RU2821402C2 - High-pressure hydrogen steel pipe, high-pressure hydrogen vessel and steel pipe manufacturing method - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to a steel pipe for high pressure hydrogen and a vessel made using such a pipe, used at automotive hydrogen stations. Pipe has a chemical composition containing the following in wt.%: C: 0.05–0.60; Si: 0.001–2.0; Mn: 0.01–5.0; P: 0.030 or less; S: 0.010 or less; N: 0.010 or less; Al: 0.0001–1.00; O: 0.010 or less; H: 0.00010 or less; optionally, at least one element selected from: Mo: 5.0 or less; Cr: 5.0 or less; Ni: 5.0 or less; Cu: 5.0 or less; Co: 5.0 or less; B: 0.01 or less; V: 1.0 or less; W: 5.0 or less; Nb: 0.1 or less; Ti: 0.1 or less; Zr: 0.2 or less; Hf: 0.2 or less; Ta: 0.2 or less; Sn: 0.2 or less; Sb: 0.2 or less; Ca: 0.01 or less; Mg: 0.01 or less and rare-earth metals: 0.5 or less, the rest is Fe and random impurities. Microstructure of steel includes residual austenite in area fractions of 3 % or less (including 0%), and the number of inclusions in the microstructure, having an aspect ratio of 2.0 or more and length of 10 mcm or more, is 15/100 mm² or less.

EFFECT: pipe has high strength and low rate of fatigue crack growth in the area in which ΔK relating to resistance to fatigue crack growth is 10 MPa·m^1/2 or less.

3 cl, 2 tbl

Description

Technical field

The present invention relates to a high pressure hydrogen steel pipe, a high pressure hydrogen vessel and a method for manufacturing a steel pipe.

State of the art

Fuel cell vehicles, which use hydrogen as fuel, do not emit carbon dioxide (CO ₂ ) and are highly energy efficient, are expected to serve as vehicles that can solve CO ₂ emissions and energy problems. To promote the use of such fuel cell vehicles, it is necessary to build hydrogen stations to refuel fuel cell vehicles with hydrogen. Therefore, vessels are being developed that are necessary for the safe storage of high-pressure hydrogen at hydrogen stations, that is, corresponding in strength and durability.

Patent Document 1 discloses a steel material for high pressure hydrogen storage, a steel material having a predetermined chemical composition and metal microstructure comprising mainly bainite, in which the area ratio of bainite is 90% or more, and in which cementite has an average grain size of 50 nm or less and an average aspect ratio of 3 or less is distributed dispersedly in bainite. Patent literature 1 aims to improve strength, toughness and hydrogen embrittlement resistance by controlling the shape of cementite. Patent Literature 2 discloses a steel pipe for a pressure vessel, a steel pipe having a specified chemical composition and metal microstructure including phases other than ferrite in an amount of 50% or more in an area fraction in which the average size of the former austenite grain is 500 μm or less. In Patent Literature 2, quench cracking resistance is improved by controlling the initial austenite grain size and P concentration. Patent Literature 3 discloses a liner for a pressure vessel made of a steel material, wherein the steel material has a predetermined chemical composition and then including in area fractions tempered martensite and bainite in a total amount of 70% or more, and ferrite in an amount of less than 30%. In patent literature 3, improvement of fatigue strength in hydrogen is realized by controlling the phase fractions. Patent Literature 4 discloses a low-alloy steel material for high-pressure hydrogen, a steel material having a certain chemical composition in which the sum of the number of sulfide-based inclusion grains and oxide-based inclusion grains having a grain size of 20 μm or more is 10/100 mm ² or less when observed in cross section. In patent literature 4, improvement in fatigue strength is realized by reducing the number of inclusions.

List of cited sources

Patent literature

PTL 1: Publication of Japanese Unexamined Patent Application No. 2012-107332.

PTL 2: International Publication No. 2018/055937.

PTL 3: Publication of pending Japanese Patent Application No. 2018-53357.

PTL 4: Publication of the pending Japanese Patent Application No. 2018-12855.

Disclosure of the invention

Technical problem

However, in the case of the methods according to Patent Literature 1 to 4, although improvement in the fatigue strength of steel materials can be achieved, there may be a case where very small defects due to inclusions or the like exist on the surface of the steel material used for the actual vessel. The service life of an actual vessel is based on the fatigue limit obtained by performing a fatigue test in which the evaluation is made using a test specimen whose surface is machined. However, this alone is not enough, and what is important is a material having such resistance to fatigue crack growth that the fatigue crack growth rate is low in the fatigue crack growth test. Generally, with regard to fatigue crack growth resistance, the fatigue crack growth rate in the region in which the Paris equation is satisfied is considered important in the region in which the stress intensity factor ΔK ranges from about 20 MPa m ^1/2 to 30 MPa m ^1/2 .

On the other hand, when designing a hydrogen pressure vessel, the case is considered where the excessive safety factor is reduced by setting a lower safety factor than for a conventional pressure vessel, which is set to 4.0, that is, for example, by setting a safety factor of about 2, 4. If the safety factor is reduced, since the vessel can be designed for higher pressure without changing its shape, the capacity increases. In addition, since the vessel can be designed with a thinner wall thickness without reducing the pressure withstand, the weight and material costs of the vessel are reduced. That is, it is possible to realize improved functionality and reduced cost while ensuring security.

To implement the design with a reduced safety factor described above, since it is necessary to fix the initial crack on the inner surface of a small-sized vessel, the fatigue crack growth rate in the region in which the Paris equation is not satisfied, the region in which ΔK is 10 MPa⋅m ^1/ is important. ² or less. However, in the case of conventional methods, reduction of the fatigue crack growth rate in the region in which ΔK is 10 MPa m ^1/2 or less has not been considered.

