel pipe, TS means the tensile strength (MPa) of the inner surface region of the steel pipe, α is a value expressed by the above formula (II), and D in the above formula (II) means the outer diameter (mm) and d means the inner diameter (mm) of the steel pipe.

5. A steel pipe material for pressure piping used in applications where autofrettage is performed, The steel pipe blank has an outer surface and an inner surface, The average hardness of an outer surface layer region from the outer surface to a depth position of 1 / 4 of the wall thickness is 1.20 times or more of the average hardness of an inner surface layer region from the inner surface to a depth position of 1 / 4 of the wall thickness, When the auto-stressing process is performed, The outer diameter of the steel pipe after the autofrettage treatment is D (mm) and the inner diameter is d (mm), The actual measured value of the residual stress on the outer surface of the steel pipe after autofrettage is σ ｏ１ (MPa), the actual measured value of the residual stress on the outer surface of the steel pipe after autofrettage treatment and halving is σ ｏ２ (MPa), the measured value of the residual stress on the inner surface of the steel pipe after autofrettage treatment and halving is σ ｉ２ When (MPa), The estimated residual stress value σ of the inner surface of the steel pipe after autofrettage treatment is calculated by the following formulas (i) to (iv): ｉ１ (MPa) is -150 MPa or less, Steel pipe material for pressure piping. s ｉ１ =(-s ｉ２ ) / (A×(t / T) ２ -1) ・・・(i) t / T=((σ ｏ２ -s ｏ１ ) / (A×(σ ｏ２ -s ｏ１ )-C×σ ｉ２ )) １／２ ・・・(ii) __________×_________________ ２ ) ・・・ (iii) C=-3.3966×exp(0.0452×(D / d) ２ ) ・・・(iv)

6. D / d is 2.0 or less; The steel pipe material for pressure piping according to claim 5.

7. D / d is 1.5 or more. The steel pipe material for pressure piping according to claim 6.

8. When the autofrettage treatment is performed, the limit internal pressure satisfies the following formula (I): A steel pipe material for pressure piping according to claim 5 or 6. IP≧0.44×TS×α...(I) α=[(D / d) 2 -1] / [0.776×(D / d) 2]...(II) In the above formula (I), IP means the limit internal pressure (MPa) of the steel pipe after autofrettage treatment, TS means the tensile strength (MPa) of the inner surface region of the steel pipe after autofrettage treatment, α is a value expressed by the above formula (II), D in the above formula (II) means the outer diameter (mm) of the steel pipe after autofrettage treatment, and d means the inner diameter (mm).

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