High-strength steel plate for hydrogen transport steel pipe and its manufacturing method, and high-strength steel pipe for hydrogen transport and its manufacturing method

Abstract

Provided is a high-strength steel plate for hydrogen transport steel pipes that has excellent fatigue crack propagation resistance, little deterioration in strength and elongation, excellent low-cycle fatigue properties, and excellent resistance to ground movement in a high-pressure hydrogen environment. The present invention provides a steel sheet having a predetermined component composition and a tensile strength (TS) in air. A ) and tensile strength in hydrogen gas environment (TS H ) is 535 MPa or more, and the tensile strength in air (TS A ) and tensile strength in hydrogen gas environment, elongation in air (EL A ) and elongation in hydrogen gas environment (EL H ), the yield ratio (YR), which is the ratio of the yield strength to the tensile strength of the steel plate in air and hydrogen gas environments, the minimum grain size of the top 20% of the largest grains at the center of the plate thickness, the number of repeated fractures in a low-cycle fatigue test with a strain amplitude of 3%, and the stress intensity factor range ΔK are all within specified ranges.

Description

[Technical Field]

[0001] The present invention relates to a high-strength steel plate for hydrogen transport pipes, particularly to a high-strength steel plate for hydrogen transport pipes suitable for use in line pipes for transporting high-pressure hydrogen gas, and a method for manufacturing the same. The present invention also relates to a high-strength steel pipe for hydrogen transport using the above-mentioned high-strength steel plate for hydrogen transport pipes, and a method for manufacturing the same. [Background technology]

[0002] Generally, line pipes are produced by forming steel plates produced in a plate mill or a hot rolling mill into steel pipes by UOE forming, press bending, roll forming, or the like.

[0003] Line pipes used to transport high-pressure hydrogen gas require not only strength, toughness, and weldability, but also resistance to hydrogen embrittlement. Because line pipes are subjected to repeated stresses due to pressure fluctuations during operation, fatigue crack propagation resistance in high-pressure hydrogen gas environments is particularly important for extending their service life. When the hydrogen pressure of the hydrogen gas transported by line pipes is up to about 15 MPa, low-alloy steels with sufficient wall thickness are used. However, at pressures above this, the risk of hydrogen embrittlement during use increases, so low-alloy steels are not used. Instead, austenitic stainless steels such as SUS316L, which are more resistant to hydrogen embrittlement than low-alloy steels, are used.

[0004] Austenitic stainless steel is expensive and has low strength, so if it is designed to withstand high hydrogen pressure, the wall thickness will be large, and the price of the hydrogen transport line pipe itself will be high. Therefore, there has been a demand for a steel material for hydrogen transport line pipe that is lower cost and can withstand high-pressure hydrogen gas environments.

[0005] Furthermore, the steel plate used as the material for the line pipe is required to have not only high strength and high toughness, but also a low yield ratio (YR) from the viewpoint of ground deformation and the like.

[0006] A commonly known method for reducing the yield ratio is to adjust the steel structure of the steel sheet. Specifically, it is known that it is effective to create a structure in which hard phases such as bainite and martensite are appropriately dispersed in soft phases such as ferrite.

[0007] In order to solve the above problems, for example, Patent Document 1 proposes an austenitic steel material with a high Mn content. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent No. 6703608 Summary of the Invention [Problem to be solved by the invention]

[0009] The technology described in Patent Document 1 makes it possible to provide steel materials that are less expensive than austenitic stainless steels such as SUS316L, but because the steel materials described in Patent Document 1 are austenitic alloys, they are more expensive than general low-alloy steels. Furthermore, the steel materials described in Patent Document 1 do not take into consideration the reduction in fatigue crack propagation resistance, strength, and elongation in a high-pressure hydrogen environment.

[0010] In view of the above problems, the present invention aims to provide a high-strength steel plate for hydrogen transport pipes, which has excellent fatigue crack propagation resistance, little deterioration in strength and elongation, excellent low-cycle fatigue properties, and excellent resistance to ground movement in a high-pressure hydrogen environment, together with an advantageous method for manufacturing the same. Another object of the present invention is to provide a high-strength steel pipe for hydrogen transport using the above-mentioned high-strength steel plate for hydrogen transport steel pipe, together with an advantageous method for producing the same. [Means for solving the problem]

[0011] The present inventors have repeatedly conducted numerous experiments and studies on the chemical composition, microstructure, and manufacturing conditions of steel materials in order to ensure fatigue crack propagation resistance, strength, and elongation properties in a high-pressure hydrogen gas environment. As a result, they have found that by appropriately selecting the chemical composition and manufacturing conditions, it is possible to obtain high-strength steel plates that are capable of manufacturing high-strength steel pipes that have excellent fatigue crack propagation resistance in a high-pressure hydrogen gas environment, a low yield ratio to obtain excellent low-cycle fatigue properties required for ground deformation, and little deterioration in strength and elongation.

[0012] It was also found that fatigue crack propagation resistance is improved by reducing the top 20% grain size in the structure at the center of the plate thickness to 30 μm or less, and that deterioration of strength and elongation is suppressed in order to obtain the excellent low-cycle fatigue properties required for ground movement by reducing the yield ratio (YR), which is the ratio of the yield strength to the tensile strength of the steel plate in both air and hydrogen gas environments, to 93% or less.

[0013] Furthermore, to achieve such a steel structure, it is necessary to strictly control the rolling and cooling conditions, and we have succeeded in finding those conditions.

[0014] The present invention has been made based on these findings, and the gist and configuration thereof are as follows. [1] In mass%, C: 0.030~0.080%, Si: 0.02 to 0.50% Mn: 0.80-2.20% P: 0.030% or less, S: 0.0030% or less, Al: 0.010~0.080%, Nb: 0.005 to 0.080%, Ti: 0.005 to 0.020%, N: 0.0020~0.0080%, and O (oxygen): 0.0050% or less and the balance being Fe and unavoidable impurities, Tensile strength in air (TS A) and tensile strength in hydrogen gas environment (TS H ) is 535 MPa or more, and the tensile strength in air (TS A ) and tensile strength in hydrogen gas environment (TS H ) is TS H / TS A ≥ 0.90 and the elongation in air (EL A ) and elongation in hydrogen gas environment (EL H ) is EL H / EL A ≧0.80, and the yield ratio (YR), which is the ratio of the yield strength to the tensile strength of the steel sheet in both air and hydrogen gas environments, is 93% or less, and the minimum grain size of the top 20% of grains with the largest grain size at the center of the sheet thickness is 30 μm or less, In a low-cycle fatigue test with a strain amplitude of 3%, the number of repeated cycles to fracture is 100 or more. Stress concentration factor range ΔK is 25 (MPa m 1 / 2 ) the fatigue crack growth rate is 5.0 × 10 -3 High-strength steel plate for hydrogen transport pipes with a fatigue life of less than (mm / cycle).