The present invention has been completed in view of the problems described above, and the object of the present invention is to provide a high pressure hydrogen steel pipe having high strength and low fatigue crack growth rate in a region in which ΔK, which is related to fatigue crack growth resistance, is 10 MPa⋅m ^1/2 or less.

Further, it is an object of the present invention to provide a high pressure hydrogen vessel using the above-described high pressure hydrogen steel pipe, and to develop a method for manufacturing the above-described high pressure hydrogen steel pipe.

Solution

The inventors of the present invention conducted studies on the influence of the chemical composition and metal microstructure of the steel material used to make a high pressure hydrogen steel pipe and a high pressure hydrogen vessel on the fatigue crack growth rate in the region in which ΔK is low, that is, 10 MPa⋅m ^1/2 or less. As a result, it is found in the microstructure of steel material that in the case where the amount of retained austenite is small, and the number of inclusions having a small radius of curvature, which greatly affects the stress concentration, is small, the fatigue crack growth resistance is suitable in the low ΔK region in high pressure hydrogen gas .

Based on the data described above, detailed studies were carried out on the chemical composition, microstructure and manufacturing conditions of steel, which led to the completion of the present invention.

That is, the subject of the present invention is as follows.

[1] A steel pipe for high-pressure hydrogen, wherein the steel pipe has a chemical composition containing, in % by mass.

C: 0.05 - 0.60%,

Si: 0.001 - 2.0%,

Mn: 0.01 - 5.0%,

P: 0.030% or less,

S: 0.010% or less,

N: 0.010% or less,

Al: 0.0001 - 1.00%,

O: 0.010% or less,

H: 0.00010% or less, the rest Fe and incidental impurities and

a microstructure comprising, in area fractions, 3% or less (including 0%) retained austenite, in which the number of inclusions having an aspect ratio of 2.0 or more and a length of 10 µm or more is 15/100 mm ² or less.

[2] Steel pipe for high-pressure hydrogen according to item [1], the chemical composition of which additionally contains in % wt. one or both elements selected from

Mo: 5.0% or less and

Cr: 5.0% or less.

[3] Steel pipe for high-pressure hydrogen according to item [1] or [2], the chemical composition of which additionally contains in % wt. one, two or more elements selected from

Ni: 5.0% or less,

Cu: 5.0% or less,

Co: 5.0% or less, and

B: 0.01% or less.

[4] Steel pipe for high-pressure hydrogen according to any of paragraphs [1] - [3], the chemical composition of which additionally contains in % by mass. one, two or more elements selected from

V: 1.0% or less,

W: 5.0% or less,

Nb: 0.1% or less,

Ti: 0.1% or less,

Zr: 0.2% or less,

Hf: 0.2% or less,

Ta: 0.2% or less,

Sn: 0.2% or less, and

Sb: 0.2% or less.

[5] Steel pipe for high-pressure hydrogen according to any of paragraphs [1] - [4], the chemical composition of which additionally contains in % by mass. one, two or all elements selected from

Ca: 0.01% or less,

Mg: 0.01% or less, and

REM: 0.5% or less.

[6] A high pressure hydrogen vessel made of high pressure hydrogen steel pipe in accordance with any of [1] to [5] above.

[7] A method for manufacturing a steel pipe for high pressure hydrogen, comprising:

a process for casting a steel material having a chemical composition in accordance with any of items [1] to [5] above, at a casting speed of 1.0 m/min or lower;

the process of heating steel material cast in a casting process at a temperature of 1350°C or lower;

a process of rolling steel material heated by a heating process at a final rolling temperature of 820°C or higher to produce a steel pipe;

the process of cooling a steel pipe obtained by rolling to a temperature of 50°C or lower at an average cooling rate of 5°C/s or higher in a temperature range of 800°C to 350°C and at an average cooling rate of 3°C/s or lower in the temperature range from 350°C to 50°C; And

the process of tempering by heating a steel pipe cooled during the cooling process to a temperature of 400°C or higher and equal to or lower than the A1 transformation temperature for tempering the heated steel pipe.

Positive effects of the invention

According to the present invention, it is possible to provide a high pressure hydrogen steel pipe having high strength and low fatigue crack growth rate in a region in which ΔK related to fatigue crack growth resistance is 10 MPa.m1 ^/2 or less.

By using the high-pressure hydrogen steel pipe according to the present invention, a high-pressure hydrogen vessel can be obtained at a lower cost, with which improved performance can be realized while ensuring safety.

Carrying out the invention

Next, the present invention will be described in detail.

In the present invention, in the case where the metal microstructure of the steel pipe has a low content of retained austenite and the metal microstructure has a low content of inclusions having a small radius of curvature, which have a strong effect on the stress concentration of the steel pipe, the fatigue crack growth resistance is appropriate in high-pressure hydrogen gas. Specifically, in the metal microstructure of the steel pipe, the area fraction of retained austenite is set to 3% or less (including 0%), and the number of inclusions having an aspect ratio of 2.0 or more and a length of 10 μm or more is set to 15/100 ^mm2 or less. The reasons for the restrictions placed on such factors will be described below. In the description, "%" related to the microstructure of the metal denotes the area fraction unless otherwise indicated. Moreover, in the present invention, the expression “high strength” means the case of a tensile strength of 800 MPa or higher.

