High-strength steel sheet for hydrogen transport steel pipe, method for producing same, and hydrogen transport steel pipe

Abstract

Provided is a high-strength steel sheet for a hydrogen transport steel pipe, which has excellent HISC resistance and excellent fatigue crack propagation resistance in a high-pressure hydrogen environment. This high-strength steel sheet for a hydrogen transport steel pipe contains, in mass%, 0.030-0.060% of C, 0.01-0.50% of Si, 0.80-1.80% of Mn, 0.015% or less of P, 0.0015% or less of S, 0.010-0.080% of Al, 0.05-0.50% of Cr, 0.005-0.080% of Nb, 0.005-0.020% of Ti, 0.0020-0.0080% of N, and 0.0005-0.0050% of Ca, the average Vickers hardness value + 3 [sigma] of 0.25 mm below the surface of the steel sheet being 225 HV or less, the upper 20% particle diameter of the center structure of the sheet thickness being 30 [mu] m or less, the fatigue crack propagation rate being less than 2.0 * 10-2 (mm / cycle) when the stress expansion coefficient range ([delta] K) is 45 ( and the tensile strength is 535 MPa or more.

Description

Technical Field

[0001] The present invention relates to a high-strength steel plate for hydrogen transport steel pipe, and more particularly to a high-strength steel plate for hydrogen transport steel pipe suitable for use in a high-pressure hydrogen transport line pipe and a method for manufacturing the same. In addition, the present invention relates to a hydrogen transport steel pipe using the high-strength steel plate for hydrogen transport. Background Art

[0002] Generally, a line pipe is manufactured by forming a steel plate manufactured by a plate rolling mill or a hot rolling mill into a steel pipe by UOE forming, press bending forming, roll forming, or the like.

[0003] Among them, the pipeline used for high-pressure hydrogen transportation requires not only strength, toughness, weldability, etc., but also hydrogen embrittlement resistance. Among them, since the pressure changes during operation repeatedly apply stress to the pipeline, fatigue crack growth resistance in a high-pressure hydrogen environment is required on the basis of extending the service life. In addition, resistance to hydrogen-induced stress cracking (HISC (Hydrogen Induced Stress Cracking) resistance) in a high-pressure hydrogen environment is required. If the hydrogen pressure is about 15MPa, low-alloy steel with sufficient wall thickness can be used. However, if the pressure is above it, low-alloy steel cannot be used because the risk of hydrogen embrittlement damage during use increases, and austenitic stainless steels such as SUS316L, which are more difficult to hydrogen embrittle than low-alloy steels, can be used.

[0004] Austenitic stainless steel has high steel material cost and low strength. Therefore, if it is designed to withstand high hydrogen pressure, the wall thickness will increase, and the price of the hydrogen transport line pipe itself will also increase. Therefore, steel materials that can withstand high-pressure hydrogen environment at a lower cost are required for hydrogen transport line pipes.

[0005] In order to solve the above-mentioned problems, for example, Patent Document 1 proposes an austenitic steel material having a high Mn content.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent No. 6703608 Summary of the invention

[0009] According to the technology described in Patent Document 1, a steel material with a lower cost than austenitic stainless steel such as SUS 316L can be provided, but the steel material described in Patent Document 1 is an austenitic alloy and therefore has a higher cost than general low-alloy steel. In addition, the steel material described in Patent Document 1 does not consider HISC resistance and fatigue crack growth resistance in a high-pressure hydrogen environment.

[0010] In view of the above problems, the present invention has an object to provide a high-strength steel plate for hydrogen transport steel pipe having excellent HISC resistance and fatigue crack growth resistance in a high-pressure hydrogen environment, and an effective method for producing the same.

[0011] Another object of the present invention is to provide a steel pipe for hydrogen transport using the high-strength steel plate for hydrogen transport steel pipe.

[0012] In order to ensure the HISC resistance and fatigue crack propagation resistance in a high-pressure hydrogen environment, the inventors of the present invention have repeatedly conducted a large number of experiments and studies on the composition, microstructure and manufacturing conditions of the steel. As a result, the following was discovered. That is, when the standard deviation of the Vickers hardness 0.25 mm below the surface of the steel plate is set to σ, the average value of the Vickers hardness 0.25 mm below the surface of the steel plate + 3σ is controlled to be less than 225HV, and the upper 20% grain size of the structure in the center of the plate thickness is made less than 30μm. As a result, the HISC resistance and fatigue crack propagation resistance are improved. Furthermore, in order to achieve such a steel structure, it is necessary to strictly control the rolling conditions and cooling conditions, and the inventors have successfully discovered this condition. The present invention is completed based on these insights.

[0013] That is, the gist of the present invention is as follows.

[0014] [1] A high-strength steel plate for hydrogen transport steel pipe having the following composition and structure:

[0015] The above ingredients are composed by mass %, including:

[0016] C: 0.030~0.060%,

[0017] Si: 0.01-0.50%,

[0018] Mn: 0.80~1.80%,

[0019] P: 0.015% or less,

[0020] S: 0.0015% or less,

[0021] Al: 0.010~0.080%,

[0022] Cr: 0.05~0.50%,

[0023] Nb: 0.005~0.080%,

[0024] Ti: 0.005-0.020%,

[0025] N: 0.0020 to 0.0080%, and

[0026] Ca: 0.0005-0.0050%, the remainder is Fe and unavoidable impurities,

[0027] The above structure is that when the standard deviation of the Vickers hardness 0.25 mm below the surface of the steel plate is σ, the average value of the Vickers hardness 0.25 mm below the surface of the steel plate + 3σ is 225 HV or less, and the upper 20% grain size in the center of the plate thickness is 30 μm or less.

[0028] The stress expansion coefficient range ΔK is 45 (MPa·m 1 / 2 ) when the fatigue crack growth rate is less than 2.0×10 -2 (mm / cycle), and the tensile strength is above 535MPa.

