Patent Application
20230140650
The present invention relates to a steel pipe for pressure piping.
As countermeasures against energy exhaustion in the future, the movement for promoting energy-saving, the movement for recycling resources, and the development of technologies to achieve these goals have gained momentum. In particular, there have recently been strong demands for the reduction of CO2 emissions with fuel combustion to prevent global warming, as worldwide efforts.
Internal combustion engines with low CO2 emissions include diesel engines used in an automobile or the like. However, although emitting less CO2, a problem with diesel engines is their generation of black smoke. Black smoke is produced due to a lack of oxygen with respect to injected fuel. In other words, part of the fuel is thermally decomposed, which causes dehydrogenation, producing a precursor of black smoke, and the precursor is thermally decomposed again, agglomerated, and combined to form black smoke. The black smoke thus produced brings about air pollution, raising concern of an adverse effect thereof on human bodies.
The emission of black smoke can be reduced by increasing the injection pressure of fuel to combustion chambers of a diesel engine. To this end, however, a steel pipe used for fuel injection is required to have high fatigue strength. For such a fuel injection pipe or a steel pipe for a fuel injection pipe, the following techniques have been disclosed.
Patent Document 1 discloses a method for producing a steel pipe to be used for fuel injection in a diesel engine, in which an inner surface of a seamless steel pipe starting material subjected to hot rolling is ground and abraded by shotblasting, and the starting material is then subjected to cold drawing. Patent Document 1 describes that, by employing this production method, high strengthening of a steel pipe to be used for fuel injection is enabled because depths of flaws on an inner surface of the steel pipe (e.g., unevenness, fracture, fine crack, etc.) are 0.10 mm or less.
Patent Document 2 discloses a steel pipe for a fuel injection pipe in which a maximum diameter of nonmetallic inclusions present at least at up to a depth of 20 μm from an inner surface of the steel pipe is 20 μm or less and that has a tensile strength of 500 MPa or more.
Patent Document 3 discloses a steel pipe for a fuel injection pipe having a tensile strength of 900 N/mm2 or more, in which a maximum diameter of nonmetallic inclusions at least present at up to a depth of 20 μm from an inner surface of the steel pipe is 20 μm or less.
The invention of Patent Document 3 achieves a tensile strength of 900 MPa or more by producing a blank pipe using a steel material from which A type, B type, and C type coarse inclusions are removed by reducing S, devising a casting method, reducing Ca, and the like, adjusting a diameter of the blank pipe to an intended diameter by cold working, and then performing quenching and tempering. In Examples of Patent Document 3, critical internal pressures of 260 to 285 MPa are achieved.
Patent Document 4 discloses a steel pipe for a fuel injection pipe having a tensile strength of 800 MPa or more, preferably 900 MPa or more and excellent in internal pressure fatigue resistance, and discloses a fuel injection pipe produced from the steel pipe. Patent Document 5 discloses a steel pipe for a fuel injection pipe that exerts a high advantageous effect of improving critical internal pressure even with a relatively low autofrettage pressure and is excellent in workability and internal pressure fatigue resistance.
A steel pipe to be used for fuel injection produced by the method disclosed in Patent Document 1 has high strength but cannot provide a fatigue life appropriate to a strength of a material of the steel pipe. A higher strength of a material of a steel pipe naturally enables a higher pressure to be applied to an inside of the steel pipe. However, in a case where pressure is applied to an inside of a steel pipe, an internal pressure to be a limit up to which no fracture due to fatigue occurs at an inner surface of the steel pipe (hereinafter, referred to as “critical internal pressure”) does not depend only on a strength of a material of the steel pipe. In other words, even when the strength of the material of the steel pipe is increased, a critical internal pressure more than expected cannot be provided. With consideration given to a reliability of an end product and the like, the longer the fatigue life is, the more preferable it is, but the lower the critical internal pressure is, the shorter the fatigue life becomes because the steel pipe is prone to be fatigued with use under high internal pressure.
The steel pipes for a fuel injection pipe disclosed in Patent Documents 2 and 3 have the advantages of their long fatigue lives and high reliabilities. However, even the steel pipes disclosed in Patent Documents 2 and 3 do not necessarily have high critical internal pressure commensurate with the tensile strengths of the steel pipes.
