Steel sheet for liquid and / or supercritical co2 transport line pipe and method for producing same

Abstract

Provided is a steel sheet for a CO2 transport line pipe that is suitably provided to a pipeline for transporting a liquid and / or supercritical state CO2. A steel sheet according to the present invention has: a component composition that contains, in mass%, 0.03-0.08% of C, 1.50-2.50% of Mn, 0.030% or less of P, 0.0100% or less of S, 0.0100% or less of N, 0.010-0.200% of Al, 0.10-1.00% of Si, and 0.0005-0.0050% of O, that further contains a prescribed amount of one or more selected from Cu, Ni, Cr, Mo, and W, that exhibits a [Si] / [O] value of 20-900, and in which the remaining portion comprises Fe and unavoidable impurities; a micro structure in which the area ratio of bainite is not less than 90% and the area ratio of island martensite is less than 3%; a strength where the tensile strength is not less than 625 MPa; and a toughness in which the Charpy impact absorption energy at -40°C is not less than 320 J.

Description

Steel plate for liquid and / or supercritical CO₂ transport pipeline and method for producing the same

[0001] The present invention relates to a steel plate for a pipeline for transporting liquid and / or supercritical CO ２ and a method for producing the same, which are suitable for use in a pipeline for transporting CO ２ .

[0002] Reduction of emissions of greenhouse gases such as carbon dioxide (hereinafter referred to as "CO ２ ") has become an important global issue as one of the measures against global warming. For example, in thermal power generation using fossil fuels such as oil and coal as energy, a large amount of CO ２ is emitted. Therefore, a shift to green power generation using renewable energy such as wind power and solar power is being promoted mainly in developed countries. On the other hand, thermal power generation that can generate electricity stably regardless of the weather is one of the power sources necessary to achieve a stable energy supply. Therefore, various measures have been taken to reduce the CO ２ emissions in thermal power generation. One of them is carbon dioxide capture and storage (CCS).

[0003] CCS is a technology for separating and collecting CO ２ emitted from power plants, chemical plants, etc. from other gases, transporting it deep underground, and storing and injecting it. In the United States, as part of CCS, a technology (Enhanced Oil Recovery: EOR) for transporting natural CO ２ collected from CO ２ fields and chemical plants by pipeline and injecting it into old oil fields to recover the crude oil remaining in the oil fields has already been carried out on a full scale. For the practical application of these technologies, the laying of a pipeline capable of safely transporting CO ２ in a high-density liquid and / or supercritical state is essential. For pipelines, welded steel pipes are usually used, which are manufactured by forming a thick steel plate into a substantially cylindrical shape and then performing welding.

[0004] CO ２Steel pipes used for laying transport pipelines require specifications from a multifaceted perspective, as described below. First, from the perspective of reducing the costs required for laying long-distance pipelines, it is not practical to use expensive steel pipes made of austenitic stainless steel, which has excellent corrosion resistance. Therefore, the steel pipes must not have high component costs. Second, CO2 from exhaust gases of thermal power plants is a source of CO2. ２ It contains sulfur oxides (hereinafter referred to as "SOx"), nitrogen oxides (hereinafter referred to as "NOx"), and water (hereinafter referred to as "H") ２ It contains impurities such as "O" and these impurities promote corrosion of steel pipes. Therefore, steel pipes should contain low-purity CO ２ Corrosion control measures are desired to enable safe transportation. In addition, there is a desire to increase the strength of steel pipes in order to improve transportation efficiency through high pressure and to improve installation work efficiency through thinner walls and lighter weight.

[0005] In addition, to avoid large-scale pipeline failures, it is not enough to suppress only brittle fracture; it is also necessary to suppress ductile fracture, also known as unstable ductile fracture. This unstable ductile fracture is a phenomenon in which fracture propagates in the axial direction of the steel pipe at a speed of 100 m / s or more, which can cause large-scale failures extending for several kilometers. In order to suppress unstable ductile fracture, it is generally considered necessary for the steel pipe to have excellent toughness. Specifically, the Charpy impact energy absorption value of the steel plate in the Charpy impact test must be 320 J or higher.

[0006] Of the aforementioned requirements, supercritical CO ２ Many studies have been conducted on the corrosion behavior of steel materials and corrosion suppression technologies. For example, Non-Patent Literature 1 describes how 2-mercaptobenzothiazole (hereinafter referred to as "MBTH") can reduce CO2 corrosion on the surface of carbon steel. ２ It is effective in suppressing corrosion caused by CO ２ It has been reported that the corrosion inhibition effect of MBTH on steel materials is more pronounced when the material is in a supercritical state compared to when it is not in a supercritical state.

