Steel pipe, method for manufacturing steel pipe, and high-strength steel pipe

Abstract

Provided is a steel pipe with which excellent fatigue characteristics can be obtained when the steel pipe is used for a high-pressure hydrogen infrastructure. The steel pipe has a predetermined component composition, and has a number density of 100 / 100 cm2 or less of voids therein having a length in the wall thickness direction of 200 μm or more.

Description

Steel pipe, steel pipe manufacturing method, and high-strength steel pipe

[0001] The present invention relates to a steel pipe, a method for manufacturing a steel pipe, and a high-strength steel pipe.

[0002] Fuel cell vehicles that use hydrogen as fuel emit little carbon dioxide (CO ２ ) and has excellent energy efficiency, ２ Fuel cell vehicles are expected to be a vehicle that can solve emissions and energy problems. In order to popularize these vehicles, it is necessary to install hydrogen stations to supply hydrogen to fuel cell vehicles. Therefore, development is underway to develop containers with excellent strength and durability, which are necessary to safely store high-pressure hydrogen at hydrogen stations.

[0003] For example, Patent Document 1 proposes a liner for a hydrogen pressure vessel having a tensile strength of 800 MPa or more and a fatigue limit of 350 MPa or more. Patent Document 2 also proposes a high-pressure hydrogen container having a tensile strength of 850 MPa or more and excellent fatigue resistance in a high-pressure hydrogen gas environment. Patent Document 2 achieves high fatigue strength by reducing inclusions.

[0004] JP 2016-172926 A JP 2018-012855 A

[0005] However, while the techniques described in Patent Documents 1 and 2 provide good fatigue properties for test specimens, it has been found that the fatigue properties are still insufficient for large-scale hydrogen infrastructure such as actual hydrogen pressure vessels. In particular, problems are likely to become apparent when the hydrogen gas pressure is 20 MPa or higher.

[0006] The present invention is intended to solve the above-mentioned problems, and aims to provide a steel pipe that exhibits excellent fatigue properties when used in large-scale hydrogen infrastructure such as actual hydrogen pressure vessels.

[0007] As a result of intensive research conducted by the inventors to solve the above problems, it was discovered that an increase in the number of voids with a thickness direction length of 200 μm or more present inside a steel pipe results in a deterioration of fatigue properties in an environment where hydrogen penetrates into the steel.

[0008] The present invention has been completed based on the above findings, and the gist and configuration of the present invention are as follows.

[0009] 1. In mass%, C: 0.05 to 0.60%, Si: 0.001 to 2.0%, Mn: 0.01 to 5.0%, P: 0.0001 to 0.060%, S: 0.010% or less, N: 0.010% or less, Al: 0.0001 to 1.00%, O: 0.010% or less, H: 0.0010% or less, Mo: 0 to 5.0%, Cr: 0 to 5.0%, Ni: 0 to 5.0%, Cu: 0 to 5.0%, Co: 0 to 5.0%, B: 0 to 0.01%, V: 0 to 1.0%, W: 0 to 5.0%, Nb: 0 to 0.1%, Ti: 0 to 0.1%, Zr: 0-0.2%, Hf: 0-0.2%, Ta: 0-0.2%, Sb: 0-0.2%, Sn: 0-0.2%, Ca: 0-0.01%, Mg: 0-0.01%, and REM: 0-0.5%, with the balance being Fe and unavoidable impurities, and the number density of voids present inside and having a length of 200 μm or more in the thickness direction is 100 / 100 cm ２ Below is a steel pipe.

[0010] 2. The chemical composition is, in mass%, S: 0.00001 to 0.010%, N: 0.00001 to 0.010%, Mo: 0.0001 to 5.0%, Cr: 0.0001 to 5.0%, Ni: 0.0001 to 5.0%, Cu: 0.0001 to 5.0%, Co: 0.0001 to 5.0%, B: 0.0001 to 0.01%, V: 0.0001 to 1.0%, W: 0.0001 to 5.0%, Nb: 0.0001 to 0.1%, Ti: 0.0001 to 0.1%, Zr: 0.0001 to 0.2%, Hf: 0.0001 to 0.2%, 2. The steel pipe according to claim 1, containing at least one selected from the group consisting of Ta: 0.0001 to 0.2%, Sb: 0.0001 to 0.2%, Sn: 0.0001 to 0.2%, Ca: 0.0001 to 0.01%, Mg: 0.0001 to 0.01%, and REM: 0.0001 to 0.5%.

[0011] 3. A steel pipe according to 1 or 2 above, having a structure in which the area fraction of austenite is 0 to 3.0%, the total area fraction of ferrite and pearlite is 50% or more, and the ferrite grain size is 200 μm or less.