[0015] [2] The component composition according to [1] further comprises, in mass %, Cu: 1.00% or less, Ni: 1.00% or less, Cr: 0.50% or less, Mo: 0.50% or less V: 0.1% or less, Ca: 0.0050% or less, B: 0.0050% or less, Zr: 0.02% or less, Mg: 0.02% or less, and REM: 0.02% or less The high-strength steel plate for hydrogen transport steel pipe according to [1] above, containing one or more selected from the following:

[0016] [3] A method for producing a high-strength steel plate for a hydrogen transport steel pipe according to [1] or [2], comprising: a heating step of heating a steel material to a temperature of 1000 to 1250°C; a hot rolling process in which the steel material heated in the heating process is hot rolled under conditions including one or more passes of rolling at 950°C or higher with a rolling reduction rate of 10% or more per pass and one or more passes of rolling at 900°C or lower with a rolling reduction rate of 15% or more per pass, a cumulative rolling reduction rate of 50% or more at or below the Tnr temperature, which is the lower limit temperature of the recrystallization temperature range, and an end-of-rolling temperature of the Ar3 transformation point or lower; and an accelerated cooling step in which the hot-rolled steel sheet obtained in the hot rolling step is accelerated-cooled under the conditions of a cooling start temperature of 600°C or higher and the Ar3 transformation point or lower, an average cooling rate of 5°C / s or higher from 750°C to 550°C, and a cooling stop temperature of 200 to 550°C.

[0017] [4] A high-strength steel pipe for hydrogen transport, which uses the high-strength steel plate for hydrogen transport steel pipe according to [1] or [2] above.

[0018] [5] A method for manufacturing a high-strength steel pipe for hydrogen transport, comprising: a cold-forming process for cold-forming a high-strength steel plate for hydrogen transport steel pipe according to [1] or [2] into a tubular shape; a welding process for welding the butt joints where the ends of the steel plate formed into a tubular shape in the cold-forming process are butted together; and an expansion process for expanding the welded pipe produced in the welding process at an expansion ratio of 0.6% or more. [Effects of the Invention]

[0019] The high-strength steel plate for hydrogen transport pipe of the present invention and the high-strength steel pipe for hydrogen transport using the high-strength steel plate for hydrogen transport pipe have excellent strength and elongation to obtain the excellent low-cycle fatigue properties required for fatigue crack propagation resistance and ground deformation in a high-pressure hydrogen environment. The steel pipe also has excellent strength and elongation in regions including welds to obtain the excellent low-cycle fatigue properties required for fatigue crack propagation resistance and ground deformation in a high-pressure hydrogen environment. Furthermore, the manufacturing method of the high-strength steel plate for hydrogen transport pipe of the present invention makes it possible to manufacture a high-strength steel plate for hydrogen transport pipe that has excellent fatigue crack propagation resistance, strength, and elongation in a high-pressure hydrogen environment. DETAILED DESCRIPTION OF THE INVENTION

[0020] (High-strength steel plate for hydrogen transport pipes) The high-strength steel sheet for hydrogen transport pipe of the present invention will be specifically described below. Note that the high-strength steel sheet for hydrogen transport pipe of the present invention will also be simply referred to as high-strength steel sheet hereinafter.

[0021] [Component composition] First, the chemical composition of the high-strength steel sheet of the present invention and the reasons for limiting it will be described. In the following description, all units shown in % are mass % unless otherwise specified. In the following description, the numerical range of "α to β" includes α and β.

[0022] C: 0.030~0.080% C effectively contributes to improving strength, but if the C content is less than 0.030%, sufficient strength cannot be ensured. Therefore, the C content is set to 0.030% or more. The C content is preferably set to 0.035% or more. On the other hand, if the C content exceeds 0.080%, the amount of precipitates increases, deteriorating the tensile properties and crack propagation resistance in a hydrogen gas environment. Therefore, the C content is set to 0.080% or less. The C content is preferably set to 0.075% or less.

[0023] Si: 0.02 to 0.50% Si is added for deoxidation, but if the Si content is less than 0.02%, the deoxidizing effect is insufficient, and many oxides are present in the steel, which act as fracture initiation sites, reducing the toughness of the steel plate and its crack propagation resistance and elongation in a hydrogen gas environment. For this reason, the Si content is set to 0.02% or more. The Si content is preferably set to 0.05% or more. On the other hand, if the Si content exceeds 0.50%, the weldability deteriorates and the tensile properties and crack propagation resistance in a hydrogen gas environment deteriorate due to the increased amount of precipitates. For this reason, the Si content is set to 0.50% or less. The Si content is preferably set to 0.45% or less.

[0024] Mn: 0.80 to 2.20% Mn effectively contributes to improving strength, but if the Mn content is less than 0.80%, this effect is not fully manifested. Therefore, the Mn content is set to 0.80% or more. The Mn content is preferably set to 1.00% or more. The Mn content is more preferably set to 1.20% or more. On the other hand, if the Mn content exceeds 2.20%, MnS inclusions are formed, deteriorating low-temperature toughness, and the increase in coarse precipitates deteriorates tensile properties and fatigue properties in a hydrogen gas environment. Therefore, the Mn content is set to 2.20% or less. The Mn content is preferably set to 2.10% or less. The Mn content is more preferably set to 2.00% or less.

