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

Abstract

The present invention provides a high-strength steel plate that can be used as a raw material for steel pipes that satisfy all of the requirements for large diameter, excellent low-temperature toughness, and strength characteristics required for high-strength steel pipes for steam transportation, and a method for manufacturing the same. The present invention also provides a steel pipe made from the high-strength steel plate and a method for manufacturing the same. The high-strength steel sheet of the present invention has a specific chemical composition and a specific microstructure, and the toughness of the steel sheet is such that the ductile fracture area ratio obtained by DWTT at -40°C is 85% or more, and the yield strength before and after aging under the condition of Larson Miller Parameter LMP = 15700, as defined by the following formula (2), is 555 MPa or more. LMP=(T+273)×(20+log(t))···(2) T: Heat treatment temperature (℃) t: Heat treatment time (hours)

Description

[Technical Field]

[0001] The present invention relates to a high-strength steel plate having excellent low-temperature toughness and a yield strength of 555 MPa or more before and after long-term aging in the intermediate temperature range, a method for manufacturing the same, and a steel pipe made of the high-strength steel plate and a method for manufacturing the same. In particular, the high-strength steel plate of the present invention can be suitably used as a material for high-strength steel pipes for steam piping. [Background technology]

[0002] Methods for recovering oil sands from oil reservoirs in Canada and elsewhere include open-cut mining and the steam injection method, in which high-temperature, high-pressure steam is pumped into the reservoir through steel pipes. Because open-cut mining is not feasible in many areas, the steam injection method is used in many regions. In the steam injection method, steam is pumped into the reservoir at temperatures in the 300–400°C range (hereinafter referred to as the medium-temperature range). In the steam injection method, medium-temperature steam is pumped into the reservoir at high pressure. As mentioned above, steel pipes are used to pump this steam. In recent years, increasing energy demand has led to demand for larger diameter and higher strength steel pipes to improve heavy oil recovery rates and reduce construction costs. Furthermore, when laying these steel pipes, construction work and hydraulic pressure testing are typically carried out in the early spring when temperatures are high. However, due to increasing demand, superior low-temperature toughness is also required to enable construction work during the cooler winter months.

[0003] Patent Documents 1 and 2 are prior art examples of steel pipes for transporting steam that can be used in the steam injection method. These patent documents disclose seamless steel pipes equivalent to API X80 grade, with the outer diameter of this seamless steel pipe being a maximum of 16 inches. It is difficult to further increase the diameter of seamless steel pipes. Furthermore, in order to obtain a seamless steel pipe with strength equal to or greater than API X80 grade, it is necessary to add a large amount of alloying elements.

[0004] Incidentally, Patent Documents 3 and 4 disclose manufacturing techniques for high-strength steel pipes that can be made larger in diameter and are manufactured by welding. More specifically, Patent Documents 3, 4 and 5 disclose techniques for manufacturing high-strength steel pipes that are manufactured by TMCP (Thermo-mechanical control process). , concerning the manufacturing technology of high strength steel pipes having strength of API X80 grade or higher. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-290728 [Patent Document 2] Patent No. 4821939 [Patent Document 3] Patent No. 5055736 [Patent Document 4] Patent No. 4741528 [Patent Document 5] Patent No. 6241569 Summary of the Invention [Problem to be solved by the invention]

[0006] The steel pipe manufactured by the steel pipe manufacturing method disclosed in Patent Document 3 has high-temperature properties in the medium temperature range that satisfy the API X80 grade. However, Patent Document 3 does not disclose the strength properties of the steel pipe when used for a long period of time in the medium temperature range.

[0007] Patent Document 4 describes the creep properties of steel pipes after long-term aging, and although the developed steel shows improved rupture strength, the safety criteria are not clearly defined. Furthermore, rupture strength values ​​are occasionally found to be below 80% (= 440 MPa) of the lower limit of the standard for yield stress in API X80 grade, making it difficult to say that the strength is sufficient.

[0008] The steel pipe disclosed in Patent Document 5 satisfies the API X80 grade in terms of high temperature properties in the medium temperature range, but has a small LMP, and no consideration is given to the strength properties when the steel pipe is used for a long period of time, nor to low temperature toughness.

[0009] As described above, the prior art has been unable to obtain a high-strength steel pipe for steam piping that satisfies all of the following requirements: a large diameter, strength characteristics required for high-strength steel pipe for steam transportation, and excellent low-temperature toughness.

[0010] The present invention has been made to solve the above-mentioned problems, and has an object to provide a high-strength steel plate that can be used as a raw material for steel pipes that satisfy all of the requirements for large diameter, excellent low-temperature toughness, and strength characteristics required for high-strength steel pipes for steam transportation, and a method for manufacturing the same. Another object of the present invention is to provide a steel pipe made from the high-strength steel plate and a method for manufacturing the same.

[0011] Furthermore, "excellent low-temperature toughness" means that the ductile fracture surface area ratio (DWTTSA-40°C) obtained by DWTT (test temperature: -40°C) in accordance with API 5L is 85% or more, and the fracture surface transition temperature is -40°C or less. The test temperature in the DWTT is set to -40°C in anticipation of a decrease in toughness due to work hardening during pipe making. [Means for solving the problem]

[0012] The present inventors have conducted extensive research into the properties of high-strength steel plates for large-diameter steel pipes in the medium temperature range, and have found that by appropriately selecting the chemical composition and manufacturing conditions, it is possible to obtain high-strength steel plates that can be used to manufacture high-strength steel pipes that have the strength characteristics and low-temperature toughness required for high-strength steel pipes for transporting steam, even though they are large in diameter.