[0012] 4. A method for producing a steel pipe, comprising heating a steel material having the chemical composition according to 1 or 2 above, piercing the heated steel material to form a hollow blank, hot working the hollow blank to form a steel pipe having a final finished wall thickness, and cooling the steel pipe, wherein in the hot working, a processing rate at 1000°C or higher, which is defined as the rate of change in wall thickness at temperatures of 1000°C or higher to the change in wall thickness throughout the hot working, is 50% or higher, and in the cooling, the average cooling rate from the end of the hot working to 500°C is 5°C / min or lower.

[0013] 5. A high-strength steel pipe having a tensile strength of 800 MPa or more, obtained by quenching and tempering the steel pipe according to any one of 1 to 3 above.

[0014] According to the present invention, it is possible to provide a steel pipe that exhibits excellent fatigue properties when used in large-scale hydrogen infrastructure such as actual hydrogen pressure vessels. The steel pipe of the present invention can be extremely suitably used as a steel pipe for high-pressure hydrogen infrastructure that is used at pressures of 30 MPa or more, for example.

[0015] The present invention will be specifically described below. Note that the following description is of preferred embodiments of the present invention, and the present invention is not limited to the embodiments described below.

[0016] [Composition] The steel pipe of the present invention has the above-mentioned composition. The reasons for limiting the composition of the steel pipe in the present invention will be explained below. In this specification, the "%" designation regarding the composition means "mass %" unless otherwise specified.

[0017] C: 0.05 to 0.60% C is an element necessary for increasing strength. High-pressure hydrogen infrastructure, such as high-pressure hydrogen gas containers, desirably has a high tensile strength of 800 MPa or more. If the C content of a steel pipe is 0.05% or more, such high strength can be obtained after heat treatment. Therefore, the C content is set to 0.05% or more, preferably 0.10% or more. On the other hand, if the C content exceeds 0.60%, quench cracks may occur during quenching. Therefore, the C content is set to 0.60% or less. Furthermore, the lower the C content, the easier it is to control the strength of the material before heat treatment and the easier it is to process, so the C content is preferably set to 0.45% or less.

[0018] Si: 0.001 to 2.0% Si is an element that contributes to improving strength and fatigue limit through solid solution strengthening. The above effects can be achieved if the Si content is 0.001% or more. Therefore, the Si content is set to 0.001% or more, preferably 0.01% or more, more preferably 0.10% or more, and even more preferably 0.15% or more. On the other hand, if the Si content exceeds 2.0%, the effect saturates, and further, the surface properties of the steel material deteriorate, and rollability also decreases. Therefore, the Si content is set to 2.0% or less, preferably 0.5% or less.

[0019] Mn: 0.01 to 5.0% Mn is an element that contributes to improving strength by improving solution strengthening and hardenability, and also has the function of improving the fatigue limit. To achieve this effect, the Mn content is set to 0.01% or more, preferably 0.10% or more, and more preferably 0.30% or more. On the other hand, if the Mn content exceeds 5.0%, not only does the effect saturate, but rolling and forming become difficult. Furthermore, austenite tends to remain. Therefore, the Mn content is set to 5.0% or less, preferably 4.0% or less, more preferably 3.0% or less, and even more preferably 1.5% or less.

[0020] P: 0.0001 to 0.060% P is an element that contributes to improving strength through solid solution strengthening. On the other hand, P also reduces toughness and increases hydrogen embrittlement susceptibility, and a P content exceeding 0.060% results in significant deterioration of properties. Therefore, the P content is set to 0.060% or less, preferably 0.025% or less, and more preferably 0.015% or less. On the other hand, excessive reduction of P content, such as to less than 0.0001%, increases manufacturing costs in the steelmaking process. Therefore, the P content is set to 0.0001% or more, preferably 0.001% or more, and more preferably 0.003% or more.

[0021] S: 0.010% or less. An increase in the S content can cause hot shortness and lead to manufacturing defects. Furthermore, S forms MnS inclusions, which reduce toughness and increase hydrogen embrittlement susceptibility. These effects are not a problem when the S content is 0.010% or less. Therefore, the S content is set to 0.010% or less, preferably 0.007% or less, and more preferably 0.003% or less. On the other hand, the lower limit of the S content is not particularly limited and may be 0%. However, excessive reduction of the S content to less than 0.00001% increases the desulfurization costs in the steelmaking process. Therefore, from a cost perspective, the S content is preferably set to 0.00001% or more, and more preferably 0.0001% or more.

[0022] In order to further improve the toughness, the total content of P and S is preferably set to 0.02% or less.

[0023] N: 0.010% or less The effect of N on the fatigue properties of steel is small, and an N content of 0.010% or less does not impair the effects of the present invention. Therefore, the N content is set to 0.010% or less, preferably 0.008% or less, and more preferably 0.004% or less. On the other hand, from the viewpoint of improving toughness, a low N content is desirable. Therefore, the lower limit of the N content is not particularly limited and may be 0%. However, excessive reduction increases the steelmaking cost. Therefore, the N content is preferably set to 0.00001% or more, more preferably 0.0001% or more, and even more preferably 0.001% or more.