Therefore, according to Patent Documents 4 and 5, autofrettage is performed to provide high critical internal pressure. The autofrettage is a process of producing compressive residual stress by applying an excessive internal pressure so as to partially subject a vicinity of an inner surface to plastic deformation. By performing autofrettage, the critical internal pressure can be increased without increasing the tensile strength of the steel pipe.
However, if a pressure applied to an inner surface of a steel pipe is excessive in autofrettage, the risk of bursting increases; therefore, the pressure is generally set to be low from the viewpoint of safety. As a result, even with autofrettage performed, the conventional techniques fail to increase the critical internal pressure sufficiently and are still susceptible to improvement.
An objective of the present invention is to solve the problem and to provide a steel pipe for pressure piping that has high critical internal pressure.
The present invention has been made to solve the problem and has a gist of the following steel pipe for pressure piping.
(1) A steel pipe for pressure piping, the steel pipe being subjected to autofrettage, wherein
when an outer diameter of the steel pipe is denoted by D (mm), an inner diameter of the steel pipe is denoted by d (mm), and a yield stress of the steel pipe is denoted by σy (MPa), and
when a measured value of a residual stress at an outer surface of the steel pipe after the autofrettage is denoted by σo1 (MPa), a measured value of a residual stress at an outer surface of the steel pipe after the autofrettage and halving is denoted by σo2 (MPa), and a measured value of a residual stress at an inner surface of the steel pipe after the autofrettage and the halving is denoted by σi2 (MPa),
D/d is 1.2 or more, and
an estimated value σ(MPa) of a residual stress at an inner surface of the steel pipe after the autofrettage satisfies Formula (v) shown below, the estimated value σi1 being determined by Formula (i) to Formula (iv) shown below:
σi1=(−σi2)/(A×(t/T)2−1) (i)
t/T=((σo2−σo1)/(A×(σo2−σo1)−C×σi2))1/2 (ii)
A=3.9829×exp(0.1071×(D/d)2) (iii)
C=−3.3966×exp(0.0452×(D/d)2) (iv)
1.1×F×σy≤σi10.8×F×σy (v)
where F in Formula (v) is a coefficient,
F=(0.3×(3−D/d)2−1) when 1.2≤D/d≤3.0, and
F=−1 when D/d>3.0.
According to the present invention, steel pipes for pressure piping having a high critical internal pressure can be provided stably.
Factors in the prior art that cause a failure to sufficiently optimize an autofrettage pressure for increasing a critical internal pressure include an unestablished method for determining a residual stress on an inner surface of a steel pipe. Note that the residual stress herein means residual stress in a circumferential direction of a steel pipe.
In conventional practices, the present inventors have evaluated a residual stress at an inner surface of a steel pipe subjected to autofrettage in a relative manner by cutting the steel pipe in half and measuring a residual stress at an inner surface of the steel pipe after the halving. Note that cutting in half means cutting a steel pipe such that the steel pipe is divided into two equal, arc-shaped members when viewed in its axial direction.
However, to increase critical internal pressure sufficiently by optimizing autofrettage pressure, a residual stress at an inner surface of a steel pipe after autofrettage and before halving needs to be evaluated quantitatively. The present inventors have thus conducted studies about a method for evaluating a residual stress at an inner surface of a steel pipe quantitatively. In the course of the studies, the present inventors investigated the evaluation of a residual stress at an inner surface of a steel pipe after autofrettage and before halving by considering a residual stress at the inner surface of a halved steel pipe as well as a residual stress at an outer surface of the steel pipe before the having and a residual stress at the outer surface of the steel pipe after the halving.
The present inventors first conducted a numerical analysis (FEM analysis) using an analytic model for a steel pipe to be evaluated under various kinds of conditions to determine residual stresses (calculated value) that are set up at portions of the steel pipe by autofrettage. Specifically, the present inventors first conducted the numerical analysis to determine a residual stress σo1 at an outer surface of the steel pipe after the autofrettage and before halving, a residual stress σi1 at an inner surface of the steel pipe after the autofrettage and before the halving, a residual stress σo2 at an outer surface of the steel pipe after the autofrettage and the halving, and a residual stress σi2 at an inner surface of the steel pipe after the autofrettage and the halving.
As a result of investigations in depth about the residual stresses obtained in the above manner, the present inventors found that the residual stress σi1 at the inner surface of the steel pipe before the halving can be estimated with high accuracy using the residual stress σo1 at the outer surface of the steel pipe before the halving, the residual stress σo2 at the outer surface of the steel pipe after the halving, and the residual stress σi2 at the inner surface of the steel pipe after the halving.