[0025] P:0.030% or less P is an unavoidable impurity element that deteriorates low-temperature toughness and tensile and fatigue properties in a hydrogen gas environment. This tendency becomes more pronounced when the P content exceeds 0.030%, so the upper limit of the P content is set to 0.030%. The P content is preferably set to 0.008% or less. While a lower P content is better, excessive dephosphorization increases refining costs. Therefore, from the perspective of refining costs, the P content is preferably set to 0.001% or more.

[0026] S: 0.0030% or less S is an unavoidable impurity element that forms MnS inclusions in steel, degrading low-temperature toughness, and also degrading tensile and fatigue properties in a hydrogen gas environment due to the increased amount of coarse precipitates. For this reason, a low S content is preferable, but up to 0.0030% is acceptable. Therefore, the S content is set to 0.0030% or less. The S content is preferably set to 0.0015% or less. Note that the lower the S content, the better, but excessive de-S leads to increased refining costs. Therefore, from the perspective of refining costs, the S content is preferably set to 0.0002% or more.

[0027] Al: 0.010 to 0.080% Al is added as a deoxidizer, but if the Al content is less than 0.010%, its effect is not fully exerted, and a large amount of oxides is present in the steel, which act as fracture initiation points, reducing the toughness of the steel sheet, as well as its resistance to crack propagation in hydrogen and elongation. For this reason, the Al content is set to 0.010% or more. The Al content is preferably set to 0.015% or more. The Al content is more preferably set to 0.025% or more. On the other hand, if the Al content exceeds 0.080%, alumina clogging of the submerged entry nozzle occurs during continuous casting, so the Al content is set to 0.080% or less. The Al content is preferably set to 0.070% or less. The Al content is more preferably set to 0.040% or less.

[0028] Nb: 0.005 to 0.080% When present as solute Nb, Nb expands the non-recrystallization temperature range during hot rolling and contributes to refining the grain size. However, if the Nb content is less than 0.005%, this effect is not fully manifested, and fatigue properties in a hydrogen gas environment deteriorate. Therefore, the Nb content is set to 0.005% or more. The Nb content is preferably set to 0.010% or more. The Nb content is more preferably set to 0.025% or more. On the other hand, if the Nb content exceeds 0.080%, coarse carbides are crystallized during solidification, which deteriorates tensile properties and fatigue properties in a hydrogen gas environment. Therefore, the Nb content is set to 0.080% or less. The Nb content is preferably set to 0.060% or less. The Nb content is more preferably set to 0.055% or less.

[0029] Ti: 0.005 to 0.020% Ti has the effect of pinning austenite grains as TiN during heating and suppressing grain growth. If the Ti content is less than 0.005%, TiN is not sufficiently generated, so this effect is not fully manifested and fatigue properties in a hydrogen gas environment deteriorate. For this reason, the Ti content is set to 0.005% or more. The Ti content is preferably set to 0.008% or more. Furthermore, if the Ti content exceeds 0.020%, the generated TiN becomes coarse and sufficient toughness of the weld heat affected zone cannot be obtained. For this reason, the Ti content is set to 0.020% or less. The Ti content is preferably set to 0.017% or less. The Ti content is more preferably set to 0.015% or less.

[0030] N: 0.0020~0.0080% N effectively contributes to improving strength, but if the N content is less than 0.0020%, sufficient strength cannot be ensured. Therefore, the N content is set to 0.0020% or more. The N content is preferably set to 0.0025% or more. The N content is more preferably set to 0.0030% or more. On the other hand, if the N content exceeds 0.0080%, the amount of precipitates such as TiN increases, which deteriorates the toughness of the weld and the tensile and fatigue properties in a hydrogen gas environment. Therefore, the N content is set to 0.0080% or less. The N content is preferably set to 0.0070% or less. The N content is more preferably set to 0.0050% or less.

[0031] O (oxygen): 0.0050% or less O exists in steel as oxides such as Al2O3 and is an element that has adverse effects, such as acting as a fracture initiation point and reducing the toughness of the steel sheet, its tensile properties in hydrogen, and its crack propagation resistance. Therefore, it is desirable to keep the O content as low as possible, but a content of 0.0050% or less is acceptable. The lower limit of the O content is not particularly limited and may be 0%, but since O is an element that is usually unavoidably contained in steel as an impurity, it may be greater than 0% industrially. Furthermore, excessive reduction of O leads to increased refining costs, so from a cost perspective, the O content is preferably 0.0005% or more.

[0032] The basic components (essential components) of the high-strength steel sheet of the present invention have been described above. In addition to the above-mentioned elements, the high-strength steel sheet of the present invention may optionally contain one or more elements selected from Cu, Ni, Cr, Mo, V, Ca, B, Zr, Mg, and REM within the following ranges. The remaining elements in the high-strength steel sheet of the present invention are Fe and unavoidable impurities. Unavoidable impurities are impurities that are inevitably mixed in from raw materials, manufacturing processes, manufacturing equipment, etc., and are permitted to be present to the extent that they do not impair the objectives of the present invention. Examples of raw materials include iron ore, reduced iron, and scrap. Examples of impurities include Co, Sn, Zn, Pb, As, Sb, Bi, and H. However, other trace elements may be present as long as they do not impair the effects of the present invention.

[0033] Cu: 1.00% or less Cu is an element effective in improving low-temperature toughness and increasing strength, and to achieve this effect, the Cu content is preferably 0.05% or more. The Cu content is more preferably 0.10% or more. However, if the Cu content exceeds 1.00%, the steel sheet is more likely to develop surface defects, so if Cu is contained, the Cu content is 1.00% or less. The Cu content is preferably 0.90% or less.

[0034] Ni: 1.00% or less Ni is an element effective in improving low-temperature toughness and increasing strength, and to achieve this effect, the Ni content is preferably 0.05% or more. The Ni content is more preferably 0.10% or more. On the other hand, since Ni is an expensive element, when Ni is contained, the Ni content is set to 1.00% or less. The Ni content is preferably 0.90% or less.

[0035] Cr:0.50% or less Like Mn, Cr is an effective element for obtaining sufficient strength even in steel with a low C content. However, if the Cr content is less than 0.05%, this effect is not fully manifested. Therefore, the Cr content is preferably 0.05% or more. The Cr content is more preferably 0.10% or more. The Cr content is even more preferably 0.15% or more. However, if the Cr content exceeds 0.50%, the amount of precipitates increases, deteriorating the tensile properties in a hydrogen gas environment. Therefore, if Cr is contained, the Cr content is 0.50% or less. The Cr content is preferably 0.45% or less. The Cr content is more preferably 0.35% or less.