[0024] Al: 0.0001 to 1.00% Al is an effective element as a deoxidizer in the steelmaking process. To achieve this effect, the Al content is set to 0.0001% or more, preferably 0.001% or more, more preferably 0.01% or more, and even more preferably 0.02% or more. When the Al content exceeds 0.06%, the deoxidizing effect saturates. However, by adding more Al, the grain structure can be refined and the material properties can be stabilized. However, this effect saturates when the Al content exceeds 1.00%. Therefore, the Al content is set to 1.00% or less, preferably 0.80% or less, more preferably 0.50% or less, and even more preferably 0.40% or less.

[0025] O: 0.010% or less O causes the formation of oxide-based inclusions, so the lower the content, the better. This effect is not a problem if the O content is 0.010% or less. Therefore, the O content is set to 0.010% or less, preferably 0.005% or less, and more preferably less than 0.0030%. On the other hand, the lower limit of the O content is not particularly limited and may be 0%. However, excessive reduction increases manufacturing costs. Therefore, from the viewpoint of cost, the O content is preferably set to 0.0001% or more, and more preferably 0.001% or more.

[0026] H: 0.0010% or less H may be introduced into steel during various manufacturing processes. If a large amount of H is introduced into the steel, the risk of cracking after solidification increases. In addition, if the amount of H is large, the fatigue properties of the steel pipe may deteriorate. This is because there is a high risk of internal voids expanding or newly forming in the steel pipe after manufacturing. These effects do not pose a problem if the H content is 0.0010% or less. Therefore, the H content is set to 0.0010% or less, preferably 0.0001% or less, and more preferably less than 0.00005%. On the other hand, the lower limit of the H content is not particularly limited and may be 0%.

[0027] The chemical composition of a steel pipe according to one embodiment of the present invention may consist of the above-mentioned elements, with the balance being Fe and unavoidable impurities. In addition to the above elements, the chemical composition may optionally contain at least one of the following elements. That is, the elements listed below are optional elements, and therefore the lower limit of their content is 0%.

[0028] Mo: 0 to 5.0% Mo is an element that improves hardenability, and adding Mo can further improve the strength of steel pipes. Mo also suppresses coarsening of prior austenite grains and further improves fatigue strength through solid solution strengthening. Mo also contributes to reducing hydrogen cracking susceptibility. However, if the Mo content exceeds 5.0%, the effect saturates and causes increased costs. Therefore, the Mo content is set to 5.0% or less, preferably 1.50% or less, more preferably 1.0% or less, and even more preferably 0.5% or less. On the other hand, since the addition of Mo is not essential, the lower limit of the Mo content is 0%. However, to achieve the above effect, the Mo content is preferably 0.0001% or more, more preferably 0.001% or more, even more preferably 0.01% or more, and most preferably 0.1% or more.

[0029] Cr: 0 to 5.0% Cr is an element that improves hardenability, and adding Cr can further improve the strength of steel pipes. Cr also has the effect of suppressing the coarsening of prior austenite grains. Adding Cr can further improve various properties in hydrogen environments, such as reducing hydrogen cracking susceptibility. However, if the Cr content exceeds 5.0%, the effect saturates and causes increased costs. Therefore, the Cr content is set to 5.0% or less, preferably 2.00% or less, and more preferably 1.50% or less. On the other hand, since the addition of Cr is not essential, the lower limit of the Cr content is 0%. However, to achieve the above effect, the Cr content is preferably set to 0.0001% or more, more preferably 0.001% or more, even more preferably 0.01% or more, and most preferably 0.5% or more.

[0030] Ni: 0-5.0% Cu: 0-5.0% Co: 0-5.0% Ni, Cu, and Co are elements that improve hardenability and contribute to further improving the strength of steel pipes. Ni, Cu, and Co also suppress the coarsening of prior austenite grains and improve various material properties. Therefore, Ni, Cu, and Co can be added arbitrarily. However, if the Ni, Cu, and Co contents exceed 5.0%, the effect saturates and costs increase. Therefore, the Ni, Cu, and Co contents are each limited to 5.0% or less, preferably 2.0% or less. However, because the addition of Ni, Cu, and Co is not essential, the lower limit of the content of these elements is 0%. However, in order to obtain the above-mentioned effects, the content is preferably 0.0001% or more, more preferably 0.001% or more, even more preferably 0.01% or more, and most preferably 0.5% or more. Note that Ni, Cu, and Co can be added independently, and the contents of these elements can be adjusted independently.

[0031] B: 0 to 0.01% B is an element that improves hardenability and contributes to further increasing the strength of steel pipes. B also suppresses the coarsening of prior austenite grains, improving various material properties. However, if the B content exceeds 0.01%, the effect saturates and causes an increase in costs. Therefore, the B content is set to 0.01% or less, preferably 0.005% or less. On the other hand, since the addition of B is not essential, the lower limit of the B content is 0%. However, to obtain the above effect, the B content is preferably set to 0.0001% or more, and more preferably 0.0010% or more.