[0007] H. Cen et al., Applied Surface Science, 476, (2019), p422-434

[0008] However, the corrosion prevention technology using MBTH disclosed in Non-Patent Document 1 is CO ２ -H ２ Although it exhibits a comprehensive corrosion-inhibiting effect in O-systems, it has not been demonstrated in corrosive environments containing NOx and SOx, which are impurities expected when using exhaust gas from thermal power plants as a source. Furthermore, MBTH exhibits corrosion protection based on chemical adsorption with the steel surface in specific chemical environments. In other words, once corrosion has progressed and the chemical environment of the steel surface has changed, it is possible that the effect of MBTH may not be fully realized. In addition, CO ２ The need to constantly supply MBTH during transportation results in significant long-term increases in running costs and operational burdens that cannot be ignored.

[0009] Furthermore, without using corrosion inhibitors such as MBTH, the properties of the material itself are as described above. ２ A steel plate made of steel that simultaneously satisfies all the specifications required for steel pipes used in transportation pipelines is not known in conventional technology.

[0010] This invention has been made in view of the above problems, and is a low-alloy steel that is cheaper than austenitic stainless steel, and is free of SOx, NOx, and H ２ Liquids containing impurities such as oxygen and / or supercritical CO ２ It has sufficient corrosion resistance against CO2, and even thin-walled, lightweight line pipes have high strength, allowing them to withstand high pressures, and in addition, it can suppress the occurrence of large-scale pipeline failures due to unstable ductile fracture. ２ The objective is to provide steel plates for transport line pipes and a method for manufacturing the same.

[0011] The inventors conducted various studies to solve the above problems.

[0012] First, the inventors identified SOx, NOx, and H ２ Supercritical CO containing oxygen ２A detailed study of the corrosion phenomenon and mechanism of steel plates under various environmental conditions yielded the following findings: namely, supercritical CO2 ２ In the environment, CO ２ When dissolved water molecule clusters collide with the steel plate surface, they become trapped on the steel plate surface based on their interaction with the surface, and CO2 is released as a trace amount of moisture. ２ CO is released from within. ２ Furthermore, the dissolution of coexisting SOx and NOx causes the moisture to become acidic, leading to overall corrosion of the steel plate surface. This phenomenon continues intermittently, resulting in severe thinning of the steel plate. In addition, while a layer of corrosion products resulting from a certain amount of corrosion acts as a corrosion barrier in that environment and suppresses further corrosion, the intermittent corrosion phenomenon results in uneven formation of corrosion products, leading to the progression of localized corrosion.

[0013] Thus, SOx, NOx, and H ２ Supercritical CO2 containing O ２ Under environmental conditions, there are two corrosion risks: severe general corrosion and localized corrosion, and it is necessary to increase resistance to both of these corrosion phenomena. Therefore, based on the above knowledge, the inventors developed supercritical CO2 ２ We have diligently conducted research toward developing steel plates that exhibit excellent resistance to both general and localized corrosion in the environment.

[0014] As a result, it was found that controlling the Si and O content in the steel sheet to an appropriate level is effective in improving overall corrosion resistance. At the same time, it was found that adding one or more of Cu, Ni, Cr, Mo, and W is important in improving localized corrosion resistance.

[0015] In addition, we conducted thorough research on the influence of manufacturing conditions on unstable ductile fracture of steel sheets for line pipes. As a result, we found that by controlling the cumulative reduction ratio and rolling temperature in the austenite non-recrystallization temperature range, and by setting the cooling stop temperature just above (Ms point - 100°C), it is possible to create a microstructure of the steel sheet that is mainly composed of bainite with island-like martensite reduced as much as possible, thereby obtaining a steel sheet with high strength and high toughness.