[0029] [2] The high-strength steel plate for hydrogen transport steel pipe according to [1], characterized in that:

[0030] The above composition is calculated by mass %, and further comprises

[0031] Cu: 0.50% or less,

[0032] Ni: 0.50% or less,

[0033] Mo: 0.50% or less,

[0034] V: 0.1% or less,

[0035] Zr: 0.02% or less,

[0036] Mg: 0.02% or less, and

[0037] REM: 0.02% or less

[0038] One or more of the following.

[0039] [3] A method for producing a high-strength steel plate for a hydrogen transport steel pipe, comprising heating a steel sheet having a composition according to [1] or [2] to a temperature of 1000 to 1250° C.,

[0040] Hot rolling is performed to produce steel sheets.

[0041] Total reduction rate in the recrystallization temperature region: 35% to 55%,

[0042] Reduction rate of the final rolling pass in the recrystallization temperature region: 10% or more,

[0043] Reduction ratio of the final rolling pass in the temperature range of (lower limit temperature of the recrystallization temperature range - 80°C) or more and less than the lower limit temperature of the recrystallization temperature range: 15% or more

[0044] Then, the steel plate is cooled.

[0045] Steel plate surface temperature at the start of cooling: Ar 3 Above the phase transition point (℃),

[0046] The cooling start time difference of the entire steel plate: within 50 seconds,

[0047] The average cooling rate from 750℃ to 550℃ with the steel plate thermometer at 0.25mm below the steel plate surface is 15~50℃ / s.

[0048] The average cooling rate from 750°C to 550°C at the steel plate temperature in the middle of the plate thickness is 15-50°C / s.

[0049] The cooling stop temperature of the steel plate thermometer at 0.25mm below the steel plate surface and in the middle of the plate thickness is: 250~550℃.

[0050] [4] A steel pipe for hydrogen transport, comprising the high-strength steel plate for hydrogen transport steel pipe according to [1] or [2] above.

[0051] The high-strength steel plate for hydrogen transport steel pipe of the present invention and the steel pipe for hydrogen transport using the same are excellent in HISC resistance and fatigue crack growth resistance in a high-pressure hydrogen environment. Regarding the above-mentioned steel pipe, even in the region including the welded portion of the steel pipe, the HISC resistance in a high-pressure hydrogen environment is excellent. In addition, according to the manufacturing method of the high-strength steel plate for hydrogen transport steel pipe of the present invention, a high-strength steel plate for hydrogen transport steel pipe having excellent HISC resistance and fatigue crack growth resistance in a high-pressure hydrogen environment can be manufactured. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 This is a schematic diagram for explaining a method of collecting test pieces used for evaluating HISC resistance in Examples. DETAILED DESCRIPTION

[0053] Hereinafter, the high-strength steel sheet for a hydrogen transport steel pipe of the present invention will be specifically described. It should be noted that the high-strength steel sheet for a hydrogen transport steel pipe of the present invention will also be simply referred to as a high-strength steel sheet hereinafter.

[0054] [Ingredients]

[0055] First, the chemical composition of the high-strength steel sheet of the present invention and the reasons for its limitation are described. In the following description, all units expressed in % are mass % unless otherwise specified.

[0056] C: 0.030~0.060%

[0057] C effectively contributes to the improvement of strength, but if the C content is less than 0.030%, sufficient strength cannot be ensured, so the C content is 0.030% or more. The C content is preferably 0.035% or more. On the other hand, if the C content exceeds 0.060%, the hardness increases during accelerated cooling, so the HISC resistance deteriorates. Therefore, the C content is 0.060% or less. The C content is preferably 0.050% or less.

[0058] Si: 0.01~0.50%

[0059] Si is added for deoxidation, but if the Si content is less than 0.01%, the deoxidation effect is insufficient, so the Si content is 0.01% or more. The Si content is preferably 0.05% or more. On the other hand, if the Si content exceeds 0.50%, the weldability deteriorates, so the Si content is 0.50% or less. The Si content is preferably 0.45% or less.

[0060] Mn: 0.80~1.80%

[0061] Mn effectively contributes to the improvement of strength, but if the Mn content is less than 0.80%, the effect cannot be fully manifested. Therefore, the Mn content is 0.80% or more. The Mn content is preferably 1.00% or more. The Mn content is more preferably 1.20% or more. On the other hand, if the Mn content exceeds 1.80%, the HISC resistance deteriorates due to the increase in hardness during accelerated cooling. Therefore, the Mn content is 1.80% or less. The Mn content is preferably 1.70% or less. The Mn content is more preferably 1.60% or less.

[0062] P: 0.015% or less

[0063] P is an inevitable impurity element that deteriorates HISC resistance by increasing hardness. If the P content exceeds 0.015%, this tendency is significant, so the upper limit of the P content is set to 0.015%. The P content is preferably 0.008% or less. It should be noted that although the lower the P content, the better, excessive P removal will lead to an increase in refining costs, so from the perspective of refining costs, the P content is preferably 0.001% or more.

[0064] S: 0.0015% or less

[0065] S is an inevitable impurity element. Since MnS inclusions are generated in steel, the low temperature toughness is deteriorated. Therefore, the S content is preferably a small amount, and up to 0.0015% is allowed. Therefore, the S content is 0.015% or less. The S content is preferably 0.0010% or less. It should be noted that the lower the S content, the better, but since excessive desulfurization leads to an increase in refining costs, from the perspective of refining costs, the S content is preferably 0.0002% or more.

[0066] Al: 0.010~0.080%

[0067] Al is added as a deoxidizer, but if the Al content is less than 0.010%, the effect cannot be fully manifested. Therefore, the Al content is 0.010% or more. The Al content is preferably 0.015% or more. The Al content is more preferably 0.025% or more. On the other hand, if the Al content exceeds 0.080%, alumina clogging of the immersion nozzle during continuous casting may occur, so the Al content is 0.080% or less. The Al content is preferably 0.070% or less. The Al content is more preferably 0.040% or less.