Metal microstructure

Retained austenite: 3% or less

In the case where retained austenite is present in the metal microstructure of the steel material, since the amount of hydrogen in the steel increases, a case of increased sensitivity to hydrogen embrittlement may occur. In addition, since transformation-induced martensite, which is formed by the transformation of retained austenite into martensite due to the loading stress applied in practice, has such a high hardness that hydrogen embrittlement tends to occur, a minute crack is formed at the transformation martensite site. which can lead to an increase in the rate of fatigue crack growth. In the present invention, in order to reduce the fatigue crack growth rate, the amount of retained austenite in the metal microstructure is set to 3% or less. The lower the amount of retained austenite in the metal microstructure, the lower the fatigue crack growth rate. Therefore, it is preferable that the amount of retained austenite is 2% or less, or more preferably 1% or less. On the other hand, the lower limit of the amount of retained austenite in the microstructure of the metal is set to 0%.

To achieve high strength (tensile strength: 800 MPa or higher), it is preferable that the metal microstructure mainly includes martensite and bainite so that the total area ratio of martensite and bainite is 80% or more. More preferably, such proportion of the total area is 90% or more, or even more preferably 95% or more. Here, such a share of the total area can be 100%. In addition, the metal microstructure may further include optional phases other than martensite, bainite, and retained austenite (hereinafter referred to as “other phases”). From the viewpoint of increasing the microstructure control effect, it is preferable that the total area fraction of the other phases described above is 10% or less. That is, it is preferable that the proportion of the total area of martensite, bainite and retained austenite in the microstructure of the metal is 90% or more. Examples of other phases described above include ferrite, pearlite and the like. In the case where the metal microstructure includes ferrite, it is preferable that the area fraction of ferrite be 10% or less. More preferably, the area ratio of ferrite is 5% or less. In addition, in the case where the metal microstructure includes pearlite, it is preferable that the area fraction of pearlite be 2% or less. More preferably, the area fraction of perlite is 1% or less. Here, the microstructure described above is determined using the method described in the examples.

Inclusion

Since an inclusion with high hardness is less susceptible to deformation than the base material, the inclusion expands the local field of elastic deformations and becomes a source of stress concentration. Thus, the inclusion acts as a powerful source of hydrogen accumulation. The degree of local stress concentration depends on the shape of the inclusion. The closer the shape is to spherical, the lower the degree of stress concentration, and the closer the shape is to needle-shaped, the higher the degree of stress concentration. In addition, since dislocation growth from the boundary between the inclusion and the substrate is facilitated by the accumulation of hydrogen in an area with a high degree of local stress concentration, there is an increased risk of hydrogen cracking. In particular, in the region in which the fatigue crack growth rate is 1.0×10 ^-7 m/cycle or less, since the growth of the crack length per loading cycle is noticeably lower than the main block of martensite, called martensite lath and shaped like thin wood chips with a thickness of about 2.0 × 10 ^-7 m, the growth rate is strongly influenced by factors other than the formation of dislocations at the fatigue crack tip. That is, inclusion has a noticeable effect on the fatigue crack growth rate.

Aspect ratio: 2.0 or more and length: 10 µm or more

In the ΔK region in which the fatigue crack growth rate is comparatively low in an environment in which hydrogen is present in the steel material, an inclusion having an aspect ratio of 2.0 or more causes hydrogen cracking, where the aspect ratio is calculated by dividing the length in the longitudinal direction (length ) of the inclusion by a length in the thickness direction (thickness) of the inclusion, and, in the case of an inclusion having a length in the longitudinal direction of 10 µm or more, there is an increase in the range of influence of local stress concentration. As a result, the fatigue crack growth rate increases. Therefore, the present invention defines the number density of inclusions having an aspect ratio of 2.0 or more and a length of 10 μm or more. Here, the aspect ratio and inclusion length are determined using the method described in the examples.

15/100 mm ² or less

By reducing the density of inclusions having a high aspect ratio and a large size that increase the fatigue crack growth rate in an environment in which hydrogen is present in the steel material, the increase in the fatigue crack growth rate can be suppressed. In the present invention, in order to reduce the fatigue crack growth rate, the density of inclusions having an aspect ratio of 2.0 or more and a length of 10 μm or more is set to 15/100 mm ² or less. The lower the density of such inclusions, the lower the fatigue crack growth rate. Therefore, it is preferable that the number density of such inclusions is 12/100 mm ² or less, more preferably 10/100 mm ² or less, even more preferably 8/100 mm ² or less, or most preferably 6/100 mm ² or less. Although there is no particular limitation on the lower limit of the inclusion number density, it is preferable that the lower limit is 0.1/100 mm ² or more. Here, the number density of inclusions is determined using the method described in the examples.

Chemical composition

Moreover, in the present invention, it is important that the high pressure hydrogen steel pipe (hereinafter also referred to simply as “steel pipe”) has a predetermined chemical composition. Therefore, the reasons for the restrictions imposed on the chemical composition of the steel pipe in the present invention will be described next. Here, "%" referring to chemical composition means "% by weight" unless otherwise specified.

C: 0.05 - 0.60%

C is an element that is needed to increase strength. To achieve the desired high strength (tensile strength: 800 MPa or higher), the C content is set to 0.05% or more. It is preferable that the C content is 0.20% or more, more preferably 0.25% or more, or even more preferably 0.33% or more. On the other hand, in the case where the C content is more than 0.60%, there may be a case where a quenching crack occurs when quenching is performed. Therefore, the C content is set to 0.60% or less. Further, it is preferable that the C content is 0.45% or less, or more preferably 0.40% or less.

Si: 0.001 - 2.0%

Si is an element that contributes to increased strength and fatigue strength through solid solution strengthening. In the case where the Si content is 0.001% or more, the effects described above are realized. Therefore, the Si content is set to 0.001% or more. It is preferable that the Si content is 0.15% or more, more preferably 0.2% or more, even more preferably 0.25% or more, or most preferably 0.3% or more. On the other hand, in the case where the Si content is more than 2.0%, the above effects are saturated, and the surface quality of the steel pipe and the rolling performance deteriorate. Therefore, the Si content is set to 2.0% or less. It is preferable that the Si content is 1.0% or less, more preferably 0.5% or less, or even more preferably 0.4% or less.