[0013] The present invention was made based on the above findings and further studies, and is therefore comprised of the following: [1] The component composition is, in mass%, C: 0.04~0.09%, Si: 0.03 to 0.25%, Mn: 1.5-2.5% P: 0.020% or less, S: 0.002% or less, Mo: 0.10 to 0.50% Nb: 0.010 to 0.055%, Ti: 0.005 to 0.020%, Ca: 0.0040% or less, Al: 0.01 to 0.04%, N: Contains 0.006% or less, with the balance being Fe and unavoidable impurities; X (%) represented by formula (1) is 0.65% or more, The microstructure has bainite in an area ratio of 80% or more at the center of the plate thickness, the average grain size of the bainite is 25 μm or less, and the minimum grain size of the top 20% of the largest grains of the bainite is 60 μm or less, and the toughness of the steel plate is such that the ductile fracture area ratio obtained by DWTT at -40 ° C is 85% or more, A high-strength steel plate having a yield strength of 555 MPa or more before and after aging under the condition of Larson Miller Parameter LMP=15700, as defined by the following formula (2). X(%)=0.35Cr+0.9Mo+12.5Nb+8V···(1) The element symbols in formula (1) represent the content (mass%) of each element, and 0 is substituted for elements that are not contained. LMP=(T+273)×(20+log(t))···(2) T: Heat treatment temperature (℃) t: Heat treatment time (hours) [2] The component composition is, in mass%, further: Cr: 0.50% or less, V: 0.070% or less, Cu: 0.50% or less, [3] The high-strength steel sheet according to [1], further comprising one or more of the following: Ni: 0.50% or less. [3] The high-strength steel sheet further comprises ferrite at an area ratio of 10% or less and island martensite at an area ratio of 10% or less at the center of the sheet thickness, At the center of the plate thickness, the total of Ti-based precipitates with a diameter of 100 nm or less, Nb-based precipitates with a diameter of 100 nm or less, V-based precipitates with a diameter of 100 nm or less, Mo-based precipitates with a diameter of 100 nm or less, Cr-based precipitates with a diameter of 100 nm or less, Al-based precipitates with a diameter of 100 nm or less, and composite precipitates with a diameter of 100 nm or less containing two or more elements of Ti, Nb, V, Mo, Cr, and Al is 1 mm2 in size. 2 There are 50,000 to 1,000,000 per 3. The high-strength steel plate according to claim 1, wherein a difference in yield strength before and after aging, obtained by subtracting the yield strength after aging from the yield strength before aging, is 50 MPa or less. [4] A steel pipe using the high-strength steel plate according to any one of [1] to [3] above. [5] A method for producing a high-strength steel plate according to any one of [1] to [3], comprising: a heating step of heating a steel material to 1000 to 1200°C; a hot rolling process in which the steel material heated in the heating process is hot-rolled under the conditions of 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, a cumulative rolling reduction of 50% or more at 900°C or lower, and a rolling finish temperature of Ar3 temperature or higher and 850°C or lower; an accelerated cooling process in which the hot-rolled steel sheet obtained in the hot rolling process is acceleratedly cooled under the conditions of a cooling start temperature of Ar3 temperature or higher, an average cooling rate of 5°C / s or higher, and a cooling stop temperature of 300 to 550°C; A method for manufacturing a high-strength steel plate having the above structure. [6] A cold forming step of cold forming the high-strength steel plate according to any one of [1] to [3] into a tubular shape; a welding process for welding the butt joints at the butt joints of the ends of the steel plates formed into a tubular shape in the cold forming process; A method for manufacturing a steel pipe having the above structure. [Effects of the Invention]

[0014] According to the present invention, it is possible to obtain a high-strength steel pipe that has the strength and toughness characteristics required for a high-strength steel pipe for transporting steam, even though it has a large diameter. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following embodiments.

[0016] <High strength steel plate> The high-strength steel plate (also simply referred to as steel plate) of the present invention contains, by mass%, C: 0.04 to 0.09%, Si: 0.03 to 0.25%, Mn: 1.5 to 2.5%, P: 0.020% or less, S: 0.002% or less, Mo: 0.10 to 0.50%, Nb: 0.010 to 0.055%, Ti: 0.005 to 0.020%, Ca: 0.0040% or less, Al: 0.01 to 0.04%, and N: 0.006% or less. In the following description, "%" representing the content of a component means "mass%."

[0017] C: 0.04 to 0.09% C is an element necessary for ensuring the strength of steel through solid solution strengthening and precipitation strengthening. In particular, an increase in the amount of solute C and the formation of precipitates contribute to ensuring strength in the mid-temperature range. In order to ensure a predetermined strength at room temperature and the mid-temperature range, the present invention specifies a C content of 0.04% or more. The C content is preferably 0.05% or more. Furthermore, a C content exceeding 0.09% leads to a deterioration in toughness and weldability. For this reason, the upper limit of the C content is set to 0.09%. Furthermore, the C content is preferably 0.08% or less, and more preferably 0.07% or less.

[0018] Si: 0.03 to 0.25% Si is added for deoxidation. If the Si content is less than 0.03%, a sufficient deoxidation effect cannot be obtained, and the toughness of the steel sheet decreases. For this reason, the Si content is set to 0.03% or more. The Si content is preferably 0.04% or more, more preferably 0.05% or more, even more preferably 0.07% or more, and most preferably 0.10% or more. On the other hand, if the Si content exceeds 0.25%, the toughness will deteriorate. Therefore, the Si content is set to 0.25% or less. Furthermore, the Si content is preferably 0.23% or less, more preferably 0.20% or less, even more preferably 0.19% or less, and most preferably 0.18% or less.

[0019] Mn: 1.5 to 2.5% Mn is an element effective in improving the strength and toughness of steel. If the Mn content is less than 1.5%, the effect is small. For this reason, the Mn content is set to 1.5% or more. The Mn content is preferably set to 1.55% or more, more preferably 1.6% or more, even more preferably 1.65% or more, and most preferably 1.7% or more. On the other hand, if the Mn content exceeds 2.5%, toughness and weldability will deteriorate significantly. Therefore, the Mn content is set to 2.5% or less. Furthermore, the Mn content is preferably set to 2.4% or less, more preferably 2.3% or less, even more preferably 2.2% or less, and most preferably 2.1% or less.

[0020] P:0.020% or less P is an impurity element that deteriorates toughness. For this reason, it is desirable to reduce the P content as much as possible. However, excessive reduction of the P content increases manufacturing costs. Therefore, to keep the deterioration of toughness within an acceptable range, the P content is set to 0.020% or less. The P content is preferably set to 0.018% or less, more preferably 0.015% or less, even more preferably 0.013% or less, and most preferably 0.010% or less. There is no particular restriction on the lower limit of the P content, and it may be 0%, but is more preferably set to 0.003% or more.

[0021] S: 0.002% or less S is an impurity element that deteriorates toughness. For this reason, it is desirable to reduce the S content as much as possible. Furthermore, even if Ca is added to control the morphology of MnS to CaS-based inclusions, in the case of X80 grade high-strength materials, finely dispersed CaS-based inclusions can also be a factor in reducing toughness. For this reason, the S content is set to 0.002% or less. The S content is preferably set to 0.0018% or less, more preferably 0.0015% or less, even more preferably 0.0013% or less, and most preferably 0.0010% or less. The lower limit of the S content is not particularly limited, and may be 0%, but is preferably 0.0003% or more.

[0022] Mo: 0.10 to 0.50% Mo solid-solution strengthens steel and improves its hardenability. Solid-solution strengthening and improved hardenability increase strength, particularly in the intermediate temperature range, due to increased tempering softening resistance. If the Mo content is less than 0.10%, the effect is small and sufficient strength cannot be obtained. Therefore, the Mo content is set to 0.10% or more. The Mo content is preferably set to 0.13% or more, more preferably 0.15% or more, even more preferably 0.18% or more, and most preferably 0.20% or more. On the other hand, if the Mo content exceeds 0.50%, the effect saturates and toughness and weldability deteriorate. Therefore, the Mo content is set to 0.50% or less. Furthermore, the Mo content is preferably set to 0.45% or less, more preferably 0.40% or less, even more preferably 0.35% or less, and most preferably 0.30% or less.