[0068] Cr: 0.05~0.50%

[0069] Cr, like Mn, is an element effective for obtaining sufficient strength even for steel with a low C content, but if the Cr content is less than 0.05%, this effect cannot be fully manifested. Therefore, the Cr content is 0.05% or more. The Cr content is preferably 0.10% or more. The Cr content is more preferably 0.15% or more. However, if the Cr content exceeds 0.50%, the hardenability becomes excessive, so the hardness increases during accelerated cooling and the HISC resistance deteriorates. Therefore, 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.

[0070] Nb: 0.005~0.080%

[0071] If Nb exists as solid solution Nb, the non-recrystallization temperature region during hot rolling is expanded, which helps to refine the grain size of the grains, but if the Nb content is less than 0.005%, this effect cannot be fully manifested. Therefore, the Nb content is 0.005% or more. The Nb content is preferably 0.010% or more. The Nb content is more preferably 0.025% or more. On the other hand, if the Nb content exceeds 0.080%, the hydrogen cracking resistance is deteriorated due to the crystallization of coarse carbides during solidification. Therefore, the Nb content is 0.080% or less. The Nb content is preferably 0.060% or less. The Nb content is more preferably 0.055% or less.

[0072] Ti: 0.005~0.020%

[0073] Ti, as TiN, pins austenite grains during heating and has the effect of inhibiting grain growth. If the Ti content is less than 0.005%, TiN cannot be fully generated, so the Ti content is 0.005% or more. The Ti content is preferably 0.008% or more. In addition, if the Ti content exceeds 0.020%, the generated TiN coarsens and sufficient toughness of the weld heat affected zone cannot be obtained, so the Ti content is 0.020% or less. The Ti content is preferably 0.017% or less. The Ti content is more preferably 0.015% or less.

[0074] N: 0.0020~0.0080%

[0075] N effectively contributes to the improvement of strength, but if the N content is less than 0.0020%, sufficient strength cannot be ensured. Therefore, the N content is 0.0020% or more. The N content is preferably 0.0025% or more. The N content is more preferably 0.0030% or more. On the other hand, if the N content exceeds 0.0080%, the hardness increases during accelerated cooling, so the HISC resistance deteriorates. Therefore, the N content is 0.0080% or less. The N content is preferably 0.0070% or less. The N content is more preferably 0.0050% or less.

[0076] Ca: 0.0005~0.0050%

[0077] Ca is an element effective in improving hydrogen cracking resistance based on morphology control of sulfide inclusions. If the Ca content is less than 0.0005%, the effect of addition is insufficient. Therefore, the Ca content is 0.0005% or more. The Ca content is preferably 0.0008% or more. The Ca content is more preferably 0.0015% or more. On the other hand, when the Ca content exceeds 0.0050%, not only the above effect is saturated, but also the cleanliness of the steel is reduced and the hydrogen cracking resistance is deteriorated. Therefore, 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.

[0078] The basic components (essential components) of the high-strength steel sheet of the present invention have been described above. In the composition of the high-strength steel sheet of the present invention, components other than the above (the remainder) may be Fe and unavoidable impurities.

[0079] The component composition of the high-strength steel sheet of the present invention may contain, in addition to the above components, one or more selected from Cu, Ni, Mo, V, Zr, Mg, and REM in the following ranges.

[0080] Cu: 0.50% or less

[0081] Cu is an element effective for improving low temperature toughness and increasing strength. In order to obtain 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 0.50%, surface defects of the steel sheet are easily generated. Therefore, when Cu is contained, the Cu content is 0.50% or less. The Cu content is preferably 0.45% or less.

[0082] Ni: 0.50% or less

[0083] Ni is an element effective for improving low temperature toughness and increasing strength. In order to obtain 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 a high-priced element, when Ni is contained, the Ni content is 0.50% or less. The Ni content is preferably 0.45% or less.

[0084] Mo: 0.50% or less

[0085] Mo is an element effective for improving low temperature toughness and increasing strength. 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 0.50% or less. The Mo content is preferably 0.45% or less.

[0086] V: 0.1% or less

[0087] V is an element that can be added arbitrarily to improve the strength and low temperature toughness of the steel plate, but if the V content is less than 0.005%, the effect cannot be fully manifested. Therefore, when V is contained, it is preferred that the V content is 0.005% or more. On the other hand, if the V content exceeds 0.1%, the toughness of the weld deteriorates, so when V is contained, it is preferred that the V content is 0.1% or less. The V content is more preferably 0.050% or less, and further preferably 0.010% or less.

[0088] Zr: 0.02% or less, Mg: 0.02% or less, REM: 0.02% or less

[0089] Zr, Mg and REM (rare earth metals) are elements that can be added arbitrarily in order to improve fatigue crack propagation resistance by grain refinement or to improve crack resistance by controlling inclusion properties. If the content of each element is less than 0.0005%, the effect cannot be fully manifested. Therefore, when these elements are contained, it is preferred that the content of each element is more than 0.0005%. On the other hand, if the content of each element exceeds 0.02%, the effect is saturated. Therefore, when Zr, Mg and REM are contained, it is preferred that the content of each element is less than 0.02%. The content of each of the above elements is preferably less than 0.0050%, and more preferably less than 0.0030%. It should be noted that REM is a general term for 15 elements from Sc, Y and lanthanum (La) with atomic number 57 to ruthenium (Lu) with atomic number 71, and the REM content mentioned here is the total content of these elements.

[0090] It should be noted that the remainder other than the above elements is composed of Fe and inevitable impurities. However, as long as the effect of the present invention is not impaired, the inclusion of other trace elements is not hindered. For example, O is an element inevitably contained in steel, but as long as its content is 0.0050% or less, preferably 0.0040% or less, it is allowed in the present invention.