[0036] Mo: 0.50% or less Mo is an element effective in improving low-temperature toughness and increasing strength, and to obtain this effect, the Mo content is preferably 0.05% or more. On the other hand, since Mo is an expensive element, when Mo is contained, the Mo content is set to 0.50% or less. The Mo content is preferably set to 0.45% or less.

[0037] V: 0.1% or less V is an element that can be added optionally to increase the strength and low-temperature toughness of steel sheets, but if the V content is less than 0.005%, this effect is not fully manifested. Therefore, if V is contained, the V content is preferably 0.005% or more. On the other hand, if the V content exceeds 0.1%, the toughness of the weld deteriorates, so if V is contained, the V content is 0.1% or less. The V content is preferably 0.050% or less, and more preferably 0.010% or less.

[0038] Ca: 0.0050% or less Ca is an effective element for improving hydrogen-induced cracking resistance by controlling the morphology of sulfide-based inclusions, but if the Ca content is less than 0.0005%, the effect of adding Ca is insufficient. Therefore, if Ca is contained, the Ca content is preferably 0.0005% or more. The Ca content is more preferably 0.0008% or more. The Ca content is even more preferably 0.0015% or more. On the other hand, if the Ca content exceeds 0.0050%, not only does the above-mentioned effect saturate, but the cleanliness of the steel decreases, resulting in an increase in precipitates, which deteriorates the tensile properties in a hydrogen gas environment. Therefore, if Ca is contained, the Ca content is 0.0050% or less. The Ca content is preferably 0.0045% or less. The Ca content is more preferably 0.0035% or less.

[0039] B: 0.0050% or less B is an element effective in improving the strength of steel sheet. However, if the B content is less than 0.0005%, this effect is poor. Therefore, when B is contained, the B content is preferably 0.0005% or more. The B content is more preferably 0.0008% or more. On the other hand, if the B content exceeds 0.0050%, the toughness of the steel sheet may deteriorate. Therefore, when B is contained, the B content is 0.0050% or less. The B content is preferably 0.0040% or less.

[0040] Zr: 0.02% or less, Mg: 0.02% or less, REM: 0.02% or less Zr, Mg, and REM (rare earth metals) are elements that can be added optionally to improve fatigue crack propagation resistance through grain refinement and tensile properties in hydrogen gas environments through control of inclusion properties. The effects of each element are not fully realized at a content of less than 0.0005%. Therefore, when these elements are contained, the content of each element is preferably 0.0005% or more. On the other hand, when the content of each element exceeds 0.02%, the effect saturates. Therefore, when Zr, Mg, and REM are contained, the content of each element is preferably 0.02% or less. The content of each of these elements is preferably 0.0050% or less, more preferably 0.0030% or less. Note that REM is a collective term for Sc, Y, and 15 elements ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content here refers to the total content of these elements.

[0041] [Tensile properties in air] The high-strength steel plate of the present invention is intended mainly for steel plates for steel pipes having a strength of API 5L X65 grade or higher, and therefore has a tensile strength of 535 MPa or higher in air. The upper limit of the tensile strength of the high-strength steel plate of the present invention is not particularly limited, but as an example, the tensile strength of the high-strength steel plate of the present invention is 760 MPa or lower. The tensile strength of the high-strength steel plate of the present invention may also be 600 MPa or lower.

[0042] [Tensile properties in a hydrogen gas environment] The high-strength steel plate of the present invention is intended mainly for steel plates for steel pipes having a strength of API 5L X65 grade or higher, and therefore has a tensile strength of 535 MPa or higher in a hydrogen gas environment. The upper limit of the tensile strength of the high-strength steel plate of the present invention is not particularly limited, but as an example, the tensile strength of the high-strength steel plate of the present invention is 760 MPa or lower. The tensile strength of the high-strength steel plate of the present invention may also be 600 MPa or lower.

[0043] Furthermore, the high-strength steel plate of the present invention is intended for steel plates for steel pipes that have the property of being able to withstand 100 or more cycles of fracture in a low-cycle fatigue test with a strain amplitude of 3% in a hydrogen gas environment, assuming ground deformation. Therefore, the deterioration of tensile strength in a hydrogen gas environment is small compared to that in air, and the tensile strength in air (TS A ) and tensile strength in hydrogen gas environment (TS H ) is TS H / TS A ≥ 0.90. Preferably, TS H / TS A ≥ 0.92, more preferably TS H / TS A ≧0.95.

[0044] Furthermore, the deterioration of elongation is small even in a hydrogen gas environment compared to air, and the elongation in air (EL A ) and elongation in hydrogen gas environment (EL H ) is EL H / EL A The relationship EL ≧ 0.80 is preferably satisfied. H / EL A ≧0.82, more preferably EL H / EL A ≧0.85. The larger these indices, the better.

[0045] Furthermore, the yield ratio (YR), which is the ratio of the yield strength to the tensile strength of the steel sheet in both air and hydrogen gas environments, is 93% or less, preferably 90% or less, and more preferably 88% or less.

[0046] [Fracture repetition rate of over 100 times in low-cycle fatigue tests with a strain amplitude of 3%] The high-strength steel plate of the present invention has a fracture repetition rate of 100 or more in a low-cycle fatigue test conducted at a strain amplitude of 3% and a strain rate of 0.1% / s or more. Preferably, the fracture repetition rate is 130 or more, and more preferably 150 or more. The higher the fracture repetition rate, the better. Even if the fracture repetition rate is the same, a higher strain rate is preferred because it is less likely to lead to fracture of the linepipe during ground movement.

[0047] [Fatigue crack growth rate] In a fatigue crack propagation test in 21 MPa high-pressure hydrogen gas, the high-strength steel plate of the present invention has a stress intensity factor range ΔK of 25 (MPa m 1 / 2 ) the fatigue crack growth rate is 5.0 × 10 -3 (mm / cycle). The fatigue crack growth rate is preferably less than 3.0×10 -3 (mm / cycle) or less. The lower the fatigue crack growth rate, the more preferable. As an example, the fatigue crack growth rate is 2.0×10 -3 (mm / cycle) or more.