[0023] Nb: 0.010 to 0.055% Nb suppresses grain growth during slab heating and rolling. This refines the microstructure and provides the steel with sufficient strength and toughness. Nb is also an essential component for forming carbides to ensure strength in the intermediate temperature range. This effect is most pronounced at an Nb content of 0.010% or more. Therefore, the Nb content is set to 0.010% or more. The Nb content is preferably set to 0.013% or more, more preferably 0.015% or more, even more preferably 0.018% or more, and most preferably 0.020% or more. On the other hand, if the Nb content exceeds 0.055%, not only does the effect saturate, but toughness and weldability also deteriorate. Therefore, the Nb content is set to 0.055% or less. The Nb content is preferably set to 0.053% or less, more preferably 0.050% or less, even more preferably 0.048% or less, and most preferably 0.045% or less.

[0024] Ti: 0.005 to 0.020% Ti forms TiN, which inhibits grain growth during slab heating and in weld heat-affected zones, and has the effect of refining the microstructure and improving toughness. To achieve this effect, the Ti content must be 0.005% or more. The Ti content is preferably 0.006% or more, more preferably 0.007% or more, even more preferably 0.008% or more, and most preferably 0.009% or more. On the other hand, if the Ti content exceeds 0.020%, toughness deteriorates. Therefore, the Ti content is set to 0.020% or less. The Ti content is preferably 0.019% or less, more preferably 0.018% or less, even more preferably 0.017% or less, and most preferably 0.016% or less.

[0025] Ca:0.0040% or less Ca functions to fix S in steel and improve the toughness of steel plate. To achieve this effect, the Ca content is preferably 0.0010% or more, and more preferably 0.0015% or more. However, if the Ca content exceeds 0.0040%, the number of inclusions in the steel increases, which may deteriorate the toughness. Therefore, the Ca content is set to 0.0040% or less. The Ca content is preferably 0.0038% or less, more preferably 0.0035% or less, even more preferably 0.0032% or less, and most preferably 0.0030% or less.

[0026] Al: 0.01 to 0.04% Al is added as a deoxidizer. If the Al content is less than 0.01%, a sufficient deoxidizing effect cannot be obtained, and the toughness of the steel sheet decreases. The Al content is set to 0.01% or more, preferably 0.013% or more, more preferably 0.015% or more, even more preferably 0.018% or more, and most preferably 0.02% or more. On the other hand, if the Al content exceeds 0.04%, the toughness decreases. Therefore, the Al content is set to 0.04% or less. The Al content is set to 0.038% or less, more preferably 0.035% or less, even more preferably 0.032% or less, and most preferably 0.03% or less.

[0027] N: 0.006% or less N combines with Ti to form TiN, which is finely dispersed in the high-temperature region of the weld heat-affected zone (HAZ) at temperatures reaching 1350°C or higher. This fine dispersion refines the prior austenite grains in the HAZ, significantly improving the toughness of the HAZ. To achieve this effect, the N content is preferably 0.002% or more, and more preferably 0.0025% or more. On the other hand, if the N content exceeds 0.006%, the toughness of the base material and the weld metal in steel pipes will deteriorate due to the coarsening of precipitates and an increase in solute N. Therefore, the N content is set to 0.006% or less. The N content is preferably 0.0058% or less, more preferably 0.0055% or less, even more preferably 0.0053% or less, and most preferably 0.005% or less.

[0028] X(X(%)=0.35Cr+0.9Mo+12.5Nb+8V...(1)):0.65% or more In the present invention, the Cr content, Mo content, Nb content, and V content are adjusted so that X (%), represented by the following formula (1), is 0.65% or more. X (%) is an important factor for obtaining a steel with excellent strength after long-term aging in the intermediate temperature range. Therefore, X (%) needs to be 0.65% or more to suppress dislocation recovery during long-term aging. X (%) is preferably 0.66% or more, more preferably 0.67% or more, even more preferably 0.68% or more, and most preferably 0.69% or more. In order to inexpensively produce the high-strength steel sheet of the present invention, X (%) is preferably 1.20% or less, more preferably 1.19% or less, even more preferably 1.18% or less, and most preferably 1.17% or less. Note that the present invention may not contain Cr or V. In such cases, 0 may be substituted for "Cr" or "V" in formula (1). X(%)=0.35Cr+0.9Mo+12.5Nb+8V···(1) The element symbols in formula (1) represent the content (mass%) of each element, and 0 is substituted for elements that are not contained.

[0029] The high strength steel sheet of the present invention may contain one or more of Cr, V, Cu and Ni for the purpose of further improving the properties.

[0030] Cr:0.50% or less Cr is one of the elements effective in increasing temper softening resistance and high-temperature strength. On the other hand, a Cr content exceeding 0.50% adversely affects weldability. Therefore, when Cr is contained, the Cr content is set to 0.50% or less. The Cr content is preferably set to 0.48% or less, more preferably set to 0.45% or less, even more preferably set to 0.43% or less, and most preferably set to 0.40% or less. Note that, as long as the temper softening resistance and high-temperature strength can be increased by, for example, ensuring that X is within the desired range, the lower limit of the Cr content is not particularly limited and may be 0%. To achieve the above effects, the Cr content is preferably set to 0.05% or more, more preferably set to 0.08% or more, and even more preferably set to 0.10% or more.

[0031] V:0.070% or less Even a small amount of V refines grains and contributes to increased strength. It also increases temper softening resistance and is one of the elements effective in increasing strength in the medium temperature range. On the other hand, if the V content exceeds 0.070%, the toughness of the weld heat-affected zone deteriorates. Therefore, when V is contained, the V content is set to 0.070% or less. The V content is preferably 0.060% or less, more preferably 0.050% or less, even more preferably 0.040% or less, and most preferably 0.030% or less. As long as high-temperature strength can be increased by, for example, ensuring that X is within the desired range, the lower limit of the V content is not particularly limited and may be 0%. To achieve the above effects, the V content is preferably 0.005% or more, more preferably 0.010% or more.

[0032] Cu: 0.50% or less Cu is one of the elements effective in improving toughness and increasing strength. To achieve these effects, the Cu content is preferably 0.05% or more, and more preferably 0.10% or more. On the other hand, if the Cu content exceeds 0.50%, weldability decreases. Therefore, when Cu is contained, the Cu content is set to 0.50% or less. The Cu content is preferably 0.48% or less, more preferably 0.45% or less, even more preferably 0.43% or less, and most preferably 0.40% or less.

[0033] Ni: 0.50% or less Ni is one of the elements that is effective in improving toughness and increasing strength. To obtain these effects, the Ni content is preferably 0.05% or more, and more preferably 0.10% or more. If the Ni content exceeds 0.50%, the effects saturate and manufacturing costs increase. Therefore, when Ni is contained, the Ni content is set to 0.50% or less. The Ni content is preferably 0.48% or less, more preferably 0.45% or less, even more preferably 0.43% or less, and most preferably 0.40% or less.

[0034] In the present invention, the Cu content, Ni content, Cr content, and Mo content are preferably adjusted so that Cu+Ni+Cr+Mo (the element symbols represent the content (mass%) of each element) is 1.00% or less. To achieve both the strength-increasing effect of these elements and production costs, Cu+Ni+Cr+Mo is preferably 1.00% or less, more preferably 0.98% or less, even more preferably 0.95% or less, and most preferably 0.90% or less. There is no particular lower limit, but Cu+Ni+Cr+Mo is preferably 0.10% or more, more preferably 0.15% or more, even more preferably 0.20% or more, and most preferably 0.25% or more.