[0032] V: 0 to 1.0% V contributes to further increasing the strength of steel pipes. However, if the V content exceeds 1.0%, the effect saturates and becomes a factor in increasing costs. Therefore, the V content is set to 1.0% or less, preferably 0.5% or less. On the other hand, since the addition of V is not essential, the lower limit of the V content is 0%. However, in order to obtain the above effect, the V content is preferably set to 0.0001% or more, and more preferably 0.0010% or more.

[0033] W: 0 to 5.0% W contributes to further increasing the strength of steel pipes. However, if the V content exceeds 5.0%, the effect saturates and becomes a factor in increasing costs. Therefore, the V content is set to 5.0% or less, preferably 0.5% or less. On the other hand, since the addition of W is not essential, the lower limit of the W content is 0%. However, in order to obtain the above effect, the W content is preferably set to 0.0001% or more, and more preferably 0.0010% or more.

[0034] Nb: 0-0.1% Ti: 0-0.1% Nb and Ti contribute to further increasing the strength of steel pipes. However, if the Nb content and Ti content each exceed 0.1%, the effect saturates and costs increase. Therefore, the Nb content and Ti content are each set to 0.1% or less, preferably 0.05% or less. On the other hand, since the addition of Nb and Ti is not essential, the lower limit of the content of these elements is 0%. However, to obtain the above effects, the content is preferably set to 0.0001% or more, and more preferably 0.0010% or more. Note that Nb and Ti can be added independently, and the contents of these elements can be adjusted independently.

[0035] Zr: 0-0.2%, Hf: 0-0.2%, Ta: 0-0.2%. Zr, Hf, and Ta contribute to further increasing the strength of steel pipes. However, if the Zr, Hf, and Ta contents exceed 0.2%, the effect saturates and costs increase. Therefore, the Zr, Hf, and Ta contents are each limited to 0.2% or less, preferably 0.01% or less. Since the addition of Zr, Hf, and Ta is not essential, the lower limit for the content of these elements is 0%. However, to achieve the above effects, the content is preferably 0.0001% or more, and more preferably 0.0010% or more. Zr, Hf, and Ta can be added independently, and the content of each element can be adjusted independently.

[0036] Sb: 0-0.2% Sn: 0-0.2% Sb and Sn are elements that improve corrosion resistance. Adding one or both of Sb and Sn can suppress corrosion of steel pipes even when there are coating defects. However, adding a large amount exceeding 0.2% degrades manufacturability and toughness. Therefore, the Sb and Sn contents are each set to 0.2% or less. On the other hand, since the addition of Sb and Sn is not essential, the lower limit of the content of these elements is 0%. However, to obtain the above effects, it is preferable to set the content to 0.0001% or more.

[0037] Ca: 0 to 0.01% Mg: 0 to 0.01% Ca and Mg contribute to improving the condition of inclusions. However, if the content exceeds 0.01%, the effect saturates and costs increase. Therefore, the contents of Ca and Mg are each set to 0.01% or less, preferably 0.0050% or less. On the other hand, since the addition of Ca and Mg is not essential, the lower limit of the content of these elements is 0%. However, to obtain the above effects, the contents are preferably set to 0.0001% or more, and more preferably 0.0010% or more.

[0038] REM: 0 to 0.5% REM (rare earth metals) contribute to improving the condition of inclusions. However, if the REM content exceeds 0.5%, the effect saturates and this leads to increased costs. Therefore, the REM content is set to 0.5% or less, preferably 0.1% or less. On the other hand, since the addition of REM is not essential, the lower limit of the REM content is 0%. However, in order to obtain the above effect, the REM content is preferably set to 0.0001% or more, and more preferably 0.0010% or more.

[0039] The remainder of the composition other than the above-mentioned components is composed of Fe and unavoidable impurity elements. Examples of the unavoidable impurities include Pb. The content of the unavoidable impurities is not particularly limited, but it is preferable that the total content of the unavoidable impurities is 0.1% or less.

[0040] [Voids] In the present invention, as described above, controlling the number density of voids present inside the steel pipe is particularly important in order to achieve the desired fatigue properties. The reason for this will be explained below.

[0041] Normally, minute voids formed during the solidification process during manufacturing exist inside a steel pipe. According to the research of the present inventors, even if the chemical composition, structure, and inclusions of the steel pipe are appropriately controlled, sufficient fatigue properties cannot be obtained if a large number of voids of a certain size are present. This is thought to be because defects tend to bond and propagate in the wall thickness direction when internal pressure is applied.

[0042] The effect of such voids is relatively small on a small scale, such as in test specimens used for laboratory-level evaluations, but becomes apparent in large-scale hydrogen infrastructure such as actual hydrogen pressure vessels. Therefore, reducing voids is effective in preventing fatigue failure when applied to actual high-pressure hydrogen infrastructure.