Mn: 0.01 - 5.0%

Mn is an element that contributes to increasing strength through solid solution strengthening and improving hardenability, and also has the function of improving fatigue strength. To realize the effects described above, the Mn content is set to 0.01% or more. It is preferable that the Mn content is 0.4% or more, more preferably 0.5% or more, or even more preferably 0.6% or more. On the other hand, in the case where the Mn content is more than 5.0%, the above effects are saturated and rolling and forming are difficult to perform. In addition, the tendency towards the preservation of austenite continues. Therefore, the Mn content is set to 5.0% or less. It is preferable that the Mn content is 1.5% or less, more preferably 1.0% or less, even more preferably 0.9% or less, or most preferably 0.8% or less.

P: 0.030% or less

P represents an element that contributes to increased strength through solid solution strengthening. On the other hand, P is also an element that causes deterioration of toughness and increases sensitivity to hydrogen embrittlement. In the case where the P content is more than 0.030%, such deterioration in properties becomes noticeable. Therefore, the phosphorus content is set to 0.030% or less. It is preferable that the P content is 0.025% or less, more preferably 0.015% or less, or even more preferably 0.010% or less. Although there is no specific limitation on the lower limit of P content, excessive reduction of P content to, for example, less than 0.0001% is accompanied by an increase in production costs in the steel production process. Therefore, it is preferable that the P content is 0.0001% or more.

S: 0.010% or less

Since increasing sulfur content causes brittleness, production problems may occur. In addition, S causes deterioration in toughness and increases sensitivity to hydrogen embrittlement due to the formation of MnS, which is an inclusion. Such influence does not cause problems in the case where the S content is 0.010% or less. Therefore, the sulfur content is set to 0.010% or less. In the case where further improvement in properties is desired, it is preferable that the S content is 0.003% or less. Although there is no specific lower limit for sulfur content, excessive reduction of sulfur content, for example to less than 0.00001%, is accompanied by an increase in the cost of desulfurization in the steelmaking process. Therefore, it is preferable that the S content is 0.00001% or more.

Incidentally, in order to stabilize the toughness at a high level, it is preferable that the sum of the P content and the S content is 0.02% or less.

N: 0.010% or less

Since N has little effect on the fatigue properties of the steel material, the effect of the present invention is not reduced in the case where the N content is 0.010% or less. Therefore, the N content is set to 0.010% or less. It is preferable that the N content is 0.005% or less, or more preferably 0.004% or less. Although there is no particular limitation on the lower limit of the N content, it is desirable that the N content be as low as possible from the viewpoint of improving toughness. Since excessive reduction of N content is accompanied by an increase in steel production costs, it is preferable that the N content be 0.00001% or more.

Al: 0.0001 - 1.00%

Al is an element that is effective as a deoxidizer in the steel production process. To realize this effect, the Al content is set to 0.0001% or more. It is preferable that the Al content is 0.02% or more, more preferably 0.03% or more, or even more preferably 0.04% or more. On the other hand, in the case where the Al content is more than 0.06%, although the deoxidation effect is saturated, since grain size separation is possible in the microstructure due to the addition of a large amount of Al, the material properties are stabilized. In the case where the Al content is more than 1.00%, this effect is saturated. Therefore, the Al content is set to 1.00% or less. It is preferable that the Al content is 0.50% or less, more preferably 0.20% or less, or even more preferably 0.10% or less.

O: 0.010% or less

Since O causes the formation of oxide-based inclusions, it is preferable to keep the O content as low as possible. This effect does not cause problems in the case where the O content is 0.010% or less. Therefore, the O content is set to 0.010% or less. It is preferable that the O content is 0.008% or less, more preferably 0.007% or less, or even more preferably less than 0.005%. Although there is no specific limitation on the lower limit of the O content, since production efficiency is reduced in the case where the O content is less than 0.0001%, it is preferable that the O content is 0.0001% or more.

H: 0.00010% or less

H can be introduced into steel material in various manufacturing processes, and in the case where the amount of introduced H is large, there is an increased risk of crack formation after solidification is completed, and an increase in fatigue crack growth rate may occur. Such effects do not cause problems when the H content is 0.00010% or less. Therefore, the H content is set to 0.00010% or less. It is preferable that the H content is 0.00008% or less, or more preferably less than 0.00005%. Although there is no specific limitation on the lower limit of the H content, since production efficiency is reduced in the case where the H content is less than 0.000001%, it is preferable that the H content is 0.000001% or more.

The remainder of the content of the steel pipe according to the present invention is Fe and incidental impurities in addition to the components described above. Moreover, in the present invention, the components described below can be further added in addition to the components described above.

Mo: 5.0% or less

Mo is an element that improves hardenability, thereby increasing the strength of steel pipe. In addition, Mo prevents an increase in the grain size of the original austenite, helps to increase fatigue strength due to solid-solution strengthening, and helps reduce sensitivity to hydrogen cracking. Although there is no specific limitation on the lower limit of the Mo content, in the case where Mo is added to realize the effects described above, it is preferable that the Mo content is 0.0001% or more. More preferably, the Mo content is 0.1% or more, or even more preferably 0.2% or more. On the other hand, in the case where the Mo content is more than 5.0%, the above-described effects become saturated and the cost increases. Therefore, in the case of adding Mo, the Mo content is set to 5.0% or less. It is preferable that the Mo content is 2.0% or less, more preferably 1.0% or less, even more preferably 0.5% or less, or most preferably 0.3% or less.