[0035] The remainder other than the above components is Fe and unavoidable impurities. These impurities are inevitably mixed in from raw materials, the manufacturing process, or manufacturing equipment, and are allowed to be present to the extent that they do not impair the objectives of the present invention. Raw materials include iron ore, reduced iron, and scrap. Examples of unavoidable impurities include Pb, Zn, Sn, As, B, Sb, Bi, Co, H, O, and REM.

[0036] Next, the structure of the high strength steel sheet of the present invention will be described.

[0037] At the center of the plate thickness, bainite is 80% or more in area ratio. Bainite is an important structure for achieving both strength and low-temperature toughness. Bainite also effectively contributes to improving the strength of steel sheets by strengthening the transformation structure. Furthermore, a uniform structure is necessary for increasing the initial dislocation density and for improving the strength of high-strength steel, particularly in the intermediate temperature range. Therefore, the structure of the high-strength steel of the present invention must be predominantly bainite. Specifically, the area fraction of bainite relative to the entire steel structure at the center of the plate thickness must be 80% or more. The area fraction of bainite is preferably 83% or more, more preferably 85% or more, even more preferably 88% or more, and most preferably 90% or more. While there are no particular limitations on the upper limit of the bainite fraction, from the viewpoint of improving deformability, the area fraction of bainite is preferably 98% or less, more preferably 95% or less.

[0038] Ferrite: area ratio 10% or less (optimal conditions) When ferrite is processed by rolling to become processed ferrite, the number of mobile dislocations increases, which can increase strength. On the other hand, the dislocation density of the ferrite phase is significantly reduced by heat treatment in the medium temperature range, which softens the ferrite phase and may reduce the strength of the steel sheet after heat treatment. Therefore, it is preferable that the area fraction of ferrite at the center of the sheet thickness is 10% or less. The area fraction of ferrite is more preferably 9% or less, even more preferably 8% or less, and most preferably 7% or less. There is no particular lower limit for the area fraction of ferrite, but from the viewpoint of improving deformability, the area fraction of ferrite is preferably 1% or more.

[0039] Island martensite (MA): Area ratio 10% or less (optimal condition) Island martensite (MA: martensite-austenite constituent) is a very hard phase and may act as a fracture initiation site, thereby reducing the low-temperature toughness of the steel sheet. Therefore, the area fraction of island martensite (MA) at the center of the sheet thickness is preferably 10% or less. The area fraction of island martensite is more preferably 8% or less, even more preferably 5% or less, and most preferably 3% or less. There is no particular lower limit for the area fraction of island martensite, but an area fraction of 1% or more is preferred, and 2% or more is more preferred. As described above, the steel structure of the steel plate serving as the base material must basically be composed of the above-mentioned bainite, but examples of the remaining structure other than bainite, ferrite, and island martensite (MA) include pearlite, martensite, cementite, and retained austenite.

[0040] The average grain size of bainite is 25 μm or less, and the minimum grain size of the top 20% of the largest grains of bainite is 60 μm or less Since bainite grain boundaries act as a resistance to brittle crack propagation, grain refinement contributes to improved low-temperature toughness. Therefore, the average grain size of all bainite is set to 25 μm or less. The average grain size of all bainite is preferably set to 23 μm or less, more preferably 20 μm or less, even more preferably 19 μm or less, and most preferably 18 μm or less. There is no particular lower limit, but a value of 5 μm or more is preferred, and a value of 6 μm or more is more preferred.

[0041] Furthermore, while reducing the average grain size improves low-temperature toughness, there is a limit to how much the average grain size can be refined when cooling is initiated near the Ar3 temperature. In the present invention, it is essential to suppress the formation of coarse grains. Coarse bainite is likely to become the origin of fracture, and if the top 20% of the largest grains among all bainite grains have a large grain size, low-temperature toughness deteriorates. In particular, if the minimum grain size of the top 20% of the largest grains exceeds 60 μm, they are likely to become the origin of fracture. Therefore, the minimum grain size of the top 20% of the largest grains at the center of the plate thickness must be 60 μm or less, preferably 55 μm or less, more preferably 50 μm or less, even more preferably 45 μm or less, and most preferably 40 μm or less. There is no particular lower limit, but 10 μm or more is preferred, and 15 μm or more is more preferred.

[0042] Here, the average grain size of bainite is determined as follows. That is, after mirror polishing the L cross section of the steel plate (a cross section parallel to the rolling direction and parallel to the normal direction of the rolling surface), a randomly selected 1 mm × 1 mm 2 The crystal orientation of this region was measured using electron backscatter diffraction (EBSD), and regions where the angle difference between adjacent pixels was 15° or more were determined to be grain boundaries through image analysis. The EBSD measurement conditions were an acceleration voltage of 17 kV and a measurement pitch of 0.8 μm. The average grain size darea (μm) is calculated by dividing the area ai (μm 2) and the circle equivalent diameter di (μm) of each crystal grain, calculated using the following formula. darea(μm)=Σ(ai·di) / Σai In addition, the minimum grain size of the top 20% of bainite grains by grain size represents the grain size of the smallest grain when the circle equivalent diameter of each grain is arranged in descending order of grain size and 20% of the total number of grains is selected from the largest.

[0043] The total of Ti-based precipitates with a diameter of 100 nm or less, Nb-based precipitates with a diameter of 100 nm or less, V-based precipitates with a diameter of 100 nm or less, Mo-based precipitates with a diameter of 100 nm or less, Cr-based precipitates with a diameter of 100 nm or less, Al-based precipitates with a diameter of 100 nm or less, and composite precipitates with a diameter of 100 nm or less consisting of two or more elements selected from Ti, Nb, V, Mo, Cr, and Al, is within an inspection area of ​​1 mm. 2 50,000 to 1,000,000 particles per unit (optimal conditions) The reasons for limiting the sizes and total number of Ti-based precipitates, Nb-based precipitates, V-based precipitates, Mo-based precipitates, Cr-based precipitates, Al-based precipitates, and composite precipitates containing two or more elements of Ti, Nb, V, Mo, Cr, and Al (for example, one or more of Ti-Nb-based precipitates, Ti-V-based precipitates, Cr-Mo-based precipitates, Cr-Mo-Nb-based precipitates, Al-Ti-based precipitates, Al-Nb-based precipitates, and Al-Ti-Nb-based precipitates) in the steel of the present invention will be explained below. Even if precipitates with a diameter of 100 nm or less (fine precipitates) are formed, the number of precipitates is 1 mm 2 If the number of precipitates is less than 50,000 per mm, the effect of suppressing austenite grain growth is weak, and the decrease in strength at medium temperatures may not be suppressed. 2 It is preferable that the number of precipitates per mm is 50,000 or more, more preferably 80,000 or more, even more preferably 100,000 or more, and most preferably 130,000 or more. 2If the number of particles exceeds 1,000,000 per mm, the fine precipitates will aggregate and coarsen, which may weaken the effect of inhibiting austenite grain growth and result in a significant decrease in strength in the medium temperature range. 2 It is preferable that the number of molecules per molecule is 1,000,000 or less, more preferably 950,000 or less, even more preferably 900,000 or less, and most preferably 850,000 or less.