[0048] [Minimum grain size of the top 20% of grains with the largest grain size at the center of the plate thickness: 30 μm or less] Although reducing the average grain size improves fatigue crack propagation resistance, there is a limit to how much the average grain size can be reduced when cooling is initiated above the Ar3 transformation temperature. In the present invention, it is essential to suppress the formation of coarse grains. Specifically, a large minimum grain size among the top 20% of grains (hereinafter also referred to as the "top 20% grain size") reduces fatigue crack propagation resistance. In particular, a structure in which the top 20% grain size in the grain size distribution at the center of the thickness exceeds 30 μm significantly reduces fatigue crack propagation resistance because fewer grains are present on the crack propagation path for the same crack propagation distance, resulting in less crack bending. Therefore, a structure in which the top 20% grain size at the center of the thickness (half the thickness position) is 30 μm or less is required. The top 20% grain size is preferably 25 μm or less. For example, the top 20% grain size is 15 μm or more. The top 20% grain size refers to the grain size corresponding to the 20% largest grain size when the grain sizes are sorted in descending order in the grain size distribution. The grain size measurement range was 1 mm × 1 mm at the center of the sheet thickness. More specifically, the grain size was determined by analyzing the structure at the center of the sheet thickness using EBSD (Electron Backscatter Diffraction) method. Boundaries with a misorientation of 15° or more were determined to be grain boundaries, and the grain size was calculated as the circle-equivalent diameter from the area of ​​each grain. In addition, in the present invention, a frequency distribution table was created for all grains measured, and the grain size corresponding to 20% of the cumulative relative frequency from the largest grain size was referred to as the "smallest grain size among the top 20% of the largest grains."

[0049] [Thickness of high-strength steel plate] The thickness of the high-strength steel plate of the present invention is not particularly limited, but is preferably 12 mm or more, and is not particularly limited, but is preferably 45 mm or less.

[0050] (Method of manufacturing high-strength steel plates for hydrogen transport pipes) The manufacturing method and manufacturing conditions for producing the high-strength steel plate for hydrogen transport steel pipe of the present invention are specifically described below. The manufacturing method for the high-strength steel plate for hydrogen transport steel pipe of the present invention involves heating a steel material (slab) having the above-mentioned chemical composition (heating step), hot rolling the steel material to form a steel plate (hot rolling step), and then subjecting the steel plate to accelerated cooling under predetermined conditions (accelerated cooling step).

[0051] [Heating process] [[Steel material heating temperature: 1000~1250℃]] If the heating temperature of the steel material (slab) is less than 1000°C, the solid solution of carbides will be insufficient, and the amount of solid solution strengthening by solute C, etc. will be reduced, making it impossible to obtain the required strength. On the other hand, if the heating temperature of the steel material exceeds 1250°C, the crystal grains will become extremely coarse and fatigue crack propagation resistance will deteriorate, so the heating temperature of the steel material is set to 1000 to 1250°C. The heating temperature of the steel material is preferably 1030°C or higher. Furthermore, the heating temperature of the steel material is preferably 1200°C or lower. The steel material (slab) is heated to the above heating temperature all the way to its center.

[0052] [Hot rolling process] Rolling at 950°C or higher with a reduction rate of 10% or more per pass is performed for one or more passes. It is necessary to ensure a sufficient reduction in the recrystallization temperature range and sufficiently promote recrystallization, so that partial recrystallization region rolling is initiated in a uniform grain state without the presence of coarse grains. If there is no rolling pass with a reduction of 10% or more per pass in the recrystallization temperature range, recrystallization is insufficient, resulting in the growth of coarse grains. By including at least one rolling pass with a reduction of 10% or more per pass at 950°C or higher, austenite grains are refined by recrystallization, improving the low-temperature toughness and fatigue crack propagation properties of the steel. Therefore, it is recommended to include at least one rolling pass with a reduction of 10% or more per pass at 950°C or higher, preferably two or more passes, and more preferably three or more passes. Note that there is no particular upper limit on the number of rolling passes with a reduction of 10% or more per pass.

[0053] [[One or more passes of rolling at 900°C or below with a reduction rate of 15% or more per pass]] By including at least one rolling pass at 900°C or below with a reduction rate of 15% or more per pass, strain is introduced into the crystal grains, leading to refinement of the austenite grains, thereby improving the low-temperature toughness and fatigue crack propagation characteristics of the steel. Therefore, the rolling pass at 900°C or below with a reduction rate of 15% or more per pass is included, preferably at least two passes, and more preferably at least three passes. Note that there is no particular upper limit on the number of rolling passes at a reduction rate of 15% or more per pass. Furthermore, rolling at a lower temperature at 900°C or below is effective for grain refinement because more strain is introduced. For this reason, it is preferable to roll at a lower temperature within the temperature range of 900°C or below, as long as the cooling start temperature described below can be observed.

[0054] [[Cumulative reduction below Tnr temperature is 50% or more]] To refine the top 20% grain size in the center of the plate thickness, hot rolling in the recrystallization temperature range must promote recrystallization of crystal grains and suppress the formation of coarse grains. If the cumulative reduction rate at 900°C or below is less than 50%, austenite grain refinement is insufficient, resulting in insufficient recrystallization, leaving coarse grains and degrading low-temperature toughness and fatigue crack propagation characteristics. Therefore, the cumulative reduction rate at the Tnr temperature or below is set to 50% or more, preferably 60% or more. Furthermore, if the cumulative reduction rate is 95% or more, recrystallization in the recrystallization temperature range is not promoted, leaving coarse grains, which may degrade low-temperature toughness and fatigue crack propagation characteristics. Therefore, the cumulative reduction rate at the Tnr temperature or below is preferably set to less than 95%, more preferably less than 92%. Here, the lower limit temperature of the recrystallization temperature range, Tnr (Non-Recrystallization Temperature) (°C), can be calculated, for example, from the steel composition using the following formula: The temperature in hot rolling is the surface temperature of the material to be rolled (steel material or steel plate), and the surface temperature can be measured with a radiation thermometer or the like. Tnr(℃)=174×log([%Nb][%C+(12 / 14)%N])+1444 In the above formula, [%X] indicates the content (mass%) of the X element in the steel.