[0016] This invention was completed after further consideration based on the above findings. In other words, the gist of this invention is as follows:

[0017] [1] Contains, by mass%, C: 0.03% to 0.08%, Mn: 1.50% to 2.50%, P: 0.030% or less, S: 0.0100% or less, N: 0.0100% or less, Al: 0.010% to 0.200%, Si: 0.10% to 1.00%, and O: 0.0005% to 0.0050%, and further contains one or more selected from Cu: 0.05% to 4.00%, Ni: 0.05% to 4.00%, Cr: 0.05% to 4.00%, Mo: 0.03% to 2.00%, and W: 0.03% to 2.00%, A liquid and / or supercritical CO2 having the following characteristics: [Si] / [O], where [Si] is the mass percent content of Si and [O] is the mass percent content of O, is between 20 and 900, and the remainder consists of Fe and unavoidable impurities; a microstructure in which, at the 1 / 2 position of the plate thickness, the area ratio of bainite is 90% or more and the area ratio of island martensite is less than 3%; a strength of tensile strength of 625 MPa or more; and toughness of Charpy impact absorption energy of 320 J or more at -40°C. ２ Steel plates for transport line pipes.

[0018] [2] The liquid and / or supercritical CO2 described in [1] above, wherein the component composition further contains one or more groups selected from groups A to D below. ２Steel plates for transport line pipes. Group A: One or more types selected from Sn: 0.50% or less, Sb: 0.50% or less, and Co: 0.50% or less by mass%. Group B: One or more types selected from Ca: 0.0100% or less, Mg: 0.0200% or less, and REM: 0.200% or less by mass%. Group C: One or more types selected from Ti: 0.100% or less, Zr: 0.100% or less, Nb: 0.100% or less, and V: 0.100% or less by mass%. Group D: B: 0.0300% or less by mass%.

[0019] [3] The liquid and / or supercritical CO2 described in [1] or [2] above. ２ A process comprising: heating a steel slab having the same component composition as steel plates for transport line pipes to 1000°C or more and 1250°C or less; and thereafter, rolling the steel slab in the austenite recrystallization temperature range and in the austenite non-recrystallization temperature range with a cumulative reduction rate of 60% or more, wherein the rolling completion temperature is (Ar ３ point +50℃) or more (Ar ３ A process of hot rolling at a temperature of 150°C or lower to form a steel sheet, and thereafter, Ar ３ points or more (Ar ３ A process for accelerated cooling of liquid and / or supercritical CO2, comprising: a step of performing accelerated cooling at an average cooling rate of 10°C / s to 80°C / s from a cooling start temperature of (Ms point - 100°C) or less to a cooling stop temperature of (Ms point - 100°C) or more to (Ms point + 100°C) or less, wherein the cooling is performed on liquid and / or supercritical CO2. ２ A method for manufacturing steel plates for transport line pipes.

[0020] According to the present invention, liquid and / or supercritical CO ２ Suitable CO2 for use in pipelines used for transporting ２ Steel plates for transport line pipes can be obtained according to the present invention. ２ Steel plates for transport line pipes are particularly free of SOx, NOx, and H ２ Liquids containing impurities such as oxygen and / or supercritical CO ２ It has excellent corrosion resistance to the environment. In addition, the CO2 according to the present invention ２Steel plates for transport line pipes possess high tensile strength and excellent toughness, which can suppress damage caused by unstable ductile fracture of pipelines, making them extremely beneficial for industry.

[0021] The present invention will be described in detail below.

[0022] [Liquid and / or supercritical CO2] ２ The steel sheet according to the present invention is a liquid and / or supercritical state CO ２ Suitable CO2 for use in pipelines transporting CO2 ２ This is a steel plate for transport line pipes. ２ The inner diameter surface of the steel pipes that make up the transport pipeline is affected by the liquid and / or supercritical CO2 flowing through the pipeline. ２ It comes into contact with. In this specification, "liquid and / or supercritical CO ２ "CO2" refers to CO2 in a state where the pressure is 7.39 MPa or higher and the temperature is -40°C or higher. ２ This refers to CO. ２ CO2 is in a liquid state when the pressure is 7.39 MPa or higher and the temperature is between -40°C and 31°C. Furthermore, it enters a supercritical state when the pressure is 7.39 MPa or higher and the temperature is above the critical point temperature of 31°C. However, CO2 transported in a pipeline... ２ In reality, CO can exist only in a liquid state, only in a supercritical state, or in a mixture of both. Therefore, in this specification, CO can exist in all of these states. ２ The term encompassing "liquid and / or supercritical CO2" is "liquid and / or supercritical CO2". ２ The term "..." is used.