[0044] Here, we will explain how to measure the density and size (diameter) of precipitates. The corroded surface at any point on the head cross section is observed using a scanning electron microscope (SEM), or an extracted replica sample or thin film sample is prepared and observed using a transmission electron microscope (TEM). The number of precipitates of 100 nm or less is counted by measuring at least 100 μm. 2 For example, when observing at a magnification of 100,000 times with one field of view being 2000nm x 2000nm, the observation area per field of view is 4μm 2 Therefore, 25 visual fields are randomly observed. The measurement results are converted into the number per unit area. When the number of precipitates of 100 nm or less is 25 visual fields (100 μm 2 ) and there are 5 precipitates, the density of the precipitates is 1 mm 2 The density of the precipitates can be calculated as 50,000 particles per 1 mm2 of the test area. 2 The diameter of a precipitate is the average value of its major axis (long side) and minor axis (short side).

[0045] Next, the strength properties of the high-strength steel sheet of the present invention will be described. The high-strength steel sheet of the present invention has a yield strength of 555 MPa or more before and after aging under the condition of Larson Miller Parameter LMP=15700. Aging treatment for LMP=15700 is performed under the conditions of heat treatment temperature and heat treatment time such that the LMP, expressed by the following formula (2), becomes 15700. LMP=15700 corresponds to the condition of heat treatment at 350°C, which is in the medium temperature range, for 20 years. The condition for Parameter (LMP) = 15700 is Larson Miller This means that the Parameter (LMP) is greater than or equal to 15650 and less than 15750. LMP=(T+273)×(20+log(t))···(2) T: Heat treatment temperature (℃) t: Heat treatment time (hours) The high-strength steel sheet of the present invention has a yield strength of 555 MPa or more before and after the aging treatment. A yield strength of 555 MPa or more has the effect of enabling stable operation as a steel pipe for steam piping. Here, the yield strength means the yield strength measured in a high-temperature tensile test at 350°C. The yield strength before and after the aging treatment is preferably 560 MPa or more, more preferably 565 MPa or more, even more preferably 570 MPa or more, and most preferably 575 MPa or more. Although there is no particular upper limit, the yield strength is preferably 840 MPa or less. As described in the Examples, in the present invention, the test specimens taken from both the steel plate and the steel pipe have a yield strength of 555 MPa or more both before and after the aging treatment.

[0046] The difference in yield strength before and after aging, calculated by subtracting the yield strength after aging from the yield strength before aging, is 50 MPa or less (suitable condition) In the present invention, it is preferable that the difference between the yield strength at 350°C measured after aging under the condition of Larson Miller Parameter (LMP) = 15700 and the yield strength at 350°C measured before aging satisfies the relationship of 50 MPa or less. The yield strength before and after aging is an index for evaluating the decrease in yield strength when held for a long time in the intermediate temperature range. If this difference is 50 MPa or less, the decrease in yield strength after long-term holding in the intermediate temperature range is within a range that is practically acceptable. If this difference is greater than 50 MPa, a significant decrease in yield strength occurs after long-term holding in the intermediate temperature range. Therefore, it is preferable that the difference in yield strength before and after aging be 50 MPa or less. The difference in yield strength before and after aging is more preferably 45 MPa or less, even more preferably 40 MPa or less, and most preferably 35 MPa or less. The lower limit is not particularly limited and may be a negative value, such as -100 MPa or more. Moreover, the aging treatment conditions under the above conditions include, for example, heat treatment at 400° C. for 2335 hours.

[0047] The toughness of the steel plate is 85% or more of the ductile fracture surface area obtained by DWTT at -40°C. The toughness of the steel plate of the present invention is such that the ductile fracture surface area (DWTSA-40°C) obtained by DWTT (test temperature: -40°C) in accordance with API 5L is 85% or more. If the ductile fracture surface area is less than 85%, the steel plate is prone to brittle fracture at low temperatures, making it difficult to use in year-round installation work, including winter when temperatures drop below 0°C, or in areas with extremely low ambient temperatures. Therefore, the ductile fracture surface area must be 85% or more. Furthermore, a ductile fracture surface area of ​​85% or more obtained by DWTT at -40°C means that the fracture transition temperature is -40°C or less. The ductile fracture surface area is preferably 86% or more, more preferably 87% or more, even more preferably 88% or more, and most preferably 89% or more. The reason for using the -40°C test temperature in the DWTT is to anticipate a decrease in toughness due to work hardening during pipemaking. The upper limit of the ductile fracture area ratio obtained by DWTT at -40°C is not particularly limited, and may be 100% or less.

[0048] <Steel pipe> The steel pipe of the present invention is manufactured using the high-strength steel plate of the present invention, and therefore has the strength characteristics and low-temperature toughness required for a high-strength welded steel pipe for steam transportation, even when it has a large diameter.

[0049] "Large diameter" means that the outer diameter (diameter) of the steel pipe is 400 mm or more. The outer diameter of the steel pipe is preferably 500 mm or more, more preferably 600 mm or more, and even more preferably 700 mm or more. There are no particular restrictions on the maximum outer diameter, but it may be 1500 mm or less, and more preferably 1400 mm or less. According to the present invention, the diameter can be increased while maintaining the strength characteristics required for high-strength welded steel pipes for steam transportation.

[0050] The thickness of the steel pipe is not particularly limited, but is 12 to 30 mm when used for transporting steam. That is, the thickness of the steel pipe is preferably 12 mm or more, more preferably 13 mm or more, even more preferably 14 mm or more, and most preferably 15 mm or more. The thickness of the steel pipe is preferably 30 mm or less, more preferably 29 mm or less, even more preferably 28 mm or less, and most preferably 27 mm or less.

[0051] The strength properties required for high-strength welded steel pipe for steam transportation are, like the above-mentioned high-strength steel, a yield strength of 555 MPa or more at 350°C before and after aging under the condition of Larson Miller Parameter (LMP) = 15700. The yield strength is preferably 560 MPa or more, more preferably 565 MPa or more, even more preferably 570 MPa or more, and most preferably 575 MPa or more. Although there is no particular upper limit, the yield strength at 350°C before and after the aging is preferably 840 MPa or less, and more preferably 830 MPa or less.

[0052] <Method of manufacturing high-strength steel sheets> Next, a method for manufacturing a high-strength steel plate according to the present invention will be described. The method for manufacturing a high-strength steel plate according to the present invention includes a heating step, a hot rolling step, and an accelerated cooling step. Each step will be described below. In the following description, unless otherwise specified, the temperature refers to the average temperature in the thickness direction. The average temperature in the thickness direction is determined by simulation calculations or the like using parameters such as the plate thickness, cooling conditions, and heat transfer coefficient from the surface temperature of the slab or steel plate. For example, the average temperature in the thickness direction can be determined by calculating the temperature distribution in the thickness direction using a finite difference method. The average temperature in the thickness direction is the average value of each temperature in the thickness direction from the surface of the steel plate to the surface of the opposite steel plate. The cooling rate is the average cooling rate obtained by dividing the temperature difference required for cooling to the cooling stop (end) temperature after the end of hot rolling by the time required for that cooling.