[0091] [Hardness 0.25mm below the surface of the steel plate]

[0092] Regarding the high-strength steel plate of the present invention, it is important that when the standard deviation of the Vickers hardness (HV0.5) at 0.25 mm below the surface of the steel plate is σ, the average value + 3σ of the Vickers hardness (HV0.5) at 0.25 mm below the surface of the steel plate is below 225 HV. By satisfying this condition, excellent HISC resistance can be obtained in a high-pressure hydrogen environment. When the average value + 3σ of the Vickers hardness (HV0.5) at 0.25 mm below the surface of the steel plate exceeds 225 HV, due to the large deviation of the hardness in the steel plate, hydrogen is locally accumulated, resulting in degradation of HISC resistance at the local hydrogen accumulation site. Here, "the Vickers hardness (HV0.5) at 0.25 mm below the surface of the steel plate" is the Vickers hardness (HV0.5) measured at 100 points at a distance of 0.25 mm from the surface of the steel plate (a depth of 0.25 mm from the surface of the steel plate to the center of the plate thickness) at equal intervals from the front end and the rear end of the rolling direction of the steel plate. It should be noted that the front end of the rolling direction of the steel plate is a position 1m downstream of the front end of the steel plate in the rolling direction. The tail end of the rolling direction of the steel plate is a position 1m upstream of the tail end of the steel plate in the rolling direction. The measurement is performed on the area except the unstable part near the end of the plate width direction. Here, the reason for measuring the hardness of the steel plate with 0.5kgf instead of the commonly used 10kgf is as follows: by measuring with 0.5kgf, the indentation becomes smaller, so it is possible to obtain hardness information at a position closer to the surface and hardness information that is more sensitive to the microstructure. If the Vickers hardness is measured with a test force smaller than 0.5kgf, the measurement deviation becomes larger due to the excessively small indentation size, so it is not preferred. The average value +3σ of the Vickers hardness 0.25mm below the surface of the steel plate is preferably less than 220HV. In addition, as an example, the average value +3σ of the Vickers hardness 0.25mm below the surface of the steel plate is more than 200HV.

[0093] [Top 20% grain size at the center of plate thickness]

[0094] By making the average grain size finer, the fatigue crack growth resistance is improved, but in Ar 3When cooling starts above the transformation point temperature, there is a limit to the refinement of the average grain size. In the present invention, it is extremely important to suppress the formation of coarse grains. That is, if the upper 20% grain size is large, the fatigue crack propagation resistance deteriorates. Especially in the center of the plate thickness, since the upper 20% of the grain size distribution exceeds 30μm, the fatigue crack propagation resistance deteriorates significantly. Therefore, a structure with an upper 20% grain size of less than 30μm in the center of the plate thickness (1 / 2 plate thickness position) is required. The above-mentioned upper 20% grain size is preferably less than 25μm. In addition, as an example, the above-mentioned upper 20% grain size is more than 15μm. It should be noted that the upper 20% grain size refers to the grain size corresponding to the 20% position when the grain size is arranged in order of large in the distribution of grain size. The measurement range of the grain size is 1mm×1mm at the center of the plate thickness. More specifically, regarding the grain size, the structure at the center of the plate thickness is analyzed by the EBSD (Electron Backscatter Diffraction) method, and the result is that the boundary with an orientation difference of more than 15° is judged as the grain boundary, and the diameter of the equivalent circle diameter is calculated according to the area of ​​each grain as the grain size. In addition, in the present invention, a degree distribution table is prepared for all the grains of the measured object, and the cumulative relative degree starting from the larger side of the calculated grain size is equivalent to 20% of the grain size is called the "upper 20% grain size".

[0095] [Fatigue crack growth rate]

[0096] In the fatigue crack growth test of the high-strength steel sheet of the present invention in 21 MPa high-pressure hydrogen, the stress expansion coefficient range ΔK is 45 (MPa·m 1 / 2 ) when the fatigue crack growth rate is less than 2.0×10 -2 (mm / cycle). It is preferred that the fatigue crack growth rate is 1.5×10 -2 (mm / cycle) or less. The lower the fatigue crack growth rate, the better. In addition, as an example, the fatigue crack growth rate is 1.0×10 -2 (mm / cycle) or above.

[0097] [Tensile strength]

[0098] The high-strength steel plate of the present invention is mainly a steel plate for steel pipes having a strength of API 5L X65 grade or higher, and therefore is a steel plate having a tensile strength of 535 MPa or higher. It should be noted that the upper limit of the tensile strength of the high-strength steel plate of the present invention is not particularly limited, and as an example, the tensile strength of the high-strength steel plate of the present invention is 760 MPa or less. In addition, the tensile strength of the high-strength steel plate of the present invention may be 600 MPa or less.

[0099] [Thickness of high-strength steel plate]

[0100] The thickness of the high-strength steel sheet of the present invention is not particularly limited, but is preferably 12 mm or more. In addition, the thickness of the high-strength steel sheet of the present invention is not particularly limited, but is preferably 39 mm or less.

[0101] [Manufacturing method]

[0102] Hereinafter, the manufacturing method and manufacturing conditions for manufacturing the above-mentioned high-strength steel sheet will be described in detail.

[0103] The method for producing the high-strength steel plate of the present invention is to heat a steel sheet (slab) having the above-mentioned composition, hot-roll the steel sheet to produce a steel plate (hot rolling step), and then cool the steel plate under predetermined conditions (cooling step).

[0104] [Heating temperature of steel sheet]

[0105] Heating temperature of steel sheet: 1000~1250℃

[0106] If the heating temperature of the steel sheet (slab) is less than 1000°C, the solid solution of carbides is insufficient, and the required strength cannot be obtained because the amount of solid solution strengthening based on the solid solution C, etc. is reduced. On the other hand, if the heating temperature of the steel sheet exceeds 1250°C, the fatigue crack growth resistance deteriorates due to the extremely coarsened grains, so the heating temperature of the steel sheet is 1000-1250°C. The heating temperature of the steel sheet is preferably above 1030°C. In addition, the heating temperature of the steel sheet is preferably below 1200°C. It should be noted that the steel sheet (slab) is heated until the center portion is the above-mentioned heating temperature.