Cr: 5.0% or less

Cr is an element that improves hardenability, thereby helping to increase the strength of steel pipe. In addition, Cr helps reduce sensitivity to hydrogen cracking by preventing an increase in the grain size of the original austenite. In addition, Cr improves various properties in a hydrogen environment by preventing the grain size of the original austenite from increasing. Although there is no specific limitation on the lower limit of the Cr content, in the case where Cr is added to realize the effects described above, it is preferable that the Cr content is 0.0001% or more. More preferably, the Cr content is 0.2% or more, or even more preferably 0.5% or more. On the other hand, in the case where the Cr content is more than 5.0%, the above effects become saturated and the cost increases. Therefore, in the case of adding Cr, the Cr content is set to 5.0% or less. It is preferable that the Cr content is 2.5% or less, more preferably 1.5% or less, even more preferably 1.0% or less, or most preferably 0.9% or less.

Ni: 5.0% or less, Cu: 5.0% or less and Co: 5.0% or less

Ni, Cu and Co are elements that improve hardenability, thereby helping to increase the strength of steel pipe, and which improve various material properties of steel pipe by preventing the grain size of the original austenite from increasing. Although there is no particular limitation on the lower limit of the content of such elements, in the case where such elements are added to realize the effects described above, it is preferable that the content of each of Ni, Cu and Co is 0.0001% or more. More preferably, the content of each of Ni, Cu and Co is 0.5% or more. On the other hand, in the case where the content of each of Ni, Cu and Co is more than 5.0%, the above effects are saturated and the cost increases. Therefore, in the case of adding Ni, Cu and Co, the content of each of Ni, Cu and Co is set to 5.0% or less. To prevent cost increases, it is preferable that the content of each of Ni, Cu and Co be 2.0% or less.

B: 0.01% or less

B represents an element that improves hardenability, thereby helping to increase the strength of the steel pipe, and which improves various material properties of the steel pipe by preventing the grain size of the original austenite from increasing. Although there is no particular limitation on the lower limit of the B content, in the case where B is added to realize the effects described above, it is preferable that the B content is 0.0001% or more. More preferably, the B content is 0.001% or more. On the other hand, in the case where the B content is more than 0.01%, the above-described effects become saturated and the cost increases. Therefore, in the case of adding B, the B content is set to 0.01% or less. To prevent an increase in cost, it is preferable that the B content is 0.008% or less, or more preferably 0.005% or less.

V: 1.0% or less

V helps to increase the strength of the steel pipe. Although there is no particular limitation on the lower limit of the V content, in the case where V is added to realize the effect described above, it is preferable that the V content is 0.0001% or more. More preferably, the V content is 0.001% or more. On the other hand, in the case where the V content is more than 1.0%, the above effect becomes saturated and the cost increases. Therefore, in the case of V addition, the V content is set to 1.0% or less. To prevent cost increases, it is preferable that the V content is 0.5% or less.

W: 5.0% or less

W helps increase the strength of the steel pipe. Although there is no particular limitation on the lower limit of the W content, in the case where W is added to realize the effect described above, it is preferable that the W content is 0.0001% or more. More preferably, the W content is 0.001% or more. On the other hand, in the case where the W content is more than 5.0%, the above effect becomes saturated and the cost increases. Therefore, in the case of adding W, the W content is set to 5.0% or less. To prevent an increase in cost, it is preferable that the W content is 1.0% or less, more preferably 0.5% or less, or even more preferably 0.4% or less.

Nb: 0.1% or less and Ti: 0.1% or less

Nb and Ti help to increase the strength of steel pipe. Although there is no particular limitation on the lower limit of the content of such elements, in the case where such elements are added to realize the effect described above, it is preferable that the content of each of Nb and Ti is 0.0001% or more. More preferably, the content of each of Nb and Ti is 0.001% or more. On the other hand, in the case where the content of each of Nb and Ti is more than 0.1%, the above effect becomes saturated and the cost increases. Therefore, in the case of adding Nb and Ti, the content of each of Nb and Ti is set to 0.1% or less. To control the increase in cost, it is preferable that the content of each of Nb and Ti is 0.09% or less, more preferably 0.07% or less, or even more preferably 0.05% or less.

Zr: 0.2% or less, Hf: 0.2% or less and Ta: 0.2% or less

Zr, Hf and Ta contribute to increasing the strength of steel pipe. Although there is no particular limitation on the lower limit of the content of such elements, in the case of adding Zr, Hf and Ta to realize the above-described effect, it is preferable that the content of each of Zr, Hf and Ta is 0.0001% or more. More preferably, the content of each of Zr, Hf and Ta is 0.001% or more. On the other hand, in the case where the content of each of Zr, Hf and Ta is more than 0.2%, the above effect becomes saturated and the cost increases. Therefore, in the case of adding Zr, Hf and Ta, the content of each of Zr, Hf and Ta is 0.2% or less. To prevent an increase in cost, it is preferable that the content of each of Zr, Hf and Ta is 0.01% or less.

Sn: 0.2% or less and Sb: 0.2% or less

Sn and Sb cause deterioration in the rolling processability of steel pipe. Such elements may be contained when scrap is used as a raw material, and there is no specific lower limit for the content of such elements. In the case where Sn and Sb are present, the content of each of Sn and Sb is set to 0.2% or less to prevent the rolling processability of the steel pipe from deteriorating. It is preferable that the content of each of Sn and Sb is 0.1% or less, or more preferably 0.05% or less. On the other hand, since it is preferable that the content of each of Sn and Sb be as small as possible, the content of each of Sn and Sb may be 0%. However, in this case the cost increases. Therefore, in the case where Sn and Sb are contained, it is preferable that the content of each of Sn and Sb is 0.001% or more. To prevent cost increases, it is more preferable that the content of each of Sn and Sb is 0.002% or more.