[0053] heating process In the method for producing a high-strength steel plate of the present invention, the heating step is a step of heating a steel material to 1000 to 1200°C. Here, the steel material is, for example, a slab obtained by casting molten steel. Since the chemical composition of the steel material will be the chemical composition of the high-strength steel plate, the chemical composition of the high-strength steel plate can be adjusted at the stage of adjusting the chemical composition of the molten steel. Note that there are no particular limitations on the steelmaking method for the steel material.

[0054] In the hot rolling process described below, in order to sufficiently promote austenitization and solid solution of carbides and obtain sufficient strength at room temperature and in the intermediate temperature range, the heating temperature of the steel material must be 1000°C or higher. The heating temperature is preferably 1010°C or higher, more preferably 1020°C or higher, even more preferably 1030°C or higher, and most preferably 1040°C or higher. On the other hand, if the heating temperature exceeds 1200°C, austenite grain growth becomes significant and the toughness of the base material deteriorates. Therefore, the heating temperature is set to 1200°C or lower. The heating temperature is also preferably 1190°C or lower, more preferably 1180°C or lower, even more preferably 1170°C or lower, and most preferably 1160°C or lower.

[0055] Hot rolling process In the method for producing a high-strength steel sheet of the present invention, the hot rolling step is a step in which the steel material heated in the heating step is hot-rolled under conditions including one or more passes of rolling at 950°C or higher with a reduction rate per pass of 10% or more and one or more passes of rolling at 900°C or lower with a reduction rate per pass of 15% or more, a cumulative reduction rate at 900°C or lower of 50% or more, and an end-of-rolling temperature of the Ar3 temperature or higher and 850°C or lower.

[0056] The upper limit of the austenite non-recrystallization temperature range rises to around 900°C due to the inclusion of Nb. By rolling in the austenite non-recrystallization temperature range of 900°C or less, the austenite grains elongate, becoming finer in the thickness and width directions, and the dislocation density within the grains introduced by rolling increases. Therefore, the cumulative reduction at 900°C or less is set to 50% or more. The cumulative reduction at 900°C or less is preferably 55% or more, more preferably 60% or more, even more preferably 65% ​​or more, and most preferably 70% or more. There are no particular limitations on the upper limit of the cumulative reduction, but it is preferably 95% or less, more preferably 90% or less, and even more preferably 85% or less.

[0057] Furthermore, this effect is significantly enhanced by setting the cumulative reduction rate at 900°C or less to 50% or more and the rolling end temperature to be between the Ar3 temperature and 850°C or less. Furthermore, the strength, particularly in the intermediate temperature range, is improved in high-strength steel plates manufactured by hot rolling and cooling (described later) and in steel pipes made from such high-strength steel plates. The Ar3 temperature can be calculated using the following formula: Ar3(℃)=910-310[C]-80[Mn]-20[Cu]-55[Ni]-15[Cr]-80[Mo] In the formula, [C], [Mn], [Cu], [Ni], [Cr], and [Mo] respectively refer to the content (mass%) of C, Mn, Cu, Ni, Cr, and Mo in the base steel plate (high-strength steel plate). When some elements are not contained in the base steel plate, the content of the element can be set to "0" to determine the Ar3 temperature. Although there is no particular upper limit to the cumulative reduction rate, an excessively large cumulative reduction rate may place an excessive load on the rolling mill, so the cumulative reduction rate is preferably 90% or less.

[0058] Furthermore, by including at least one rolling pass at 950°C or higher with a reduction rate of 10% or more per pass, austenite grains are refined by recrystallization, thereby improving the low-temperature toughness of the steel. Therefore, it is necessary to include at least one rolling pass at 950°C or higher with a reduction rate of 10% or more per pass. It is also preferable to include at least two rolling passes at 950°C or higher with a reduction rate of 10% or more per pass. It is more preferable to include at least three rolling passes at 950°C or higher with a reduction rate of 10% or more per pass, and even more preferable to include at least four rolling passes. The upper limit of the number of rolling passes at a reduction rate of 10% or more per pass is not particularly limited, but may be 50 passes or less, more preferably 30 passes or less, and even more preferably 15 passes or less. The upper limit of the rolling reduction rate per pass at 950°C or higher is not particularly limited, but is preferably 40% or less.

[0059] Furthermore, 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, thereby promoting the refinement of austenite grains and improving the low-temperature toughness of the steel. Therefore, it is necessary to include at least one rolling pass at 900°C or below with a reduction rate of 15% or more per pass. Furthermore, it is preferable to include at least two rolling passes at 900°C or below with a reduction rate of 15% or more per pass. It is more preferable to include at least three rolling passes at 900°C or below with a reduction rate of 15% or more per pass, and even more preferable to include at least four rolling passes. Note that the upper limit of the number of rolling passes at a reduction rate of 15% or more per pass is not particularly limited, but may be 50 passes or less, more preferably 30 passes or less, and even more preferably 15 passes or less. Furthermore, the upper limit of the rolling reduction rate per pass at 900°C or below is not particularly limited, but is preferably 40% or less.

[0060] If the cumulative reduction rate at 900°C or less is less than 50% or the rolling end temperature exceeds 850°C, austenite grains are not sufficiently refined, resulting in poor low-temperature toughness. Furthermore, the intragranular dislocation density decreases, resulting in poor strength in the intermediate temperature range. Furthermore, if the rolling end temperature is less than the Ar3 temperature, a structure containing ferrite and bainite is formed, resulting in poor strength in the intermediate temperature range. Therefore, the cumulative reduction rate at 900°C or less is set to 50% or more, and the rolling end temperature is set to be Ar3 temperature or higher and 850°C or lower. The rolling end temperature is set to be Ar3 temperature or higher, preferably Ar3 temperature + 5°C or higher, more preferably Ar3 temperature + 10°C or higher, and even more preferably Ar3 temperature + 15°C or higher. Furthermore, the rolling end temperature is set to be 850°C or lower, preferably 840°C or lower, more preferably 830°C or lower, and even more preferably 820°C or lower.

[0061] Accelerated cooling process In the method for producing a high-strength steel sheet according to the present invention, the accelerated cooling step is a step of cooling a hot-rolled steel sheet (hot-rolled sheet) obtained in the hot rolling step at a cooling start temperature of Ar3 temperature or higher and an average cooling rate of 5 ° C. / s or higher. This is a process of accelerated cooling under the condition that the cooling stop temperature is 300 to 550° C. In this case, the average cooling rate refers to the average cooling rate from the cooling start temperature to the cooling stop temperature, and means the cooling rate obtained by dividing the difference between the cooling start temperature and the cooling stop temperature by the time required from the start of cooling to the stop of cooling.