[0055] [[Rolling finish temperature]] In the hot rolling process, in order to refine the crystal grains, the lower the rolling end temperature, the better, and the better the fatigue crack propagation characteristics. Furthermore, by setting the rolling end temperature below the Ar3 transformation point, the cooling start temperature is lowered, ferrite is formed, and the yield ratio (YR), which is the ratio of yield strength to tensile strength, is reduced, thereby improving low-cycle fatigue characteristics. The lower the rolling end temperature and the cooling start temperature, the smaller the YR. Therefore, taking into account the need for the cooling start temperature in the cooling process after the hot rolling process to be below the Ar3 transformation point at the steel sheet surface temperature, it is necessary to set the rolling end temperature below the Ar3 transformation point. Here, the Ar3 transformation point means the ferrite transformation start temperature during cooling, and can be calculated, for example, from the steel composition using the following formula. The surface temperature of the steel sheet can be measured using a radiation thermometer or the like. The rolling end temperature is preferably the Ar3 transformation point (°C) - 10°C, more preferably the Ar3 transformation point (°C) - 30°C. Ar3 transformation point (℃) = 910-310[%C]-80[%Mn]-20[%Cu]-15[%Cr]-55[%Ni]-80[%Mo] In the above formula, [%X] indicates the content (mass%) of element X in the steel, and elements that are not contained are represented as 0 (zero).

[0056] [[Cooling start temperature: Steel plate surface temperature 600°C or higher and Ar3 transformation point (°C) or lower]] The steel sheet after the hot rolling process is subjected to cooling (controlled cooling). If the steel sheet surface temperature at the start of cooling exceeds the Ar3 transformation point (°C), a bainite single-phase structure is formed, and the YR increases, resulting in poor low-cycle fatigue properties. For this reason, the steel sheet surface temperature at the start of cooling is set to the Ar3 transformation point (°C) or lower. The cooling start temperature is preferably the Ar3 transformation point (°C) -10°C or lower, more preferably the Ar3 transformation point (°C) -30°C or lower. If the steel sheet surface temperature at the start of cooling is lower than 600°C, sufficient water cooling cannot be performed, resulting in poor strength and toughness of the steel sheet. For this reason, the steel sheet surface temperature at the start of cooling is set to 600°C or higher. The cooling start temperature is preferably 630°C or higher, more preferably 650°C or higher. The steel sheet surface temperature at the start of cooling is the temperature of the steel sheet surface region where the cooling start temperature is highest. Specifically, the surface temperature of the steel sheet at the start of cooling is the surface temperature of the steel sheet at the leading end when the steel sheet is cooled while traveling in one direction relative to the cooling device. Furthermore, when the entire steel sheet is cooled in fixed regions and the cooling start times differ between the regions, the surface temperature of the steel sheet is the surface temperature of the region that was cooled last. As an example, the upper limit of the surface temperature of the steel sheet at the start of cooling is the rolling end temperature.

[0057] [[Average cooling rate of steel plate: 5℃ / s or more]] If the cooling rate is too slow, coarse ferrite and pearlite are formed, leading to insufficient strength, deterioration of toughness, and deterioration of fatigue crack propagation characteristics, so in order to prevent this, the average cooling rate from 750°C to 550°C at the steel plate temperature is set to 5°C / s or more, preferably 7°C / s or more. Note that while a faster average cooling rate is preferable, from the viewpoint of strength and toughness, it is preferably set to 300°C / s or less.

[0058] Cooling stop temperature: 200 to 550°C at the surface of the steel plate If the cooling stop temperature exceeds 550°C at the steel sheet surface temperature, the bainite transformation becomes incomplete, and sufficient strength cannot be obtained. Furthermore, the yield ratio (YR) increases due to a decrease in the amount of dislocations introduced or the progress of dislocation fixation, resulting in deterioration of low-cycle fatigue properties. Therefore, the cooling stop temperature is set to 550°C or lower, preferably 500°C or lower. Furthermore, if the cooling stop temperature is lower than 200°C, the amount of MA (martensite-austenite constituent) produced increases, resulting in reduced elongation in a hydrogen gas environment. Therefore, the cooling stop temperature is set to 200°C or higher, preferably 250°C or higher.

[0059] (High-strength steel pipes for hydrogen transportation) The high-strength steel pipe for hydrogen transport according to the present invention is a high-strength steel pipe for hydrogen transport that uses the high-strength steel plate for hydrogen transport according to the present invention described above.

[0060] The high-strength steel pipe for hydrogen transport of the present invention uses the high-strength steel plate for hydrogen transport steel pipe of the present invention described above, and is excellent in strength and elongation to obtain the excellent low-cycle fatigue properties required for fatigue crack propagation resistance in a high-pressure hydrogen environment and for dealing with ground deformation. Furthermore, the high-strength steel pipe for hydrogen transport of the present invention is also excellent in strength and elongation in the region including the welds to obtain the excellent low-cycle fatigue properties required for fatigue crack propagation resistance in a high-pressure hydrogen environment and for dealing with ground deformation.

[0061] (Method of manufacturing high-strength steel pipes for hydrogen transportation) The high-strength steel plate for hydrogen transport steel pipe of the present invention is cold-formed into a tubular shape by press bending, roll forming, UOE forming, or the like (cold forming process), followed by welding the butt joint (welding process). The welded pipe produced in the welding process is then expanded at an expansion ratio of 0.6% or more (expansion process), thereby enabling the production of high-strength steel pipe for hydrogen transport (UOE steel pipe, electric resistance welded steel pipe, spiral steel pipe, etc.) suitable for transporting high-pressure hydrogen gas. Furthermore, by producing steel pipe using the high-strength steel plate of the present invention, it is possible to produce steel pipe whose properties in hydrogen are less deteriorated compared to their properties in air, even if a high-hardness region exists in the weld. In the present invention, high-pressure hydrogen refers, for example, to a hydrogen gas environment of 15 MPa or more.