Ca: 0.01% or less and Mg: 0.01% or less

Ca and Mg help improve the condition of inclusions. Although there is no particular limitation on the lower limit of the content of such elements, in the case where Ca and Mg are added to realize the above-described effect, it is preferable that the content of each of Ca and Mg be 0.0001% or more. More preferably, the content of each of Ca and Mg is 0.001% or more. On the other hand, in the case where the content of each of Ca and Mg is more than 0.01%, the above effect becomes saturated and the cost increases. Therefore, in the case of adding Ca and Mg, the content of each of Ca and Mg is set to 0.01% or less. To prevent cost increases, it is preferable that the content of each of Ca and Mg be 0.005% or less.

REM: 0.5% or less

Rare earth metals (REM) help improve the state of inclusions. Although there is no specific limitation on the lower limit of the REM content, in the case where REM is added to realize the effect described above, it is preferable that the REM content is 0.0001% or more. More preferably, the REM content is 0.001% or more. On the other hand, in the case when the REM content is more than 0.5%, the effect described above is saturated and the cost increases. Therefore, in the case of adding REM, the REM content is set to 0.5% or less. To prevent cost increases, it is preferable that the REM content be 0.1% or less. Incidentally, REM is a general term used to refer to Sc and Y, and fifteen elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the expression “REE content” here means the total content of these elements.

Preparation method

Next, a method for manufacturing a steel pipe according to the present invention will be described. In the following description, a method for manufacturing a steel pipe will be described by taking the case where the steel pipe is a seamless steel pipe as an example. However, needless to say, it is possible to produce a resistance welded steel pipe and a UOE steel pipe by performing processing in such a way as to reproduce the thermal history realized in the case of a seamless steel pipe. For example, in the case of resistance welded steel pipe, similar properties can be achieved by performing the casting process and heating process described below, by subsequently rolling the steel sheet under a final rolling temperature of 820°C or higher, followed by the cooling process and process tempering described below, and then welding is performed to obtain a resistance welded steel pipe.

The steel pipe according to the present invention can be manufactured by performing the following processes (1) to (5) in the given order:

(1) the process of casting steel material, with the selected chemical composition;

(2) the process of heating the cast material (steel material) cast in the casting process;

(3) the process of rolling the cast material heated by the heating process to produce a steel pipe;

(4) the cooling process of the steel pipe produced by the rolling process;

(5) the process of tempering steel pipe cooled in the cooling process.

The processes will be described below. Here in the description below, the term "temperature" means the surface temperature of the steel material or steel pipe unless otherwise specified.

Casting process

Casting at a casting speed of 1.0 m/min and below

The hydrogen concentration and the number of inclusions in steel decrease with decreasing casting speed, and this effect is observed at a casting speed of 1.0 m/min and lower. Therefore, the casting speed is set to 1.0 m/min or lower. Since the number of inclusions decreases as the casting speed decreases, it is preferable that the casting speed is 0.5 m/min or lower, or more preferably 0.1 m/min or lower. On the other hand, although there is no particular limitation on the lower limit of the casting speed, it is preferable that the casting speed is 0.01 m/min or higher from the point of view of productivity.

Heating process

To perform hot rolling, a steel material having the chemical composition described above is heated. Although there is no particular limitation on the steel material described above, examples of the steel material include a workpiece and the like that are manufactured using a conventional continuous casting method. In addition, to eliminate defects in the form of pores formed in the central part of the cross section perpendicular to the casting direction of the casting in continuous casting and to add a heat treatment process for hydrogen desorption, a workpiece made by hot forging of the cast material having a rectangular cross section is used to obtain a round cross section sections.

Heating at 1350°C or lower

In the case where the heating temperature during the heating process is higher than 1350°C, since the average grain size of the original austenite increases excessively, various properties deteriorate. Therefore, the heating temperature is set to 1350°C or lower. It is preferable that the heating temperature is 1300°C or lower. On the other hand, in the case where the heating temperature is excessively low, since the final rolling temperature decreases, rolling is difficult to perform. Therefore, it is preferable that the heating temperature is 950°C or higher. Although there is no particular limitation on the heating time (holding time at heating temperature described above), since excessively long heating time causes performance deterioration, it is preferable that the heating time is 180 minutes or less, or more preferably 120 minutes or less.

Rolling process

Then, in the rolling process, the steel material that has been heated in the heating process described above is rolled so that it has the shape of a steel pipe. For such rolling, a conventional hot rolling process involving piercing may be used, such as a Mannesmann mandrel piercing mill or Mannesmann mandrel piercing mill process.

Final rolling temperature: 820°C or higher

In the case where the final rolling temperature is below 820°C, because there is an excessive increase in rolling load, there is an increased risk of rolling problems. Therefore, the final rolling temperature is set to 820°C or higher. It is preferable that the final rolling temperature is 850°C or higher. On the other hand, although there is no specific limitation on the upper limit of the final rolling temperature, since the microstructure of the metal tends to be non-uniform in the case where the final rolling temperature is excessively high, it is preferable that the final rolling temperature is 1200° C. or lower.