[0023] CO in CCS ２ The gases emitted from thermal power plants, one of the sources of CO2, generally contain more than 95 vol% CO2. ２ It contains gas, as well as SOx at 5000 ppmv or less, NOx at 500 ppmv or less, and H at 1500 ppmv or less. ２ It may contain O, where "ppmv" is parts per million by volume. As mentioned above, liquids and / or supercritical CO2 containing these impurities. ２ On the surface of the steel plate in contact with it, H released from the gas２ O is attached, and this H ２ The dissolution of SOx and NOx in oxygen acidifies the free water. This creates an environment where the surface of the steel plate is susceptible to corrosion, and therefore, special considerations against corrosion are necessary for the steel plate according to the present invention.

[0024] 1. Liquid and / or supercritical CO ２ In one embodiment of a steel plate for transport line pipes, the present invention contains, by mass%, C: 0.03% to 0.08%, Mn: 1.50% to 2.50%, P: 0.030%, S: 0.0100%, N: 0.0100%, Al: 0.010% to 0.200%, Si: 0.10% to 1.00%, and O: 0.0005% to 0.0050%, and further, Cu: 0.05% to 4.00%, Ni: 0.05% to 4.00%, Cr: 0.05% to 4.00%, Mo: 0.03% to 2.00%, and W: 0.03% to 2.0%. A liquid and / or supercritical CO2 having the following characteristics: it contains one or more elements selected from those present in amounts of 0% or less, and the ratio of [Si] to [O] expressed as mass percent, where [Si] is the Si content and [O] is the O content, is [Si] / [O], is between 20 and 900, with the remainder being Fe and unavoidable impurities; it has a microstructure in which, at the 1 / 2 position of the plate thickness, the area ratio of bainite is 90% or more and the area ratio of island martensite is less than 3%; it has a strength of tensile strength of 625 MPa or more and toughness of Charpy impact absorption energy of 320 J or more at -40°C. ２ This invention concerns steel plates for transport line pipes.

[0025] [Component composition] CO of the present invention ２ The component composition of steel plates for transport line pipes will be described below. In this specification, the component composition of steel plates will be expressed in "mass percent," and unless otherwise specified, it will simply be shown as "%."

[0026] C: 0.03% to 0.08% C forms a microstructure mainly composed of bainite after accelerated cooling, and effectively contributes to increased strength through transformation strengthening. However, if the C content is too low, ferrite or pearlite transformation is more likely to occur during cooling, resulting in an insufficient amount of bainite being obtained, and CO ２ In some cases, a tensile strength of 625 MPa or higher, which is desirable for steel plates used in transport line pipes, may not be obtained. Therefore, the carbon content should be 0.03% or more, preferably 0.04% or more. On the other hand, if the carbon content is too high, hard martensite is likely to form after accelerated cooling, which may result in poor toughness. Therefore, the carbon content should be 0.08% or less, preferably 0.07% or less.

[0027] Mn: 1.50% to 2.50% Like carbon, Mn forms a bainite-dominant structure after accelerated cooling, effectively contributing to increased strength through transformation. However, if the Mn content is too low, ferrite or pearlite transformations are more likely to occur during cooling, resulting in an insufficient amount of bainite being obtained, and CO ２ In some cases, a tensile strength of 625 MPa or higher, which is desirable for steel plates used in transport line pipes, may not be obtained. Therefore, it is preferable that the Mn content be 1.50% or more, and 1.55% or more if further improvement in toughness is required. On the other hand, if the Mn content is excessive, Mn becomes concentrated in the segregation areas that are inevitably formed during casting, causing poor toughness in those areas. Therefore, it is preferable that the Mn content be 2.50% or less, and 2.00% or less if further improvement in toughness is required.

[0028] P: 0.030% or less. Since P degrades toughness and weldability, the P content should be 0.030% or less. Preferably, the P content should be 0.025% or less. Since a lower P content is preferable, there is no particular lower limit. However, since it is industrially difficult to reduce the P content to less than 0.001%, it is preferable that the P content be 0.001% or more.

[0029] S: 0.0100% or less. Since sulfur (S) is an element that degrades the toughness and weldability of steel, it is preferable to reduce its content as much as possible. In particular, if the S content exceeds 0.0100%, the deterioration of toughness in the base material and weld becomes significant. For this reason, the S content should be 0.0100% or less. Preferably, the S content should be 0.0080% or less, and more preferably 0.0060% or less. Since a lower S content is preferable, there is no particular lower limit. However, since it is industrially difficult to reduce the S content to less than 0.0001%, it is preferable that the S content be 0.0001% or more.