The present inventors then came to obtain a finding that steel pipes having high critical internal pressure can be provided stably by adjusting autofrettage conditions such that an estimated value σi1 of the residual stress at the inner surface of the steel pipe before the halving satisfies a predetermined condition.
The present invention is made based on the findings described above. Requirements of the present invention will be described below in detail.
1. Steel Pipe for Pressure Piping
The present invention relates to a steel pipe for pressure piping. The pressure piping includes a hydraulic cylinder, an airbag steel pipe, an accumulator, a pipe for hydrogen, a fuel injection pipe, and the like. To withstand high internal pressure, it is desirable to increase a wall thickness of a steel pipe with an increase in an inner diameter of the steel pipe. With an inner diameter of a steel pipe constant, an outer diameter of the steel pipe is increased as a wall thickness of the steel pipe is increased. In other words, to withstand high internal pressure, it is desirable to increase an outer diameter of a steel pipe with an increase in an inner diameter of the steel pipe. When D (mm) denotes an outer diameter of a steel pipe, and d (mm) denotes an inner diameter of the steel pipe, a steel pipe being a subject matter in the present invention brings D/d to 1.2 or more. D/d is preferably 1.5 or more and is more preferably 2.0 or more.
The other dimensions may be selected according to usage of the steel pipe and need not be limited to particular dimensions. For example, in a case where the steel pipe is used as a hydraulic cylinder, it is generally preferable to select an inner diameter of the steel pipe within the range of 15 to 580 mm in accordance with working pressure (internal pressure) to secure the output (load) of a piston. In addition, it is preferable to select a wall thickness of the steel pipe within the range of 5 to 60 mm and select an outer diameter of the steel pipe within the range of 30 to 700 mm so that the steel pipe is capable of withstanding repetitive internal pressure. The higher an internal pressure fatigue strength, the thinner the wall thickness can be made to be, which determines the outer diameter accordingly.
In a case where the steel pipe is used as an airbag inflator, the outer diameter of the steel pipe is desirably 20 to 100 mm and is more desirably 20 to 60 mm. The wall thickness of the steel pipe is desirably 1 to 5 mm and is more desirably 1 to 4 mm.
In a case where the steel pipe is used as an accumulator, the outer diameter of the steel pipe is desirably 25 to 500 mm and is more desirably 50 to 400 mm. The wall thickness of the steel pipe is desirably 2 to 40 mm and is more desirably 4 to 30 mm.
Further, in a case where the steel pipe is used as a pipe for hydrogen gas or a fuel injection pipe, the steel pipe is required to have a certain amount of volume to reduce fluctuations in inside pressure in its use. For that reason, the inner diameter of the steel pipe is desirably 2.5 mm or more and is more desirably 3.0 mm or more. In addition, because of requirement of withstanding high internal pressure, the wall thickness of the steel pipe is desirably 1.5 mm or more and is more desirably 2.0 mm or more. On the other hand, the outer diameter of the steel pipe is desirably 20 mm or less, is more desirably 15 mm or less, and is still more desirably 10 mm or less.
Mechanical properties of the steel pipe may be selected according to usage of the steel pipe and need not be limited to particular mechanical properties. However, in a case where the steel pipe is used in pressure piping such as one for a fuel injection pipe or a hydraulic cylinder, a tensile strength of the steel pipe is preferably 500 MPa or more, is more preferably 800 MPa or more, and is still more preferably 900 MPa or more. A yield stress of the steel pipe is preferably 300 MPa or more, is more preferably 400 MPa or more, and is still more preferably 500 MPa or more.
A yield ratio of the steel pipe is preferably 0.50 to 0.95; the yield ratio is more preferably 0.60 or more and is still more preferably 0.70 or more to provide a high compressive residual stress by performing autofrettage with higher pressure. To introduce a compressive residual stress more efficiently by performing autofrettage with low pressure, the yield ratio is preferably 0.90 or less and is more preferably 0.85 or less.
In the present invention, the tensile strength of the steel pipe is determined by cutting out a straight pipe section of the steel pipe, chucking regions extending by a certain length from both end faces of the straight pipe section (hereinafter, referred to as “grip portions”), and conducting a tensile test with an extensometer set up on a parallel portion of the straight pipe section between the grip portions. The chucking is performed in such a manner as to press chuck jaws formed with a V groove or an R groove shallower than an external diameter of the steel pipe against the grip portions by means of oil pressure or bolting, or using a wedge jig.