Cooling process

Cooling to a temperature of 50°C or lower with an average cooling rate of 5°C/s or higher in the temperature range from 800°C to 350°C and at an average cooling rate of 3°C/s or lower in the temperature range from 350°C to 50°C

In the case where the average cooling rate in the temperature range from 800°C to 350°C is high, a uniform and fine carbide microstructure can be achieved, which effectively reduces the fatigue crack growth rate. Therefore, the average cooling rate in the temperature range from 800°C to 350°C is set to 5°C/s or higher. It is preferable that the average cooling rate in the temperature range from 800°C to 350°C is 6°C/s or higher, more preferably 8°C/s or higher, or even more preferably 10°C/s or higher. Although there is no particular limitation on the upper limit of such an average cooling rate, it is preferable that the average cooling rate in the temperature range from 800°C to 350°C is 15°C/s or lower from the point of view of the cost of the refrigerant. Moreover, by performing cooling to a temperature of 50°C or lower with an average cooling rate of 3°C/s or lower in a temperature range of 350°C to 50°C, the amount of hydrogen in the steel can be reduced. Therefore, the average cooling rate in the temperature range from 350°C to 50°C is set to 3°C/s or lower. Preferably, the average cooling rate in the temperature range from 350°C to 50°C is 2.8°C/s or lower, or more preferably 2.5°C/s or lower. Although there is no particular limitation on the lower limit of such an average cooling rate, it is preferable that the average cooling rate in the temperature range from 350°C to 50°C be 0.5°C/s or higher from a performance point of view.

There is no particular limitation on the method used for cooling and the water cooling method, oil cooling method, air cooling method and the like. can be used separately or together. It is preferable to use a water cooling method or an oil cooling method in a temperature range of 800°C to 350°C, and use an air cooling method in a temperature range of 350°C to 50°C.

Vacation process

Heating to a temperature of 400°C or higher and equal to or lower than the transformation temperature A1

After cooling is carried out as described above, heating is carried out to a temperature of 400°C or higher and equal to or lower than the transformation temperature A1 in the tempering process. In the case when the tempering temperature is 400°C and higher, the proportion of retained austenite decreases and the amount of hydrogen in the steel decreases. On the other hand, in the case where heating is carried out to a temperature above the transformation temperature A1, an increase in the proportion of retained austenite may be observed and the amount of hydrogen in the steel may increase. Here, although it depends on the size and shape of the steel material, it is preferable that heating is carried out to a temperature of 500° C. or higher during the tempering process. In addition, it is preferable that heating is carried out to a temperature equal to or lower (transformation temperature A1 -30°C). Although there is no particular limitation on the tempering time (holding time at the tempering temperature), since the proportion of retained austenite and the amount of hydrogen in the steel material decreases as the tempering time increases, it is preferable that the tempering time is 60 minutes or more, or more preferably 90 minutes or more.

Here, in the present invention, the transformation temperature A1 (°C) is calculated using the following equation.

Transformation temperature A1 (°C) = 751 - 16.3 [% C] × 34.9 [% Si] - 27.5 [% Mn],

where [ %C], [ %Si] and [ %Mn] in the above equation denote the content (% wt) of C, Si and Mn respectively.

The high pressure hydrogen steel pipe according to the present invention has a corresponding fatigue crack growth resistance property represented by a fatigue crack growth rate of 8.0×10 ^-8 m/cycle or lower when ΔK is 10 MPa m ^1/2 . In addition, the high pressure hydrogen steel pipe of the present invention has a tensile strength of 800 MPa or higher. It is preferable that the tensile strength is 850 MPa or higher. In addition, although there are no specific restrictions, it is preferable that the tensile strength is 1200 MPa or lower in terms of hydrogen embrittlement. Here, the tensile strength is determined using the method described in the examples. It is preferable that the high pressure hydrogen steel pipe according to the present invention is used for a high pressure hydrogen vessel (high pressure hydrogen gas storage vessel). The high pressure hydrogen vessel can be manufactured, for example, by shaping a high pressure hydrogen steel pipe, which is manufactured as described above, into a predetermined shape. Here in the present invention, the term “high pressure hydrogen” means, for example, a hydrogen gas medium having a pressure of 1.0 MPa or higher.

Examples

Hereinafter, the present invention will be described more specifically in accordance with examples. The examples described below show preferred examples of the present invention, and the present invention is in no way limited to such examples.

By manufacturing billets having the chemical composition indicated in Table 1 (with the exception of H), continuous casting, heating the billets, hot rolling the heated billets and subsequent cooling and tempering, steel pipes are obtained. The manufacturing conditions are shown in Table 2. Each of the resulting steel pipes was evaluated with respect to metal microstructure, hydrogen content, tensile strength, and fatigue crack growth rate. The assessment methods are described below. Here, the H content (% wt) given in Table 1 was determined using the hydrogen amount estimation method described below.

Inclusion

The inclusion test was carried out using an inclusion test sample having a length in the longitudinal direction of 20 mm, a length in the width direction of 5 mm, and a length in the wall thickness direction of 15 mm, which was taken from the central part in the wall thickness direction of the tempered steel pipe. By placing such a test piece in the resin so that the surface having a side in the longitudinal direction, that is, in the rolling direction of the steel pipe, and a side in the wall thickness direction (the so-called “L - cross section”) constitutes the surface to be analyzed, then performed mirror polishing, and then visually observed using an optical microscope, the number density of inclusions having an aspect ratio of 2.0 or more and a length of 10 μm or more in an area of 10 mm × 10 mm (100 mm ² ). Here, the number of test specimens for determining inclusions was 10 for each test number, the number of inclusions having an aspect ratio of 2.0 or more and a length of 10 μm or more in such 10 test specimens was determined as described above, and the arithmetic mean of the total quantities in such 10 test samples were determined as the number of inclusions (numerical density of inclusions) of the corresponding test number. Incidentally, the aspect ratio and length of the inclusions were determined in accordance with JIS G 0555:2020 (Method for microscopic examination of non-metallic inclusions in steel).

Area fraction in metal microstructure

A test sample was taken from each of the resulting steel pipes so that a position located at 1/4 of the wall thickness in the central portion in the longitudinal direction of the corresponding steel pipe was the observation position. The cross section of such a sample was etched in a 3% nital solution. This cross section was then analyzed using a scanning electron microscope (SEM) at an appropriate magnification of 1000 - 5000 times. By analyzing the resulting image, the types of phases and the area fraction of each phase were assessed. The area fraction of retained austenite was determined by chemical polishing of the cross-section of the test specimen described above and by X-ray diffraction analysis. Using a Co-Kα incident X-ray source, the area fraction of retained austenite was calculated from the ratio of the peak intensities of the (200), (211), and (220) planes of ferrite. to the (200) plane, (220) plane and (311) plane of austenite.