[0043] Number density of voids: 100 / 100 cm ２ Specifically, the number density of voids present inside the steel pipe and having a length in the wall thickness direction of 200 μm or more (hereinafter simply referred to as "number density of voids") is 100 / 100 cm ２ If the number density of the voids exceeds 100 / 100 cm, the fatigue properties will deteriorate. ２ Preferably 50 pieces / 100cm or less ２ Less than 20 pieces / 100cm, more preferably ２ More preferably, 10 pieces / 100cm or less ２ By reducing the number density of voids in this way, the probability of defects linking together is reduced, and therefore fatigue properties are improved. On the other hand, since the fewer the voids, the better, there is no particular lower limit to the number density of voids, and it is not particularly limited to 0 voids / 100 cm. ２ It may be.

[0044] The number density of the voids can be measured by ultrasonic flaw detection. In this case, if the wall thickness of the steel pipe is 20 mm or more, the measurement is made in a region of 20 mm in the wall thickness direction centered at the half-wall position of the steel pipe, and if the wall thickness is less than 20 mm, the measurement is made over the entire wall thickness. This is because voids are particularly likely to occur near the center of the wall thickness of the steel pipe. More specifically, the number density can be measured by the method described in the examples.

[0045] [Microstructure] The steel pipe of the present invention achieves excellent fatigue properties by controlling the chemical composition and the number density of voids as described above, and therefore the microstructure of the steel pipe is not particularly limited. However, from the viewpoint of further improving performance, it is preferable that the steel pipe has a microstructure that satisfies the following conditions. In this specification, the microstructure of the steel pipe is defined as the microstructure at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe. Furthermore, the "%" designation regarding the metal microstructure means the area fraction unless otherwise specified.

[0046] Austenite area fraction: 0 to 3.0%. If a large amount of austenite is present in the structure of a steel pipe, hydrogen is more likely to be trapped in the steel. As a result, when austenite transforms into ferrite, martensite, or the like during the process of manufacturing a final product from the steel pipe, hydrogen is released, increasing the risk of voids being generated and enlarged. Therefore, from the perspective of further improving fatigue properties in a hydrogen environment, the austenite area fraction is preferably 3.0% or less, more preferably 2.0% or less, and even more preferably 1.0% or less. On the other hand, the less austenite, the better, so the lower limit of the austenite area fraction is set to 0%. The austenite area fraction can be measured by X-ray diffraction measurement. More specifically, it may be measured using the method described in the Examples.

[0047] - Total area fraction of ferrite and pearlite: 50% or more. If the total area fraction of ferrite and pearlite is less than 50%, the reverse transformation during final heat treatment of the steel pipe will vary greatly, restricting the heat treatment conditions required to obtain a uniform structure. Therefore, the total area fraction of ferrite and pearlite is preferably 50% or more, more preferably 80% or more, even more preferably 90% or more, and most preferably 95% or more. On the other hand, the upper limit of the total area fraction of ferrite and pearlite is not particularly limited and may be 100%. The structure may contain either ferrite or pearlite or both, but typically contains both ferrite and pearlite. The total area fraction of ferrite and pearlite can be determined by analyzing a structure image taken using an optical microscope. More specifically, it can be measured using the method described in the Examples.

[0048] Average particle size of ferrite: 200 μm or less. If the average particle size of ferrite exceeds 200 μm, the structure after final heat treatment becomes coarse, restricting the manufacturing conditions for obtaining the properties required for hydrogen infrastructure. Therefore, the average particle size of ferrite is preferably 200 μm or less, and more preferably 50 μm or less. On the other hand, fine grains are preferable from the viewpoint of material control after heat treatment, so the lower limit of the average particle size of the ferrite is not particularly limited. However, since achieving fine grains increases manufacturing constraints, the average particle size of the ferrite is preferably 5 μm or more, and more preferably 10 μm or more. The average particle size of the ferrite can be determined by a sectioning method from a structural image taken with an optical microscope. More specifically, it can be measured using the method described in the Examples.

[0049] The wall thickness of the steel pipe of the present invention is not particularly limited and may be any thickness. However, from the viewpoint of further enhancing compatibility with high-pressure hydrogen infrastructure, the wall thickness is preferably 30 mm or more. On the other hand, the upper limit of the wall thickness is not particularly limited, but from the viewpoint of production using existing facilities, it is preferably 100 mm or less, and more preferably 65 mm or less. This technology can be effectively applied to steel pipes with an outer diameter of 300 mm or more, but can also be applied to steel pipes with an outer diameter of less than 300 mm, because the smaller the outer diameter of the steel pipe, the smaller the stress applied to the steel pipe at the same gas pressure.