[0107] [Total reduction ratio in the recrystallization temperature range: 35% to 55%]

[0108] In order to make the upper 20% grain size of the structure in the center of the plate thickness fine, it is necessary to promote the recrystallization of the grains and suppress the formation of coarse grains by hot rolling in the recrystallization temperature range. When the total reduction rate in the recrystallization temperature range is less than 35%, coarse grains remain due to insufficient recrystallization. Therefore, the total reduction rate in the recrystallization temperature range is 35% or more, preferably 38% or more. On the other hand, if the total reduction rate in the recrystallization temperature range exceeds 55%, although the coarsening of the grains can be suppressed, the grain refinement cannot be achieved due to insufficient reduction in the non-recrystallization area. Therefore, the total reduction rate in the recrystallization temperature range is 55% or less, preferably 52% or less. Here, the lower limit temperature Tnr (℃) of the recrystallization temperature range can be calculated, for example, from the composition of the steel by the following formula. It should be noted that the hot rolling temperature is the surface temperature of the rolled material (steel sheet or steel plate), and the above surface temperature can be measured by a radiation thermometer or the like.

[0109] Tnr(℃)=174×log[%Nb][%C+(12 / 14)%N]+1444

[0110] In the above formula, [%X] represents the content (mass %) of element X in steel.

[0111] [Reduction ratio of the final rolling pass in the recrystallization temperature region: 10% or more]

[0112] In addition to making the total reduction rate in the recrystallization temperature zone 35% to 55%, it is necessary to fully promote recrystallization by fully ensuring the reduction rate of the final rolling pass in the recrystallization temperature zone, so as to start the partial recrystallization zone rolling in the state of uniform grains without coarse grains. When the reduction rate of the final rolling pass in the recrystallization temperature zone is less than 10%, due to insufficient recrystallization, coarse grains grow during the holding time from rough rolling to the start of finish rolling. Therefore, the reduction rate of the final rolling pass in the recrystallization temperature zone is 10% or more, preferably 11% or more. The upper limit of the reduction rate of the final rolling pass in the recrystallization temperature zone is not particularly limited, and the higher the better. As an example, the reduction rate of the final rolling pass in the recrystallization temperature zone is 20% or less.

[0113] [Reduction ratio of the final rolling pass in the temperature range of (lower limit temperature of the recrystallization temperature range - 80°C) or more and less than the lower limit temperature of the recrystallization temperature range: 15% or more]

[0114] Since partial recrystallization occurs after the rolling in the recrystallization temperature zone is completed, recrystallization can be promoted by further increasing the reduction rate in the temperature zone above (the lower limit temperature of the recrystallization temperature zone - 80°C) and below the lower limit temperature of the recrystallization temperature zone. As a result, the upper 20% grain size of the structure in the center of the plate thickness can be effectively refined. Therefore, the reduction rate of the final rolling pass in the temperature zone above (the lower limit temperature of the recrystallization temperature zone - 80°C) and below the lower limit temperature of the recrystallization temperature zone is 15% or more, preferably 16% or more. The upper limit of the reduction rate of the final rolling pass in the above-mentioned temperature zone is not particularly limited, and the higher the better. As an example, the reduction rate of the final rolling pass in the above-mentioned temperature zone is 25% or less.

[0115] Regarding rolling at a temperature lower than (the lower limit temperature of the recrystallization temperature range - 80°C), rolling at a low temperature causes more deformation and is therefore effective for grain refinement. Therefore, within the range of the cooling start temperature that can be followed, rolling is preferably performed at a low temperature lower than (the lower limit temperature of the recrystallization temperature range - 80°C).

[0116] [Rolling end temperature]

[0117] In the hot rolling process, in order to make the grains fine, the lower the rolling end temperature, the better. On the other hand, in a high-pressure hydrogen environment, from the viewpoint of ensuring HISC resistance, it is necessary to make the cooling start temperature in the cooling process after the hot rolling process as Ar2 / 2. 3 The rolling end temperature needs to be set if the temperature is above the phase transformation point. 3 The transformation point refers to the temperature at which ferrite transformation starts during cooling, and can be determined from the composition of the steel using the following formula, for example. The surface temperature of the steel plate can be measured using a radiation thermometer or the like.

[0118] Ar 3 Phase transition point (℃) = 910-310[%C]-80[%Mn]-20[%Cu]-

[0119] 15[%Cr]-55[%Ni]-80[%Mo]

[0120] In the above formula, [%X] represents the content (mass %) of the element X in the steel, and the element not contained is 0.

[0121] [Cooling start temperature of cooling]

[0122] Cooling start temperature: Steel plate surface thermometer Ar 3 Phase change point (℃) or above

[0123] After the hot rolling process, the steel sheet is cooled (controlled cooling). The surface temperature of the steel sheet at the start of cooling is less than Ar 3 In the case of the transformation point (°C), ferrite is generated before cooling, and the strength reduction becomes larger. Therefore, the surface temperature of the steel plate at the start of cooling is Ar 3 Phase transformation point (°C) or above. It should be noted that the surface temperature of the steel plate at the start of cooling is the temperature of the steel plate surface area where the cooling start temperature becomes the lowest. Specifically, regarding the surface temperature of the steel plate at the start of cooling, for example, when the steel plate is moved in one direction relative to the cooling device while being cooled, it is the surface temperature of the steel plate at the rear end of the steel plate. In addition, for example, when the entire steel plate is cooled at regular intervals and the cooling start time between the above-mentioned areas is different, it is the surface temperature of the steel plate in the last cooled area. It should be noted that, as an example, the upper limit of the steel plate surface temperature at the start of cooling is the above-mentioned rolling end temperature.