[0062] In order to suppress the formation of ferrite on the front and back surfaces of the steel sheet and increase the bainite fraction, the cooling start temperature is set to the Ar3 temperature or higher. The cooling start temperature is preferably set to the Ar3 temperature + 10°C or higher, more preferably the Ar3 temperature + 15°C or higher, even more preferably the Ar3 temperature + 20°C or higher, and most preferably the Ar3 temperature + 25°C or higher. In order to prevent deterioration of low-temperature toughness due to an increase in the average crystal grain size, the upper limit is preferably set to the Ar3 temperature + 150°C or lower, more preferably the Ar3 temperature + 140°C or lower, and even more preferably the Ar3 temperature + 130°C or lower.

[0063] The strength of high-strength steel plates tends to increase with increasing average cooling rate during accelerated cooling. When the average cooling rate during accelerated cooling is less than 5°C / s, transformation begins at high temperatures, resulting in the formation of ferrite and pearlite in addition to bainite, and dislocation recovery also occurs during cooling. Therefore, when the average cooling rate is less than 5°C / s, sufficient strength cannot be obtained at room temperature or in the intermediate temperature range. Furthermore, when the average cooling rate is less than 5°C / s, the effect of refining the structure is reduced, the grain size does not decrease, and low-temperature toughness deteriorates. Therefore, the average cooling rate during accelerated cooling is set to 5°C / s or more. Furthermore, the average cooling rate during accelerated cooling is preferably 8°C / s or more, more preferably 10°C / s or more, even more preferably 15°C / s or more, and most preferably 20°C / s or more. While there is no particular upper limit to the cooling rate, to avoid an excessive increase in the martensite fraction, the average cooling rate is preferably 80°C / s or less, and more preferably 50°C / s or less.

[0064] The strength of steel sheets tends to increase as the cooling stop temperature of accelerated cooling decreases. If the cooling stop temperature of accelerated cooling exceeds 550°C, carbide growth is promoted and the amount of solute carbon is reduced. As a result, the high-strength steel after cooling and the steel pipe made from that high-strength steel do not achieve sufficient strength, especially in the mid-temperature range. For this reason, the cooling stop temperature of accelerated cooling is set to 550°C or lower. It is preferably set to 540°C or lower, more preferably set to 530°C or lower, and even more preferably set to 520°C or lower. On the other hand, if the cooling stop temperature is lower than 300°C, the formation of low-temperature transformation products such as martensite, which has many mobile dislocations, becomes significant. As a result, long-term aging in the mid-temperature range promotes dislocation recovery, significantly reducing strength. For this reason, the cooling stop temperature of accelerated cooling is set to 300°C or higher. It is preferably set to 310°C or higher, more preferably set to 320°C or higher, even more preferably set to 330°C or higher, and most preferably set to 340°C or higher.

[0065] <Steel pipe manufacturing method> The method for producing a steel pipe of the present invention includes a cold forming step and a welding step.

[0066] Cold forming process The cold forming step is a step of cold forming the high-strength steel plate of the present invention into a tubular shape. When manufacturing a steel pipe for transporting steam, the thickness of the steel plate is preferably 12 mm or more. Also, it is preferably 30 mm or less. More preferred ranges are the same as the thickness of the steel pipe described above. The method for cold forming the steel plate into a tubular shape is not particularly limited. Examples of forming methods include UOE forming, press bending, and roll forming.

[0067] Welding process The welding process is a process of welding the butt joints where the ends (one end and the other end) of the steel plates formed into a tubular shape in the cold forming process are butted together. The welding method is not particularly limited, but submerged arc welding or the like may be used for welding. It is preferable to expand the steel pipe after welding, as this improves the roundness of the pipe cross section. Heat treatment after steel pipe production may be performed according to the desired properties, and is not particularly specified. [Example]

[0068] Steel plates (15-25 mm thick) manufactured under the manufacturing conditions shown in Tables 2-1 and 2-2 using steels A to AH with the chemical compositions shown in Table 1 were cold-formed, seam-welded, and then expanded by 1.0% to produce steel pipes with an outer diameter of 610 mm and a thickness of 15-25 mm. Regarding the manufacturing conditions shown in Tables 2-1 and 2-2, "reduction rate" refers to the cumulative reduction rate at 900°C or less, "FT" refers to the rolling finish temperature, and "heat treatment" refers to long-term aging treatment. A 1.0% expansion rate means that the inner diameter of the steel pipe was expanded by 1.0% in the radial direction from the inner surface to the outer surface using a pipe expander. Microstructure observation of the steel plate, evaluation of low-temperature toughness, tensile tests before and after aging, and tensile tests of the steel pipe before and after aging were carried out by the methods described below.

[0069] Observation of the microstructure of steel sheets A sample for observing the steel structure was taken from the center of the sheet width of the steel sheet manufactured as described above, and the L-section of the steel sheet (a cross section parallel to the rolling direction and parallel to the normal direction of the rolling surface) was mirror-polished.The crystal orientation of a randomly selected 1 mm x 1 mm area at the center of the sheet thickness was then measured using electron backscatter diffraction (EBSD).Image analysis was performed to determine the grain size of bainite, with areas where the angle difference between adjacent pixels was 15° or more being considered as grain boundaries. The average grain size darea (μm) is calculated by dividing the area ai (μm 2 ) and the circle equivalent diameter di (μm) of each crystal grain, calculated using the following formula. darea(μm)=Σ(ai·di) / Σai The top 20% grain size of bainite represents the grain size of the grains that are 20% from the largest when the circle equivalent diameters of each grain are arranged in order of grain size and 20% of the total number of grains are selected from the largest. In addition, the L-section of the steel sheet (a cross section parallel to the rolling direction and normal to the rolling surface) was mirror-polished and then subjected to nital etching to reveal the microstructure. -2 mm 2 The steel structure was photographed in five fields of view (magnification: 400x), and the bainite fraction, ferrite fraction, and island martensite fraction in the photographed fields were measured as area fractions using an image analyzer (Fiji), and the bainite grain size was measured using an image analyzer (TSL OIM Analysis). The average values ​​of the five fields of view were used as the bainite fraction, ferrite fraction, island martensite fraction, and bainite grain size.

[0070] The number of precipitates was determined using the evaluation method used in the embodiment, as described above.

[0071] Low temperature toughness evaluation of steel plates In addition, DWTT conforming to API 5L was performed to evaluate the low-temperature toughness of the steel plates. DWTT test specimens were taken so that the thickness direction of the steel plate coincided with the thickness direction of the test specimen and the width direction of the steel plate coincided with the longitudinal direction of the test specimen. A ductile fracture area ratio of 85% or more at a test temperature of -40°C and a fracture transition temperature of -40°C or less were evaluated as good.

[0072] Tensile tests of steel sheets before and after aging For the steel sheet properties (before aging), tensile test pieces were taken so that the longitudinal direction of the tensile test piece was perpendicular to the rolling direction of the steel sheet and perpendicular to the sheet thickness direction, and a tensile test was carried out at 350°C to determine the yield strength. In the tensile test, a round bar test piece with a diameter of 6 mm was used, and the crosshead speed was 0.15 mm / min. A yield strength of 555 MPa or more in the tensile test at 350°C was evaluated as good. did.