The length of the grip portions is only required to be determined with consideration given to a pressing pressure and a test load so that the steel pipe subjected to the test does not slip during the test. A length of the parallel portion is only required to be ensured to the extent that the extensometer can be set up, and that necking deformation occurring immediately before rupture is not influenced by chucks. If the steel pipe has no straight pipe section with a sufficient length, the tensile test may be conducted with a small test specimen having a thin dumbbell shape as described in Non-Patent Document 1 cut out from the steel pipe.
The steel pipes for pressure piping according to the present invention preferably has a critical internal pressure satisfying Formula (I) shown below.
IP≥0.44×TS×α (I)
α=[(D/d)2−1]/[0.776×(D/d)2] (II)
Note that, in Formula (I) above, IP denotes the critical internal pressure (MPa) of the steel pipe, TS denotes the tensile strength (MPa) of the steel pipe, and α is a value given by Formula (II) above. Further, in Formula (II) above, D denotes an outer diameter (mm) of the steel pipe, and d denotes an inner diameter (mm) of the steel pipe. The value a is a coefficient for compensating for changes in the relation between an internal pressure and an initiation stress at the inner surface of the steel pipe in accordance with the ratio of the outer diameter to the inner diameter of the steel pipe.
The term “critical internal pressure” in the present invention means a highest internal pressure (MPa) that causes no breakage (leak) to occur, in an internal pressure fatigue test in which an internal pressure fluctuation following a sine wave over time is repeatedly applied with a lowest internal pressure set to 18 MPa, even when the number of repetitions of the internal pressure fluctuation reaches 107. Specifically, the critical internal pressure is determined, on an S—N curve with its vertical axis representing maximum internal pressure and its horizontal axis representing the number of repetitions at breakage, as a middle value between a minimum value of maximum internal pressures at which a breakage occurs and a maximum value of maximum internal pressures which cause no breakage to occur even when the number of repetitions reaches 107.
2. Residual Stress Estimation Model
A model for estimating the residual stress αi1 at an inner surface of a steel pipe before halving will be described in detail.
In this model, measured values of a residual stress σo1 at an outer surface 20a of the steel pipe 20 after the autofrettage, a residual stress σo2 at the outer surface 22a of the halved sample 22, and a residual stress σi2 at an inner surface 22b of the halved sample 22 are used. Note that, as described above, the residual stresses each mean residual stress in a circumferential direction of the steel pipe 20.
Referring to
Measurement of the residual stresses is conducted after removing outer layers of the outer surface 20a of the steel pipe 20 and the inner surface 22b of the halved sample 22 within the range of 10 μm or less by electropolishing. As a method for the measurement, a sin 2ψ method based on an X-ray diffraction technique can be used, and the method can be performed in conformity with Non-Patent Document 2.
An estimated value of the residual stress αi1 is calculated from a multivariable function of variables including the residual stresses σo1, σo2, and αi2, actually measured by the method described above as well as the outer diameter D and the inner diameter d.
Specifically, as illustrated in
First, to simulate the steel pipe 20, as illustrated in
In the initial state, it is assumed that no stress occurs in a region between a position P that is away from the inner surface 40c in a radial direction of the analytic model 40 (a position indicated with an arc-shaped broken line) and an outer surface 40d. Further, in the initial state, a stress distribution in a region between the inner surface 40c and the position P is set as a linear distribution such that a compressive stress is gradually decreased from the inner surface 40c toward the position P.
After the volume force is provided as described above, an elastic analysis is conducted to redistribute the stress. This causes a change in the stress state in the analytic model 40 as illustrated in
Next, to simulate the halved sample 22 (halved steel pipe 20), an elastic analysis is conducted with the constraint on the end portion 40a eliminated, as illustrated in
In the analytic model 40 illustrated in
The analysis described with reference to
As a result of various investigations conducted by the present inventors, it has been found that the thickness T of the steel pipe 20 as well as the distance t in the steel pipe 20 after the autofrettage (see
Specifically, the present inventors found that there is a certain correlation between a value of (t/T)2 and a value of (σi2/−σi1) in the steel pipe 20 after the autofrettage. The present inventors made a linear approximation of the relation between the value of (t/T)2 and the value of (σi2/−σi1) by least squares method, deriving Formula (1) shown below. Note that, in Formula (1) below, A and B are coefficients.