[0124] [Cooldown start time of cooldown]

[0125] The cooling start time difference of the entire steel plate: within 50 seconds

[0126] When the cooling start time difference of the entire steel plate exceeds 50 seconds, the temperature difference in the steel plate becomes larger, so the deviation of the steel plate temperature when cooling stops becomes larger, and the deviation of the Vickers hardness of 0.25mm below the surface of the steel plate becomes larger, and the HISC resistance deteriorates. Therefore, the cooling start time difference of the entire steel plate is within 50 seconds, preferably within 45 seconds. Specifically, for example, when the steel plate is moved in one direction relative to the cooling device and cooled, the difference between the cooling start time of the front end of the steel plate and the cooling start time of the tail end of the steel plate is within 50 seconds. In addition, for example, with respect to the entire steel plate, cooling is performed at intervals of a certain area, and when the time for starting cooling between the above-mentioned areas is different, the difference between the cooling start time of the initial area and the cooling start time of the last area is within 50 seconds. It should be noted that when the entire steel plate can be cooled at one time, the cooling start time difference of the entire steel plate can be 0 seconds.

[0127] [Cooling speed of cooling]

[0128] In order to obtain excellent HISC resistance and achieve high strength, it is necessary to control the cooling rate at 0.25 mm below the surface of the steel plate and in the middle of the plate thickness.

[0129] Average cooling rate from 750℃ to 550℃ at 0.25mm below the steel plate surface: 15~50℃ / s

[0130] It is important to slow down the average cooling rate of 750°C to 550°C at the steel plate temperature 0.25mm below the steel plate surface as much as possible to form granular bainite. Since the temperature range of 750°C to 550°C is an important temperature range in the bainite transformation, it is important to control the cooling rate in this temperature range. If the average cooling rate in the above temperature range exceeds 50°C / s, there is a possibility of hardness deviation, and the HISC resistance after pipe making deteriorates. Therefore, the average cooling rate is 50°C / s or less. It is preferably 45°C / s or less. On the other hand, if the cooling rate is excessively reduced, the strength is insufficient due to the formation of ferrite and pearlite. From the viewpoint of preventing this, the average cooling rate in the above temperature range is 15°C / s or more, preferably 17°C / s or more. It should be noted that with regard to cooling in the temperature range of 550°C or less at the steel plate temperature 0.25mm below the steel plate surface, if the cooling rate is slow, it cannot be cooling in a stable nucleate boiling state, and there is a possibility of hardness deviation in the extreme surface part of the steel plate. Therefore, the average cooling rate from 550°C to the cooling stop temperature is preferably 150°C / s or more, based on the steel sheet temperature 0.25 mm below the steel sheet surface. From the perspective of making it easier to suppress hardness variations, the average cooling rate is preferably 250°C / s or less.

[0131] Average cooling rate from 750°C to 550°C in the center of the plate thickness: 15-50°C / s

[0132] If the average cooling rate from 750°C to 550°C in the center of the plate thickness is less than 15°C / s, the granular bainite structure cannot be obtained and the strength is reduced. Therefore, the average cooling rate from 750°C to 550°C in the center of the plate thickness is 15°C / s or more. From the viewpoint of suppressing the deviation of the structure, the above average cooling rate is preferably 17°C / s or more. On the other hand, in order to suppress the deviation of the grain size, the above average cooling rate is 50°C / s or less, preferably 45°C / s or less. It should be noted that there is no particular limitation on cooling in a temperature range below 550°C based on the steel plate thermometer in the center of the plate thickness. From the viewpoint of suppressing the deviation of the structure and grain size, the average cooling rate in the above temperature range is preferably 15°C / s or more. In addition, from the above viewpoint, the average cooling rate in the above temperature range is preferably 50°C / s or less.

[0133] It should be noted that the temperature of the steel plate at 0.25 mm below the surface of the steel plate and at the center of the plate thickness cannot be measured directly physically. However, based on the surface temperature at the start of cooling measured by a radiation thermometer and the target surface temperature at the end of cooling, for example, the temperature distribution in the plate thickness section is calculated by differential calculation using a process computer, and the temperature can be obtained in real time based on the result. The temperature at 0.25 mm below the surface of the steel plate under this temperature distribution is referred to as the "steel plate temperature at 0.25 mm below the surface of the steel plate" in this specification, and the temperature at the center of the plate thickness under this temperature distribution is referred to as the "steel plate temperature at the center of the plate thickness" in this specification.

[0134] [Cooling stop temperature]

[0135] Cooling stop temperature: 250~550℃ with the steel plate temperature at 0.25mm below the steel plate surface and at the center of the plate thickness

[0136] If the cooling stop temperature exceeds 550°C, the bainite transformation becomes incomplete and sufficient strength cannot be obtained, as measured by the steel plate temperature at 0.25 mm below the surface of the steel plate and at the center of the plate thickness. Therefore, the cooling stop temperature is 550°C or less, preferably 500°C or less. In addition, if the cooling stop temperature is less than 250°C, the hardness increases, so the HISC resistance deteriorates. Therefore, the cooling stop temperature is 250°C or more, preferably 300°C or more.

[0137] [Steel pipe for hydrogen transportation]

[0138] After the high-strength steel plate of the present invention is formed into a tubular shape by pressure bending, roll forming, UOE forming, etc., by welding the butt joint, a hydrogen transport steel pipe (UOE steel pipe, electric resistance welded steel pipe, spiral steel pipe, etc.) suitable for transporting high-pressure hydrogen can be manufactured. In addition, by using the high-strength steel plate of the present invention to manufacture a steel pipe, even if there is a high hardness area in the weld, a steel pipe with excellent HISC resistance can be manufactured. It should be noted that in the present invention, as an example, high-pressure hydrogen refers to a hydrogen environment of 15 MPa or more.

[0139] For example, regarding UOE steel pipes, the ends of steel plates are grooved, and after being formed into a steel pipe shape by C-pressing, U-pressing, and O-pressing, the butt joints are seam welded by inner surface welding and outer surface welding, and then manufactured through a pipe expansion process as needed. In addition, the welding method may be any method as long as it can obtain sufficient joint strength and joint toughness. From the viewpoint of excellent welding quality and manufacturing efficiency, submerged arc welding is preferably used. In addition, after being formed into a tubular shape by pressure bending, the steel pipe with seam welded butt joints may be expanded.