[0073] To evaluate the properties after long-term aging (heat treatment at 400°C for 2335 hours), tensile test specimens were taken so that the direction perpendicular to the rolling direction of the steel sheet and perpendicular to the thickness direction was the longitudinal direction of the test specimen, and heat treatment was carried out at 400°C for 2335 hours in a N (nitrogen) atmosphere furnace, followed by a tensile test at 350°C to evaluate the yield strength. The tensile test was carried out using a round bar test specimen with a diameter of 6 mm at a crosshead speed of 0.15 mm / min. If the yield strength in the tensile test at 350°C was 555 MPa or more, was rated as good.

[0074] Furthermore, the change in yield strength when held in the medium temperature range for a long time was evaluated by measuring the difference in yield strength before and after the long-term aging, that is, (yield strength before aging - yield strength after aging).

[0075] In addition, to evaluate the properties after long-term aging (heat treatment at 400°C for 2335 hours), 6 mm diameter round bar test specimens were also taken from steel pipes, with the circumferential direction of the pipe aligned with the longitudinal direction of the test specimen. These were then heat treated in a furnace at 400°C for 2335 hours in an N (nitrogen) atmosphere furnace, and tensile tests were conducted at 350°C. The yield strength was evaluated in the same way as for steel plates, and a yield strength of 555 MPa or more at 350°C was evaluated as good.

[0076] As mentioned above, Tables 2-1 and 2-2 show the manufacturing conditions of the steel plates and the test results of the steel plates and steel pipes. In the invention examples (Nos. 1 to 29) in which the chemical composition and steel plate manufacturing conditions are both within the ranges of the invention, the steel plates and steel pipes have a yield strength of 555 MPa or more at 350°C, and the steel plates and steel pipes also have a yield strength of 555 MPa or more at 350°C after long-term aging, with (yield strength before aging - yield strength after aging) being 50 MPa or less, and the ductile fracture area ratio (DWTTSA-40°C) obtained by DWTT (test temperature: -40°C) in accordance with API 5L being 85% or more, and a fracture transition temperature being -40°C or less.

[0077] On the other hand, the comparative examples (Nos. 30 to 59) whose chemical compositions or steel sheet manufacturing conditions were outside the scope of the present invention were inferior to the examples of the present invention in at least one of the following: a yield strength at 350°C and / or a yield strength at 350°C before and after long-term aging of 555 MPa or more, a ductile fracture area ratio obtained by DWTT (test temperature: -40°C) of 85% or more, and a fracture transition temperature of -40°C or less.

[0078] [Table 1]

[0079] [Table 2-1]

[0080] [Table 2-2]

Claims

1. The component composition is, in mass%, C: 0.04-0.09%, Si: 0.03-0.25%, Mn: 1.5-2.5%, P: 0.020% or less, S: 0.002% or less, Mo: 0.10-0.50%, Nb: 0.010-0.055%, Ti: 0.005 to 0.020%, Ca: 0.0040% or less, Al: 0.01-0.04%, N: 0.006% or less, the balance being Fe and unavoidable impurities; X (%) represented by formula (1) is 0.65% or more, the microstructure has bainite in an area ratio of 80% or more at the center of the plate thickness, the average grain size of the bainite is 25 μm or less, and the minimum grain size of the top 20% of the bainite grains with the largest grain size is 60 μm or less, the toughness of the steel plate is such that the ductile fracture area ratio obtained by DWTT at -40°C is 85% or more, A high-strength steel plate having a yield strength of 555 MPa or more before and after aging under the condition of Larson Miller Parameter LMP = 15700, as defined by the following formula (2). X (%) = 0.35Cr + 0.9Mo + 12.5Nb + 8V... (1) The element symbols in formula (1) represent the content (mass%) of each element. Elements that are not contained are substituted with 0. LMP=(T+273)×(20+log(t))...(2) T: Heat treatment temperature (°C) t: heat treatment time (hours)

2. The component composition is further expressed in mass % as follows: Cr: 0.50% or less, V: 0.070% or less, Cu: 0.50% or less, 2. The high-strength steel plate according to claim 1, further comprising one or more of the following: Ni: 0.50% or less.

3. Furthermore, the sheet has an area ratio of ferrite of 10% or less and an area ratio of island martensite of 10% or less at the center of the sheet thickness, At the center of the sheet thickness, the total of Ti-based precipitates with a diameter of 100 nm or less, Nb-based precipitates with a diameter of 100 nm or less, V-based precipitates with a diameter of 100 nm or less, Mo-based precipitates with a diameter of 100 nm or less, Cr-based precipitates with a diameter of 100 nm or less, Al-based precipitates with a diameter of 100 nm or less, and composite precipitates with a diameter of 100 nm or less containing two or more elements selected from Ti, Nb, V, Mo, Cr, and Al is within an inspection area of ​​1 mm ２ There are 50,000 to 1,000,000 per 2. The high-strength steel plate according to claim 1, wherein a difference in yield strength before and after aging, obtained by subtracting the yield strength after aging from the yield strength before aging, is 50 MPa or less.

4. Furthermore, the sheet has an area ratio of ferrite of 10% or less and an area ratio of island martensite of 10% or less at the center of the sheet thickness, At the center of the sheet thickness, a total of 50,000 to 1,000,000 Ti-based precipitates with a diameter of 100 nm or less, Nb-based precipitates with a diameter of 100 nm or less, V-based precipitates with a diameter of 100 nm or less, Mo-based precipitates with a diameter of 100 nm or less, Cr-based precipitates with a diameter of 100 nm or less, Al-based precipitates with a diameter of 100 nm or less, and composite precipitates with a diameter of 100 nm or less containing two or more elements selected from Ti, Nb, V, Mo, Cr, and Al are present per mm2 of the test area, 3. The high-strength steel plate according to claim 2, wherein a difference in yield strength before and after aging, obtained by subtracting the yield strength after aging from the yield strength before aging, is 50 MPa or less.

5. A steel pipe using the high strength steel plate according to any one of claims 1 to 4.

6. The method for producing a high-strength steel plate according to any one of claims 1 to 4, comprising: a heating step of heating a steel material to 1000 to 1200 ° 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 at 900°C or lower is 50% or more, and the rolling end temperature is Ar. ３ a hot rolling process of hot rolling under conditions of a temperature of 850°C or higher and 850°C or lower; The hot-rolled steel sheet obtained in the hot rolling step is cooled to a temperature of Ar ３ and an accelerated cooling step of performing accelerated cooling under the conditions of a temperature or higher, an average cooling rate of 5°C / s or higher, and a cooling stop temperature of 300 to 550°C.

7. a cold forming step of cold forming the high strength steel plate according to any one of claims 1 to 4 into a tubular shape; a welding process for welding the butt joints at the butt joints of the ends of the steel plates formed into a tubular shape in the cold forming process; A method for manufacturing a steel pipe having the above structure.

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