σi2/(−σi1)=A×(t/T)2−B (1)
Rearrangement of Formula (1) above for σi1 gives Formula (i) shown below. Note that, in the present invention, A is set to a value given by Formula (iii) shown below, and B is set to one.
σi1=(−σi2)/(A×(t/T)2−1) (i)
A=3.9829×exp(0.1071×(D/d)2) (iii)
Further, the present inventors found that there is also a certain correlation between the value of (t/T)2 and a value of ((σo2−σo1)/(−σi1)) in the steel pipe 20 after the autofrettage. The present inventors made a linear approximation of the relation between the value of (t/T)2 and the value of ((σo2−σo1)/(−σi1)) by least squares method, deriving Formula (2) shown below. Note that, in Formula (2) below, C and E are coefficients.
(σo2−σo1)/(−σi1)=−C×(t/T)2−E (2)
From Formula (1) and Formula (2) above, (t/T) can be given by Formula (3) shown below.
t/T=((B×(σo2−σo1)−E×σi2)/(A×(σo2−σo1)−C×σi2))1/2 (3)
Further, in the present invention, B is set to one, and E is set to zero, which gives Formula (ii) shown below. C is set to a value given by Formula (iv) shown below.
t/T=((σo2−σo1)/(A×(σo2−σo1)−C×σi2))1/2 (ii)
C=−3.3966×exp(0.0452×(D/d)2) (iv)
From Formula (i) to Formula (iv) obtained based on the estimation model, an estimated value σi1 of the residual stress at the inner surface 20b of the steel pipe 20 after the autofrettage can be calculated.
In addition, the steel pipe according to the present invention makes a value of σi1 satisfy Formula (v) shown below:
1.1×F×σy≤σi1≤0.8×F×σy (v)
where F in Formula (v) is a coefficient,
F=(0.3×(3−D/d)2−1) when 1.2≤D/d≤3.0, and
F=−1 when D/d>3.0.
By bringing σi1 to not more than 0.8×F×σy in the relation with the yield stress of the steel pipe through the autofrettage, high critical internal pressure can be provided. If σi1 falls below 1.1×F×σy, a tensile residual stress on an outer surface side becomes high, which not only makes a rise in the critical internal pressure saturate but also rather decreases the critical internal pressure.
3. Production Method
There are no special limitations on a method for producing the steel pipe for pressure piping according to the present invention; however, the steel pipe for pressure piping can be produced by performing steps including performing autofrettage under various conditions on a plurality of steel pipe starting materials that bring D/d to 1.2 or more, and determining σi1 of each of the resultant steel pipes by the method described above and selecting steel pipes that make σi1 satisfy Formula (v).
Note that the autofrettage conditions can be adjusted such that σi1 satisfies Formula (v), by controlling an autofrettage pressure and/or an autofrettage duration, for example. As described above, by estimating a residual stress at an inner surface of a steel pipe after autofrettage and before halving accurately, autofrettage conditions can be optimized, and steel pipes having high critical internal pressure can be provided stably.
Further, for example, by forming connection heads at both end portions of the steel pipe, the steel pipe can be made into a high-pressure fuel injection pipe.
The present invention will be described below more specifically with reference to Examples, but the present invention is not limited to these Examples.
A steel having a chemical composition shown in Table 1 was melted in a vacuum furnace, by which an ingot was obtained. The ingot was heated to 1250° C. and then subjected to hot forging, by which round bars each having a diameter of 20 mm were obtained. Further, the round bars were subjected to solution treatment in which the round bars were heated to 1000° C., retained for 10 minutes, and then allowed to cool, by which the round bars were made into starting materials for test specimens. The starting materials were subjected to rough machining, heat treatment, and finishing, by which internal pressure fatigue test specimens each having a shape illustrated in
There were two types of heat treatment conditions: normalizing treatment in which the starting material is heated to 1000° C., retained for 10 minutes, and then air-cooled; and quenching and tempering treatment in which the starting material is heated to 1000° C. by high-frequency heating, rapidly cooled, heated to 640° C., retained for 15 minutes, and then allowed to cool. Here, the former will be referred to as a normalized item, and the latter will be referred to as a quenched item. In the shape of the test specimen illustrated in
From one of the internal pressure fatigue test specimens, a small test specimen having a thin dumbbell shape described in Non-Patent Document 1 was cut out and subjected to a tensile test to evaluate mechanical properties. Results of the evaluation are shown in Table 2. It is understood that quenched items had higher tensile strengths, higher yield stresses, and higher yield ratios than those of normalized items.