[0140] Example

[0141] The steel (steel grades A to W) having the component composition shown in Table 1 is made into a steel sheet (slab) by a continuous casting method, and after being heated to the heating temperature shown in Table 2, hot rolling and cooling are carried out under the conditions shown in Table 2 to make a steel plate with the final thickness shown in Table 2. In the cooling process, the steel plate is moved in one direction while being controlled cooled using a water-cooled controlled cooling device. Then, the end of the steel plate is grooved, and after being formed into a steel pipe shape by C pressing, U pressing, and O pressing, the butt joints of the inner and outer surfaces are seam welded by submerged arc welding, and a steel pipe is made through a pipe expansion process. It should be noted that Ar in Table 1 3 The transformation point and the lower limit temperature Tnr of the recrystallization temperature range are respectively obtained from the above-mentioned formulas.

[0142] [Measurement of Vickers hardness]

[0143] Regarding the cross-sections perpendicular to the rolling direction from the front end and the tail end of the steel plate in the rolling direction, according to JIS Z2244 (2009), 100 points of Vickers hardness (HV0.5) were measured at equal intervals along the width direction of the plate at a position 0.25 mm below the surface of the steel plate. And the average value and standard deviation σ of the Vickers hardness (HV0.5) of 200 points were calculated. It should be noted that the front end of the rolling direction of the above-mentioned steel plate is a position 1m away from the downstream side of the rolling direction from the front end of the steel plate. The tail end of the rolling direction of the above-mentioned steel plate is a position 1m away from the upstream side of the rolling direction from the tail end of the steel plate. In addition, the measurement is performed on the area other than the unstable part near the end of the plate in the width direction. The average value of the Vickers hardness 0.25 mm below the surface of the steel plate + 3σ is shown in Table 3.

[0144] [Calculation of the top 20% particle size]

[0145] A sample for metal structure observation is collected from the central part of the plate width of the steel plate obtained as described above. After the cross section of this sample perpendicular to the width direction of the plate is mirror-polished, it is etched with colloidal silica. Then, crystal data is collected by EBSD (Electron Backscatter Diffraction) method with a field of view of 1mm×1mm at the center of the plate thickness (measurement step: 0.8μm). After data collection, OIM-Analysis (manufactured by EDAX, OIM Analysis software) is used to judge the boundary with an azimuth difference of more than 15° as a grain boundary, and the diameter of the equivalent circle diameter is calculated according to the area of ​​each grain as the grain size. In addition, a degree distribution table is prepared for all grains of the measured object, and the grain size equivalent to 20% of the cumulative relative degree of the larger side of the calculated grain size is taken as the "upper 20% grain size". The results of the measurement are shown in Table 3.

[0146] [Derivation of fatigue crack growth rate]

[0147] From the steel plate obtained as described above, CT test pieces were collected in accordance with ASTM E 647 in a manner that the load direction and the rolling direction were parallel. The CT test piece is a 10 mm thick test piece collected from the 1 / 2 position of the plate thickness. In addition, the length of the fatigue crack was measured by the compliance method using a clamp-type extensometer, and the fatigue crack growth rate in 21 MPa high-pressure hydrogen was calculated. In addition, the stress expansion coefficient range ΔK was evaluated to be 45 (MPa·m 1 / 2 The results are shown in Table 3.

[0148] [Determination of tensile strength]

[0149] The full-thickness test piece in the direction perpendicular to the rolling direction was used as a tensile test piece, and a tensile test was performed in accordance with the provisions of JIS Z2241 (2011) to measure the tensile strength and yield strength.

[0150] [Evaluation of HISC resistance]

[0151] Regarding HISC resistance, Figure 1 As shown, after the test piece (coupon) cut out from the obtained steel pipe is flattened, a test piece of 3mm×10mm×50mm is collected from the inner surface of the steel pipe. At this time, in addition to collecting a test piece that does not include the weld but only includes the base material, a test piece containing both the weld and the base material is also collected. In order to retain the state of the outermost layer, the inner surface of the test surface remains with black skin. That is, the portion 0.25mm below the surface of the steel plate is included in the test piece. A stress of 90% of the actual yield strength (0.5% YS) of each steel pipe is loaded on the test piece collected in this way, and a 4-point bending test is performed in 21MPa high-pressure hydrogen. After 720 hours of exposure, in both the test piece that does not include the weld but only includes the base material and the test piece that includes both the weld and the base material, the case where no cracks are found is judged to be excellent (good) in HISC resistance and is set to ○. In addition, the case where cracks occur in at least one of the test pieces is judged to be poor and is set to ×. The results are shown in Table 3.

[0152] The target range of the present invention is as follows. As a high-strength steel plate for hydrogen transport steel pipe, the average value of the Vickers hardness 0.25mm below the surface of the steel plate +3σ is less than 225HV. The upper 20% grain size of the structure in the center of the plate thickness is less than 30μm. The stress expansion coefficient range ΔK is 45 (MPa·m 1 / 2 ) when the fatigue crack growth rate is less than 2.0×10 -2 (mm / cycle). The tensile strength was 535 MPa or more. Furthermore, no cracks were found in the evaluation of the HISC resistance (4-point bending test).

[0153]

[0154]

[0155] Table 3

[0156]

[0157] Note 1: The underline indicates the area outside the scope of the present invention. *1 The position 0.25 mm below the surface of the steel plate

[0158] *2 Structure in the center of plate thickness

[0159] As shown in Table 2, No. 1 to No. 9, No. 33 to No. 35 are invention examples whose component compositions and manufacturing conditions meet the appropriate range of the present invention. As shown in Table 3, any one of No. 1 to No. 9, No. 33 to No. 35 is a high-strength steel plate, and the average value + 3σ of the Vickers hardness 0.25 mm below the surface of the steel plate is less than 225 HV. The upper 20% grain size of the structure in the center of the plate thickness is less than 30 μm. The stress expansion coefficient range ΔK is 45 (MPa·m 1 / 2 ) when the fatigue crack growth rate is less than 2.0×10 -2 (mm / cycle). The tensile strength is 535 MPa or more. In addition, the HISC resistance is also good.