[0050] [Manufacturing method] Next, a method for manufacturing a steel pipe of the present invention will be described. In the following description, the manufacturing method will be described using an example in which the steel pipe is a seamless steel pipe, but it goes without saying that electric resistance welded pipes and UOE steel pipes can also be manufactured by performing treatments to achieve a similar thermal history. In the following description, temperatures refer to the temperatures at the surface of the steel material or steel pipe, unless otherwise specified.

[0051] First, a steel material having the above-described chemical composition is heated, and the heated steel material is pierced to form a hollow blank. Next, the hollow blank is hot worked to form a steel pipe having a final finished wall thickness, and the steel pipe is cooled. The steel material may be, for example, a billet. A series of processes from the piercing to the hot working can be performed, for example, by a Mannesmann plug mill system or a Mannesmann mandrel mill system. The hot working may typically be hot rolling, but forging or drawing can also be used.

[0052] The heating temperature when heating the steel material is not particularly limited, and any heating temperature can be used as long as it satisfies the working ratio conditions described below. Since the lower the heating temperature, the more hydrogen content in the steel can be reduced, it is desirable to lower the heating temperature from the perspective of reducing the amount of hydrogen. Therefore, the heating temperature is preferably 1350°C or lower. However, if the heating temperature is too low, it may be difficult to achieve a working ratio of 50% or higher at 1000°C or higher. Therefore, the heating temperature is preferably 1150°C or higher.

[0053] Although the heating time is not particularly limited, if it is too long, there is a high risk of increasing the amount of hydrogen introduced into the steel material. Therefore, the heating time is preferably 180 minutes or less. On the other hand, although the lower limit of the heating time is not particularly limited, it is preferably 10 minutes or more, and more preferably 30 minutes or more.

[0054] Processing ratio R at 1000°C or higher: 50% or higher In the above hot processing, it is important that the processing ratio R at a temperature of 1000°C or higher is 50% or higher. If the processing ratio R is less than 50%, it becomes difficult to compress the internal defects generated during solidification, so the number density of voids should be 100 / 100cm. ２ Therefore, the processing ratio R is set to 50% or more, preferably 70% or more. On the other hand, the upper limit of the processing ratio R is not particularly limited, but may typically be 100% or less, 90% or less, or 80% or less.

[0055] Here, the processing ratio R is a value defined as the ratio (percentage) of the change in thickness at a temperature of 1000°C or higher to the change in thickness throughout the hot processing, and can be calculated by the following formula (1): R (%) = (t Ｓ -t １０００ ) / (t Ｓ -t Ｆ ) × 100 (1) where the definitions of the symbols in the formula are as follows: Ｓ t: wall thickness of hollow blank before hot working Ｆ t: Final thickness (thickness after hot working) １０００ : Wall thickness when the temperature reaches 1000°C during hot working

[0056] In the case of UOE steel pipe or electric resistance welded steel pipe, 50% or more of the change in thickness between the slab and the final plate thickness may be achieved by rolling at 1000° C. or higher. The change in thickness between the slab and the final plate thickness is the difference between the slab thickness and the final plate thickness when a steel plate is produced by rolling as a material for press or roll forming.

[0057] The end temperature of the hot working is not particularly limited. However, when the steel pipe is subjected to heat treatment to adjust its material properties for application in hydrogen infrastructure, in order to obtain a structure with better properties, the end temperature of rolling is preferably set to Ar3 or higher to prevent ferrite transformation during rolling and to improve properties by homogenizing the structure after heat treatment. On the other hand, the end temperature of rolling is preferably set to 950°C or lower to prevent the gamma grain size from becoming too coarse and to improve properties by refining the structure. The Ar3 point can be calculated using the following formula (1): Ar3 (°C) = 910 - 203√C - 30Mn + 44.7Si + 700P + 100Al + 31.5Mo - 11Cr - 15.2Ni - 20Cu + 104V ... (1) where the element symbols in the formula represent the content (mass%) of the element.

[0058] Average cooling rate: 5°C / min or less During the cooling, the average cooling rate from the end of the hot working to 500°C is 5°C / min or less. If the average cooling rate is 5°C / min or less, the amount of hydrogen introduced into the steel during casting and heating is 0.0010% or less, which can prevent the expansion or new formation of voids that could not be closed by rolling. The average cooling rate is preferably 2°C / min or less. On the other hand, if the cooling rate is extremely slow, austenite-stabilizing elements will concentrate in the austenite portion that remains at lower temperatures during cooling. As a result, non-uniformity of alloying elements is likely to occur during heat treatment, and reverse transformation to austenite may also be delayed. Therefore, the average cooling rate is preferably 0.5°C / min or more.

[0059] [High-Strength Steel Pipe] A high-strength steel pipe according to one embodiment of the present invention is a high-strength steel pipe obtained by quenching and tempering the above-described steel pipe, and has a tensile strength of 800 MPa or more. The upper limit of the tensile strength is not particularly limited, but it may typically be 1250 MPa or less, or may be 1200 MPa or less. Furthermore, as shown in the examples described below, particularly excellent fatigue properties are obtained when the tensile strength is 1000 MPa or less. Therefore, from the viewpoint of fatigue properties, it is more preferable that the tensile strength is 1000 MPa or less.