[0062] [Cold forming process] [Welding process] [Pipe expansion process] For example, UOE steel pipes are manufactured by groove-forming the ends of steel plates, forming them into a tubular shape using a C press, a U press, or an O press, seam-welding the butt joints using internal and external welding, and then expanding the welded pipe. Any welding method may be used as long as it provides sufficient joint strength and joint toughness, but submerged arc welding is preferred from the viewpoint of excellent weld quality and manufacturing efficiency. Pipe expansion can also be performed on steel pipes that have been press-bent into a tubular shape and then seam-welded at the butt joints.

[0063] [[Pipe expansion rate: 0.6% or more]] If the expansion ratio of a steel pipe is too small, the roundness of the steel pipe will deteriorate, and when high-pressure hydrogen gas is passed through the steel pipe, stress will be applied unevenly to the steel pipe, causing stress concentration and possibly leading to rupture. To prevent this, the expansion ratio is set to 0.6% or more. Furthermore, the expansion ratio is preferably set to 1.0% or more, more preferably 1.2% or more. On the other hand, if the expansion ratio is too large, excessive tensile stress will be applied to the outer surface of the steel pipe, resulting in a decrease in the circumferential compressive yield strength due to the Bauschinger effect. Therefore, the expansion ratio is preferably set to 5.0% or less. [Example]

[0064] Steels (steel grades A to AA) with the chemical compositions shown in Table 1 were made into steel materials (slabs) by continuous casting. These were then heated to the temperatures shown in Table 2 (heating process), followed by hot rolling (hot rolling process) and cooling (accelerated cooling process) under the conditions shown in Table 2 to produce steel plates with the final thicknesses shown in Table 2. In the accelerated cooling process, the steel plates were run in one direction while undergoing controlled cooling using a water-cooled controlled cooling device. The edges of the steel plates were then beveled and formed into tubular shapes using a C press, U press, or O press (cold forming process). The inner and outer surfaces of the butt joints were seam-welded by submerged arc welding (welding process), and the steel pipes were then produced through a pipe expansion process. The Ar3 transformation point and the lower limit temperature Tnr of the recrystallization temperature range in Table 1 were calculated using the above-mentioned formulas.

[0065] [Calculation of the top 20% particle size] A sample for metallographic observation was taken from the center of the steel plate width obtained as described above. The cross section of this sample perpendicular to the plate width direction was mirror-polished and then etched with colloidal silica. Crystal data was then collected at the center of the plate thickness using the electron backscatter diffraction (EBSD) method over a 1 mm × 1 mm field of view (measurement step: 0.8 μm). After data collection, boundaries with a misorientation of 15° or more were determined to be grain boundaries using OIM-Analysis (EDAX, OIM Analysis software). The grain size was calculated as the circle-equivalent diameter from the area of ​​each grain. A frequency distribution table was also created for all grains measured, and the grain size corresponding to 20% of the cumulative relative frequency from the largest grain size was designated the "top 20% grain size." The results of the top 20% grain size measurements are shown in Table 3.

[0066] [Measurement of tensile strength, yield strength, and elongation in air] The full-thickness test piece perpendicular to the rolling direction was used as a tensile test piece, and a tensile test was carried out in air in accordance with the provisions of JIS Z2241 (2022) to measure the tensile strength, yield strength, and elongation. The results are shown in Table 3.

[0067] [Measurement of tensile strength, yield strength, and elongation in a hydrogen gas environment] The full-thickness test piece perpendicular to the rolling direction was used as a tensile test piece, and a tensile test was carried out in 21 MPa high-pressure hydrogen gas in accordance with the provisions of JIS Z2241 (2022) to measure the tensile strength, yield strength, and elongation. The results are shown in Table 3.

[0068] [Evaluation of low-cycle fatigue properties at a strain amplitude of 3%] Test specimens were prepared from the steel sheets obtained as described above in accordance with ASTM E 606, with the load direction being the width direction of the sheet. A low-cycle fatigue test was then conducted in 21 MPa high-pressure hydrogen gas at a strain amplitude of 3%, and the number of repetitions to fracture was measured. A specimen with a repetition rate of 100 or more was considered to have good low-cycle fatigue properties. The results are shown in Table 3.

[0069] [Derivation of fatigue crack growth rate] CT test pieces conforming to ASTM E 647 were taken from the steel plates obtained as described above, with the load direction parallel to the rolling direction. The CT test pieces were 10 mm thick and taken from the half-thickness position of the plate. The fatigue crack length was measured using a clip gauge by the compliance method, and the fatigue crack growth rate in 21 MPa high-pressure hydrogen gas was calculated. The stress intensity factor range ΔK was 25 (MPa m 1 / 2 The fatigue crack growth rate (mm / cycle) was evaluated at 1000 kJ / s. The results are shown in Table 3.

[0070] [Table 1]

[0071] [Table 2]

[0072] [Table 3]

[0073] As shown in Table 2, Nos. 1 to 15 are examples of the present invention whose component compositions and manufacturing conditions satisfy the appropriate ranges of the present invention. As shown in Table 3, Nos. 1 to 15 all have tensile strength (TS A ) and tensile strength in hydrogen gas environment (TS H ) was 535 MPa or more. A ) and tensile strength in hydrogen gas environment (TS H ) is TS H / TS A ≥ 0.90 and the elongation in air (EL A ) and elongation in hydrogen gas environment (EL H ) is EL H / EL A ≧0.80. Furthermore, the yield ratio (YR), which is the ratio of the yield strength to the tensile strength of the steel plate in both air and hydrogen gas environments, was 93% or less. Furthermore, the top 20% grain size in the structure at the center of the plate thickness was 30 μm or less, and the number of repetitions to fracture in a low-cycle fatigue test with a strain amplitude of 3% was 100 or more. In addition, the stress intensity factor range ΔK was 25 (MPa m 1 / 2 ) the fatigue crack growth rate is 5.0 × 10 -3 (mm / cycle).