[0160] In contrast, the composition of the steel sheets of No. 10 to No. 20 is outside the scope of the present invention. No. 10, No. 12, No. 15 and No. 19 have insufficient solid solution strengthening and insufficient strength. Since the Vickers hardness of 0.25 mm below the surface of the steel sheets of No. 11, No. 13, No. 14, No. 16 and No. 20 increases, the HISC resistance is poor. No. 17 and No. 18 have insufficient grain growth inhibition based on precipitates and poor fatigue crack growth resistance.

[0161] The component compositions of No.21 to No.32 are within the scope of the present invention, but the manufacturing conditions are comparative examples outside the scope of the present invention. Regarding No.21, since the heating temperature of the steel sheet (slab) is low, the solid solution of carbides is insufficient, that is, the strength is low. Regarding No.22, since the heating temperature of the steel sheet is high, the grains are coarsened and the fatigue crack growth resistance is deteriorated. Regarding No.23, since the total reduction rate in the recrystallization temperature range is insufficient, coarse grains remain and the fatigue crack growth resistance is deteriorated. Regarding No.24, since the total reduction rate in the recrystallization temperature range is too much, the upper 20% grain size of the structure in the center of the plate thickness is large, and the fatigue crack growth resistance is deteriorated. Regarding No.25, since the reduction rate of the final rolling pass in the recrystallization temperature range is insufficient, coarse grains remain and the fatigue crack growth resistance is deteriorated. Regarding No. 26, since the reduction rate in the final rolling pass in the temperature range above (lower limit temperature of the recrystallization temperature range - 80°C) and lower than the lower limit temperature of the recrystallization temperature range is insufficient, the upper 20% grain size of the structure in the center of the plate thickness is large, and the fatigue crack growth resistance is deteriorated. Regarding No. 27, since the cooling start temperature is low, part of ferrite is generated, so the strength is low. Regarding No. 28, since the cooling start time difference of the entire steel plate is large, the deviation of the Vickers hardness 0.25mm below the surface of the steel plate becomes large, and the HISC resistance is deteriorated. Regarding No. 29, since the average cooling rate from 750°C to 550°C is low, part of ferrite is generated, so the strength is low. Regarding No. 30, since the average cooling rate from 750°C to 550°C is high, the deviation of the Vickers hardness 0.25mm below the surface of the steel plate becomes large, and the HISC resistance is poor. Regarding No. 31, since the cooling stop temperature was low, the variation of the Vickers hardness 0.25 mm below the steel sheet surface became large, and thus the HISC resistance was deteriorated. Regarding No. 32, since the cooling stop temperature was high, part of ferrite was generated, and thus the strength was low.

[0162] Industrial Application Possibility

[0163] According to the present invention, it is possible to provide a high-strength steel plate for hydrogen transport steel pipe having excellent HISC resistance and fatigue crack growth resistance in a high-pressure hydrogen environment.

Claims

1. A high-strength steel plate for hydrogen transport steel pipe, having the following composition and structure, The composition of the ingredients contains, in mass %, the following: C：0.030～0.060％、 Si: 0.01-0.50%, Mn: 0.80~1.80%, P: 0.015% or less, S: 0.0015% or less, Al：0.010～0.080％、 Cr：0.05～0.50％、 Nb: 0.005~0.080%, Ti: 0.005-0.020%, N: 0.0020~0.0080%, and Ca: 0.0005~0.0050%, The remainder is Fe and unavoidable impurities; The structure is such that when the standard deviation of the Vickers hardness at 0.25 mm below the surface of the steel plate is set to σ, the average value of the Vickers hardness at 0.25 mm below the surface of the steel plate + 3σ is less than 225 HV, and the upper 20% grain size at the center of the plate thickness is less than 30 μm; In addition, the stress expansion coefficient range ΔK is 45MPa·m 1 / 2 The fatigue crack growth rate is less than 2.0×10 -2 mm / cycle, and the tensile strength is above 535MPa.

2. The high-strength steel plate for hydrogen transport steel pipe according to claim 1, characterized in that: The composition of the ingredients, in terms of mass%, also contains Cu: 0.50% or less, Ni: 0.50% or less, Mo: 0.50% or less, V: 0.1% or less, Zr: 0.02% or less, Mg: 0.02% or less and REM: 0.02% or less One or more of the following.

3. A method for manufacturing a high-strength steel plate for a hydrogen transport steel pipe, comprising heating a steel sheet having a composition as claimed in claim 1 or 2 to a temperature of 1000 to 1250° C., The steel sheet is hot rolled and then cooled. During the hot rolling, Total reduction rate in the recrystallization temperature region: 35% to 55%, Reduction rate of the final rolling pass in the recrystallization temperature region: 10% or more, The reduction ratio of the final rolling pass in the temperature range of (lower limit temperature of the recrystallization temperature range - 80°C) or more and less than the lower limit temperature of the recrystallization temperature range: 15% or more, During the cooling, Steel plate surface temperature at the start of cooling: above the Ar3 transformation point (°C), The cooling start time difference of the entire steel plate: within 50 seconds, The average cooling rate from 750℃ to 550℃ with the steel plate thermometer at 0.25mm below the steel plate surface is 15~50℃ / s. The average cooling rate from 750°C to 550°C at the steel plate temperature in the middle of the plate thickness is 15-50°C / s. The cooling stop temperature of the steel plate thermometer at 0.25mm below the steel plate surface and in the middle of the plate thickness is: 250~550℃.

4. A steel pipe for hydrogen transportation, using the high-strength steel plate for hydrogen transportation steel pipe according to claim 1 or 2.

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