† 1.1 × F × σy ≤ σi1 ≤ 0.8 × F × σy . . . (ν)
‡ IP ≥ 0.44 × TS × α . . . (1)
Subsequently, the resultant internal pressure fatigue test specimens were subjected to autofrettage. The autofrettage was performed in such a manner as to seal one end face of the internal pressure fatigue test specimen illustrated in
One of the test specimens after the autofrettage was subjected to a residual stress measurement. First, an outer layer of an outer surface of the test specimen at a center position in a longitudinal direction was removed within the range of 10 μm or less by electropolishing, and then a circumferential residual stress σo1 was measured. As a method for the measurement, the sin2ψ method based on an X-ray diffraction technique was used, and the method was performed in conformity with Non-Patent Document 2. Detailed measurement conditions are as follows.
Note that all conditions for a residual stress measurement described below were as described above.
Next, the test specimen which was measured for the residual stress at its outer surface was cut in half in its pipe axis direction by wire cut electrical discharge. When a position of the residual stress measurement at the outer surface was determined to be 0° in a circumferential direction, a position of the cutting was set to a vicinity of ±90°. A thickness t between a cut surface and an outer surface at a center position in a longitudinal direction of each of the samples after the halving was set within the range of D/2±0.2 mm.
Then, on each of the samples after the halving, the circumferential residual stress σo2 was measured again at the position of the residual stress measurement at the outer surface. Further, after the halving, an outer layer of an inner surface of each test specimen at the center position in the longitudinal direction was removed within the range of 10 μm or less by electropolishing, and then a circumferential residual stress σi2 at an inner surface of the steel pipe at its center position was measured.
The residual stress measured values σo1, σo2, and σi2 obtained in such a manner are shown in Table 2. These measured values are substituted into Formulas (i) to (iv), which gives the estimated value σi1 of the residual stress at the inner surface after the autofrettage and before the halving. The estimated value failed to satisfy Formula (v) in comparative examples and satisfied Formula (v) in inventive examples of the present invention. As shown in Table 2, it is estimated that the inventive examples of the present inventions caused higher compressive residual stresses than those caused by the comparative examples.
Further, the other test specimens were subjected to an internal pressure fatigue test to determine their critical internal pressures. The internal pressure fatigue test is performed in such a manner as to cause an internal pressure to fluctuate within the range of 18 MPa at the minimum from a maximum internal pressure such that the internal pressure follows a sine wave over time. The frequency of the internal pressure fluctuations was set to 8 Hz. A maximum internal pressure that caused no breakage (leak) to occur even when the number of repetitions reaches 107 as the result of the internal pressure fatigue test was evaluated as the critical internal pressure.
Results of the evaluation are shown together in Table 2. Note that an indication of “≥400” in the column of Critical Internal Pressure in Table 2 means that no breakage occurred even at an upper limit internal pressure (400 MPa), up to which a testing machine used for the internal pressure fatigue test was capable of applying a load.
As is clear from the results shown in Table 2, the inventive examples of the present invention provided higher critical internal pressures than those provided by the comparative examples. Since the normalized items each have high D/d and a low yield ratio, the normalized items are likely to cause high compressive residual stress at their inner surfaces and generally have higher critical internal pressures than those of the quenched items. Therefore, the normalized items enhanced an advantageous effect of improving the critical internal pressure of the inventive examples of the present invention compared with the comparative examples. Although the quenched items had high tensile strength, since the quenched items each have low D/d and a high yield ratio, the quenched items are unlikely to cause compressive residual stress at their inner surfaces. However, even under such conditions, improvement in the critical internal pressure was recognized in the inventive examples of the present invention compared with the comparative examples.
According to the present invention, steel pipes for pressure piping having a high critical internal pressure can be provided stably. The steel pipe for pressure piping according to the present invention is therefore suitably applicable particularly to a hydraulic cylinder or a fuel injection pipe for an automobile.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2020-069080 | Apr 2020 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2021/014435 | 4/5/2021 | WO |