[0074] In contrast, the chemical compositions of steel sheets Nos. 16 to 30 are outside the scope of the present invention. Nos. 17, 21, and 30 were insufficient in solid solution strengthening, resulting in insufficient strength. Nos. 16, 18, 20, 22 to 24, 26, and 29 had a large amount of precipitates in the steel sheets, which made hydrogen more likely to accumulate, resulting in poor tensile properties in hydrogen, low-cycle fatigue properties at a strain amplitude of 3%, and fatigue crack propagation resistance. Nos. 19 and 25 had poor fatigue crack propagation resistance due to oxides that made cracks more likely to connect. Nos. 27 and 28 had poor grain growth inhibition due to precipitates, resulting in poor fatigue crack propagation resistance.

[0075] Nos. 31 to 39 are comparative examples whose chemical compositions were within the scope of the present invention but whose manufacturing conditions were outside the scope of the present invention and therefore did not satisfy the desired properties. No. 31 was produced at a high heating temperature for the steel material (slab), resulting in coarsening of crystal grains and poor fatigue crack propagation resistance. No. 32 was produced at a low heating temperature for the steel material, resulting in insufficient solid solution of carbides and poor strength. No. 33 had no passes with a rolling reduction of 10% or more in the rolling temperature range of 950°C or higher, and the maximum rolling reduction per pass was low, resulting in residual coarse grains and poor fatigue crack propagation resistance. No. 34 had no passes with a rolling reduction of 15% or more in the rolling temperature range of 900°C or lower, resulting in a low maximum rolling reduction per pass, resulting in large grains in the top 20% of the structure at the center of the plate thickness, resulting in poor fatigue crack propagation resistance. In No. 35, the cumulative reduction rate below Tnr was low, resulting in a large top 20% grain size in the microstructure at the center of the thickness, and thus reduced fatigue crack propagation resistance. In No. 36, the rolling end temperature and cooling start temperature were high, resulting in a large top 20% grain size in the microstructure at the center of the thickness, and thus reduced fatigue crack propagation resistance. Furthermore, the microstructure became a single bainite phase, resulting in a high YR and reduced low-cycle fatigue properties. In No. 37, the average cooling rate from 750°C to 550°C was low, resulting in a large top 20% grain size in the microstructure at the center of the thickness, and thus reduced fatigue crack propagation resistance. In No. 38, the cooling stop temperature was high, resulting in incomplete bainite transformation and low strength. Furthermore, reduced dislocation introduction and increased dislocation locking resulted in a large YR, which reduced low-cycle fatigue properties. In No. 39, the cooling stop temperature was low, resulting in increased MA formation, which reduced elongation in a hydrogen gas environment. [Industrial Applicability]

[0076] According to the present invention, it is possible to provide a high-strength steel plate for hydrogen transport pipes, which has excellent fatigue crack propagation resistance, little deterioration in strength and elongation, excellent low-cycle fatigue properties, and excellent resistance to ground movement in a high-pressure hydrogen environment, together with an advantageous method for manufacturing the same.

Claims

1. In mass%, C: 0.030-0.080%, Si: 0.02-0.50%, Mn: 0.80 to 2.20%, P: 0.030% or less, S: 0.0030% or less, Al: 0.010-0.080%, Nb: 0.005-0.080%, Ti: 0.005 to 0.020%, N: 0.0020 to 0.0080%, and O (oxygen): 0.0050% or less and the balance being Fe and unavoidable impurities, Tensile strength in air (TS Ａ ) and tensile strength in hydrogen gas environment (TS Ｈ ) is 535 MPa or more, and the tensile strength in air (TS Ａ ) and tensile strength in hydrogen gas environment (TS Ｈ ) is TS Ｈ / TS Ａ ≧0.90, and the elongation in air (EL Ａ ) and elongation in hydrogen gas environment (EL Ｈ ) is EL Ｈ / EL Ａ ≧0.80, and the yield ratio (YR), which is the ratio of the yield strength to the tensile strength of the steel sheet in air and in a hydrogen gas environment, is 93% or less in both cases, and the steel sheet has a structure in which the minimum grain size of the top 20% of grains with large grain sizes at the center of the sheet thickness is 30 μm or less, The number of repeated cycles to fracture in a low-cycle fatigue test with a strain amplitude of 3% is 100 or more, The stress intensity factor range ΔK is 25 (MPa m １／２ ) the fatigue crack growth rate is 5.0 × 10 －３ (mm / cycle) or less.

2. The component composition according to claim 1 further comprises, in mass %, Cu: 1.00% or less, Ni: 1.00% or less, Cr: 0.50% or less, Mo: 0.50% or less, V: 0.1% or less, Ca: 0.0050% or less, B: 0.0050% or less, Zr: 0.02% or less, Mg: 0.02% or less, and REM: 0.02% or less 2. The high-strength steel plate for hydrogen transport steel pipe according to claim 1, which contains one or more selected from the following:

3. 3. The method for producing a high-strength steel plate for a hydrogen transport steel pipe according to claim 1, comprising: a heating step of heating a steel material to a temperature of 1000 to 1250°C; The steel material heated in the heating step is subjected to one or more passes of rolling at 950°C or higher with a rolling reduction of 10% or more per pass, and one or more passes of rolling at 900°C or lower with a rolling reduction of 15% or more per pass, and the cumulative rolling reduction is 50% or more at or below the Tnr temperature, which is the lower limit temperature of the recrystallization temperature range, and the rolling end temperature is Ar. ３ a hot rolling process of hot rolling under conditions below the transformation point; The hot-rolled steel sheet obtained in the hot rolling step is cooled to a temperature of 600°C or higher in Ar. ３ and an accelerated cooling step in which accelerated cooling is performed under the conditions of an average cooling rate of 5°C / s or more from 750°C to 550°C below the transformation point, with a cooling stop temperature of 200 to 550°C.

4. A high-strength steel pipe for hydrogen transport, which uses the high-strength steel plate for hydrogen transport steel pipe according to claim 1 or 2.

5. 3. A method for manufacturing a high-strength steel pipe for hydrogen transport, comprising: a cold-forming step of cold-forming a high-strength steel plate for hydrogen transport steel pipe according to claim 1 or 2 into a tubular shape; a welding step of butting together ends of the steel plate formed into a tubular shape in the cold-forming step and welding the butt joint; and an expansion step of expanding the welded pipe produced in the welding step at an expansion ratio of 0.6% or more.

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