[0060] The quenching and tempering conditions are not particularly limited, and conditions common in the art, i.e., a temperature at which an austenite single-phase structure is obtained before quenching, can be used. Typically, the heating temperature during quenching (quenching temperature) is preferably 850°C or higher, more preferably 900°C or higher. The tempering temperature is preferably 500°C or higher.

[0061] Here, the tensile strength refers to the tensile strength at a position 1 / 4 of the wall thickness from the inner surface of the high-strength steel pipe. The tensile strength can be measured by a tensile test in accordance with JIS Z 2241 using a test piece taken from a position 1 / 4 of the wall thickness from the inner surface of the high-strength steel pipe. More specifically, it may be measured by the method described in the Examples. If the tensile strength at a position close to the inner surface of the steel pipe is sufficiently high, it can be determined that the strength of the steel pipe as a whole is sufficiently high.

[0062] Next, the present invention will be described in more detail based on examples. The following examples are intended to illustrate preferred examples of the present invention, and the present invention is not limited to these examples in any way.

[0063] A steel material (billet) having the chemical composition shown in Table 1 was prepared, and a seamless steel pipe having an outer diameter of 300 mm or more and a wall thickness of 30 mm or more was obtained from the steel material. Specifically, the steel material was heated and pierced to form a hollow blank. The hollow blank was then hot worked to form a steel pipe having a final finished wall thickness. The steel pipe was then cooled. Detailed manufacturing conditions are shown in Table 2.

[0064] Next, the number density and structure of voids in each of the obtained steel pipes were measured by the following procedure. The measurement results are shown in Table 2.

[0065] (Number density of voids) The number density of voids was measured in a 20 mm thick region in the wall thickness direction centered at the half position of the wall thickness of the steel pipe. Specifically, test pieces 60 mm wide, 200 mm or longer, and 20 mm thick were cut out from the steel pipe by processing, and the number density of voids 200 μm or longer in the wall thickness direction was measured by ultrasonic flaw detection.

[0066] (Austenite Area Fraction) A test specimen was taken from the steel pipe so that the observation position was the longitudinal center of the steel pipe, located 1 / 4 of the way down from the inner surface to the wall thickness. The cross section of the test specimen was buffed, and then the surface of the test specimen was chemically polished to a thickness of 50 μm or more using oxalic acid. The austenite area fraction was then measured by X-ray diffraction measurement using the test specimen. A Co-Kα radiation source was used for incident X-rays, and the austenite area fraction was calculated from the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite.

[0067] (Total area fraction of ferrite and pearlite, ferrite grain size) A test specimen was taken from the steel pipe so that the observation position was the longitudinal center of the steel pipe, at a position 1 / 4 of the wall thickness from the inner surface. The surface of the test specimen was etched using a nital solution to reveal the structure. Then, the surface of the test specimen was observed using an optical microscope at an appropriate magnification of 50 to 400 times to obtain a structure image. Ferrite and pearlite were distinguished from the structure image, and image analysis was performed to calculate the total area fraction of ferrite and pearlite. In addition, the ferrite grain size was determined by a cutting method.

[0068] Next, test specimens taken from the steel pipe were quenched and tempered to evaluate the properties after the heat treatment. The properties of the test specimens after quenching and tempering can be considered to be equivalent to the properties of a high-strength steel pipe obtained by quenching and tempering the steel pipe.

[0069] Specifically, the steel pipe was sliced ​​into rings to obtain annular test pieces of the required length. Next, ultrasonic testing was performed on the cross section of the annular test piece, and the longest length of the detected defects in the wall thickness direction was determined. The obtained longest length of the defects was used in the subsequent fatigue property test.

[0070] Next, the annular test specimens were quenched and tempered. In the quenching, the annular test specimens were heated at 900°C for 60 minutes, and then oil-cooled under conditions that would prevent cracking. Subsequent tempering was performed for 90 minutes at the tempering temperature shown in Table 2. The quenching and tempering conditions were selected so that the tensile strength measured in the subsequent tensile test would be 800 MPa or more. It was separately confirmed that the test specimens after the quenching and tempering had a structure consisting of tempered martensite or a composite structure of tempered martensite and bainite.

[0071] The tensile strength and fatigue properties of each of the heat-treated test pieces were evaluated according to the following procedures. The evaluation results are shown in Table 2.

[0072] (Tensile Strength) A round bar test piece having a diameter of 7 mm was taken from a position ¼ of the wall thickness from the inner surface of each heat-treated test piece in accordance with JIS Z 2201. A tensile test was performed using the round bar test piece in accordance with JIS Z 2241 to measure the tensile strength.