Incidentally, the properties of a high pressure hydrogen vessel can also be determined by taking a sample from the central portion in the longitudinal direction of the corresponding vessel, as in the case of the steel pipe described above.

Hydrogen quantity

The amount of hydrogen was determined by taking a round rod with a diameter of 15 mm and a length of 15 mm from the central part in the direction of the wall thickness of the tempered steel pipe and performing thermal desorption analysis. Here, the amount of hydrogen was defined as the time-integrated amount of hydrogen desorption (the amount of hydrogen detected in one minute) from the time when hydrogen desorption began to be detected (at room temperature) to the time once hydrogen desorption fell below the lower detection limit after increasing the temperature by during the analysis. On the other hand, depending on the steel grade, there was a case where hydrogen desorption did not fall below the lower limit of detection, for example, as a result of strong hydrogen trapping within the retained austenite grains. In this case, the amount of hydrogen was determined as the result of integration from the temperature at which detection began to a temperature of 400°C. In addition, the initial detection temperature was set to −100 °C and the heating rate for the assay was set to 200 °C/h to improve the assay efficiency. In this case, the number of round rods taken from one steel pipe was three, and the average amount of hydrogen in such three round rods was determined as the amount of hydrogen in steel (hydrogen content is given in Table 1).

Tensile Strength (TS)

A test sample in the form of a round rod with a diameter of 7 mm was selected from a part located at 1/4 of the wall thickness of the resulting steel pipe (a place located at 1/4 of the thickness from the outer surface of the steel pipe) in accordance with JIS Z 2201, and the tensile strength tensile test was determined in accordance with the standard “Metallic Materials – Tensile Test Method” prescribed in JIS Z 2241.

Fatigue crack growth rate

The fatigue crack growth rate was determined in high pressure hydrogen gas, at a pressure of 93 MPa. A 10 mm thick CT sample was taken from the inner surface of the steel pipe; using such a test sample, the fatigue crack growth rate was determined with a gradual decrease in ΔK from 15 MPa m ^1/2 to 8 MPa m ^1/2 , and the value was recorded when ΔK was 10 MPa m ^1/2 . Here, the cyclic stress frequency used in the fatigue crack growth process was set to 1 Hz. Here, the fatigue crack growth rate is determined using a CT test piece with a thickness of 10 mm, which is taken from the inner surface side of the steel pipe in the case of seamless steel pipe as described above, and, in the case of resistance welded steel pipe, UOE steel pipe and etc. (steel pipes made from steel strips or steel sheets), the fatigue crack growth rate is determined using a CT test piece of 10 mm thickness, which is taken from the steel pipe so that the center of the test piece corresponds to a position at 1/2 the wall thickness of the steel pipe .

As shown in Table 2, steel pipes (examples of the present invention) that satisfied the conditions of the present invention in terms of chemical composition and metal microstructure had suitable properties represented by a sufficient tensile strength of 800 MPa or higher and a fatigue crack growth rate at ΔK of 10 MPa. m ^1/2 , amounting to 8.0 × 10 ^-8 m/cycle or less.

As described above, the steel pipe according to the present invention has high strength and suitable resistance to fatigue crack growth. Similarly, a high pressure hydrogen vessel manufactured using such a steel pipe also has high strength and corresponding resistance to fatigue crack growth.

Claims (38)

1. A steel pipe for high-pressure hydrogen, wherein the steel pipe has a chemical composition containing, by weight. %: C: 0.05 - 0.60%, Si: 0.001 - 2.0%, Mn: 0.01 - 5.0%, P: 0.030% or less, S: 0.010% or less, N: 0.010% or less, Al: 0.0001 - 1.00%, O: 0.010% or less, H: 0.00010% or less, optionally at least one element selected from: Mo: 5.0% or less, Cr: 5.0% or less, Ni: 5.0% or less, Cu: 5.0% or less, Co: 5.0% or less, B: 0.01% or less, V: 1.0% or less, W: 5.0% or less, Nb: 0.1% or less, Ti: 0.1% or less, Zr: 0.2% or less, Hf: 0.2% or less, Ta: 0.2% or less, Sn: 0.2% or less, Sb: 0.2% or less, Ca: 0.01% or less, Mg: 0.01% or less and REM: 0.5% or less, the rest is Fe and random impurities, and a microstructure comprising, in area fractions, 3% or less (including 0%) retained austenite, in which the number of inclusions having an aspect ratio of 2.0 or more and a length of 10 μm or more is 15/100 mm ² or less. 2. A high pressure hydrogen vessel made using a high pressure hydrogen steel pipe according to claim 1 3. A method for manufacturing a steel pipe for high pressure hydrogen, including: a process for casting a steel material having the chemical composition of claim 1 at a casting speed of 1.0 m/min or lower; the process of heating steel material cast in a casting process at a temperature of 1350°C or lower; a process of rolling steel material heated by a heating process at a final rolling temperature of 820°C or higher to produce a steel pipe; the process of cooling a steel pipe obtained by rolling to a temperature of 50°C or lower at an average cooling rate of 5°C/s or higher in a temperature range of 800°C to 350°C and at an average cooling rate of 3°C/s or lower in the temperature range from 350°C to 50°C; And tempering process by heating a steel pipe cooled in a cooling process to a temperature of 400°C or higher and equal to or lower than the A1 transformation temperature for tempering the heated steel pipe.