[0073] (Fatigue Properties) The fatigue life of the heat-treated test specimens was measured to evaluate their fatigue properties. Specifically, three fatigue test specimens were taken from each of the heat-treated test specimens in a direction parallel to the longitudinal direction of the test specimen. In this case, if the longest length in the thickness direction of the defects detected by the ultrasonic flaw detection described above was 200 μm or more, the test specimens were processed so that the voids were located in the parallel portion of the test specimen. The diameter of the parallel portion of the test specimen was 5 mm and the length was 14 mm, and the test specimens were buffed and polished.

[0074] The fatigue test was carried out while hydrogen charging the fatigue test specimen. The hydrogen charging was carried out by cathodic charging with only the parallel part of the fatigue test specimen in contact with the solution. The solution was 1M NaOH + 3 g / L NH ４ An SCN solution was used, and the applied voltage was -1.125 V (vs. SHE). The cathodic charging introduced an amount of hydrogen equivalent to that in high-pressure hydrogen of 105 MPa into the specimen. The fatigue test conditions were a stress ratio of -1 and a frequency of 2 Hz. The applied stress was 0.55 times the tensile strength in the smooth tensile test.

[0075] The above fatigue test was carried out on each of the three fatigue test pieces, and the minimum fatigue life among the three measurement results is shown in Table 2. Here, if the minimum fatigue life was 100,000 cycles or more, the fatigue characteristics were judged to be good.

[0076] As shown in Table 2, the steel pipes satisfying the conditions of the present invention had both a high strength of 800 MPa or more and excellent fatigue properties with a fatigue life of 100,000 cycles or more in a hydrogen environment after heat treatment. In particular, for steel pipes with a tensile strength of 1,000 MPa or less, the number density of voids was 20 / 100 cm. ２ In the following cases, the minimum fatigue life is 200,000 cycles or more, which is an especially excellent characteristic. As can be seen from this, the steel pipe of the present invention can be extremely suitably used as a material for various members to be used in a high-pressure hydrogen environment.

[0077]

[0078]

Claims

1. In mass%, C: 0.05 to 0.60%, Si: 0.001 to 2.0%, Mn: 0.01 to 5.0%, P: 0.0001 to 0.060%, S: 0.010% or less, N: 0.010% or less, Al: 0.0001 to 1.00%, O: 0.010% or less, H: 0.0010% or less, Mo: 0 to 5.0%, Cr: 0 to 5.0%, Ni: 0 to 5.0%, Cu: 0 to 5.0%, Co: 0 to 5.0%, B: 0 to 0.01%, V: 0 to 1.0%, W: 0 to 5.0%, Nb: 0 to 0.1%, Ti: 0 to 0.1%, Zr: 0-0.2%, Hf: 0-0.2%, Ta: 0-0.2%, Sb: 0-0.2%, Sn: 0-0.2%, Ca: 0-0.01%, Mg: 0-0.01%, and REM: 0-0.5%, with the balance being Fe and unavoidable impurities, and the number density of voids present inside and having a length of 200 μm or more in the thickness direction is 100 / 100 cm ２ Below is a steel pipe.

2. The chemical composition is, in mass%, S: 0.00001 to 0.010%, N: 0.00001 to 0.010%, Mo: 0.0001 to 5.0%, Cr: 0.0001 to 5.0%, Ni: 0.0001 to 5.0%, Cu: 0.0001 to 5.0%, Co: 0.0001 to 5.0%, B: 0.0001 to 0.01%, V: 0.0001 to 1.0%, W: 0.0001 to 5.0%, Nb: 0.0001 to 0.1%, Ti: 0.0001 to 0.1%, Zr: 0.0001 to 0.2%, Hf: 0.0001 to 0.2%, 2. The steel pipe according to claim 1, containing at least one selected from the group consisting of Ta: 0.0001 to 0.2%, Sb: 0.0001 to 0.2%, Sn: 0.0001 to 0.2%, Ca: 0.0001 to 0.01%, Mg: 0.0001 to 0.01%, and REM: 0.0001 to 0.5%.

3. A steel pipe according to claim 1 or 2, having a structure in which the area fraction of austenite is 0 to 3.0%, the total area fraction of ferrite and pearlite is 50% or more, and the average grain size of ferrite is 200 μm or less.

4. A method for manufacturing a steel pipe, comprising heating a steel material having the chemical composition defined in claim 1 or 2, piercing the heated steel material to form a hollow blank, hot working the hollow blank to form a steel pipe having a final finished wall thickness, and cooling the steel pipe, wherein in the hot working, a processing rate at 1000°C or higher, defined as the ratio of the amount of change in wall thickness at temperatures of 1000°C or higher to the amount of change in wall thickness throughout the entire hot working, is 50% or higher, and in the cooling, the average cooling rate from the end of the hot working to 500°C is 5°C / min or lower.

5. A high-strength steel pipe having a tensile strength of 800 MPa or more, obtained by quenching and tempering the steel pipe according to any one of claims 1 to 3.