[0030] N: 0.0100% or less. Since N is a harmful element that reduces toughness, it is preferable to reduce it as much as possible. In particular, if the N content exceeds 0.0100%, the reduction in toughness becomes significant. Therefore, the N content should be 0.0100% or less. Preferably, the N content should be 0.0080% or less, and more preferably 0.0070% or less. Since a lower N content is preferable, there is no particular lower limit. However, since it is industrially difficult to reduce the N content to less than 0.0005%, it is preferable that the N content be 0.0005% or more.

[0031] Al: 0.010% or more and 0.200% or less. Al is added to molten steel as an element that acts as a deoxidizing agent. However, if the Al content is too low, the deoxidation of the molten steel will be insufficient. Therefore, the Al content should be 0.010% or more, preferably 0.020% or more. On the other hand, if the Al content is too high, the toughness of the steel sheet will decrease. Therefore, the Al content should be 0.200% or less. It is preferable that the Al content be 0.080% or less, and more preferably 0.040% or less.

[0032] Si: 0.10% to 1.00% Si is in the liquid and / or supercritical state of CO ２ From the perspective of suppressing total corrosion in the environment, it is a very effective element. That is, Si is a liquid and / or supercritical CO ２ As steel plates corrode in the environment, Si dissolves into the free moisture on the surface of the steel plate. After dissolution, Si reacts with free water to form SiO ２ It forms SiO.２ As this formation occurs, free water is consumed, the surface of the steel plate dries, the progression of the corrosion reaction is suppressed, and the susceptibility of the steel plate to overall corrosion is reduced. If the Si content is too low, this effect will not occur. Therefore, the Si content should be 0.10% or more, preferably 0.12% or more, and more preferably 0.15% or more. On the other hand, if the Si content is too high, it will lead to deterioration of toughness and weldability. Therefore, the Si content should be 1.00% or less, preferably 0.80% or less.

[0033] O: 0.0005% or more and 0.0050% or less. O is the liquid and / or supercritical state of CO due to Si as described above. ２ It is an important element that promotes the effect of suppressing overall corrosion in the environment. Like Si, oxygen in steel plates dissolves into the free moisture on the surface of the steel plate as corrosion occurs. The dissolved Si reacts with free water to form SiO2. ２ The rate at which it forms is not inherently fast, and corrosion progresses before the dehydration effect of Si is fully realized. On the other hand, the oxygen that dissolves from the steel plate along with Si quickly forms SiO ２ It is possible to form the SiO formed in this way. ２ The oxygen (O) content acts as a crystal nucleus, promoting the reaction between Si and free water, and manifesting the dehydration effect of Si. If the O content is too low, this effect will not occur. Therefore, the O content should be 0.0005% or more, preferably 0.0008% or more. On the other hand, if the O content is too high, it will cause deterioration of the weldability and toughness of the steel plate. Therefore, the O content should be 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0030% or less.

[0034] [Si] / [O]: 20 to 900 as mentioned above, SiO ２ The formation reaction is strongly influenced by the O content in the steel sheet; therefore, in order to stably obtain the effect of suppressing overall corrosion by Si, it is necessary to appropriately control the ratio of Si content to O content in the steel sheet. In this specification, CO ２When the Si content in steel plates for transport line pipes is expressed as [Si] and the O content is [O], the ratio of [Si] to [O] is denoted as "[Si] / [O]". If the value of [Si] / [O] is too small, SiO2 will be released from the Si and O that have leached out of the steel plate. ２ The formation of SiO continues to proceed, with Si and free water forming SiO ２ The formation reaction does not proceed, and the dehydrating effect of Si does not occur. Therefore, the [Si] / [O] value should be 20 or more, preferably 30 or more, and more preferably 100 or more. On the other hand, if the [Si] / [O] value is too large, the amount of Si dissolved during anode dissolution becomes excessive, resulting in thermodynamic stability of the formation of elemental Si, and SiO with free water. ２ The dehydrating effect caused by the formation will not occur. Therefore, the [Si] / [O] value should be 900 or less, preferably 800 or less, and more preferably 700 or less.

[0035] One or more elements selected from Cu: 0.05% to 4.00%, Ni: 0.05% to 4.00%, Cr: 0.05% to 4.00%, Mo: 0.03% to 2.00%, and W: 0.03% to 2.00%. Cu, Ni, Cr, Mo, and W are liquid and / or supercritical CO2. ２ These are important elements for suppressing localized corrosion of steel plates in contact with them, and it is necessary to include one or more of these elements. Liquid and / or supercritical CO ２ In this environment, free water is easily formed discretely on the surface of steel plates, making it easy for localized corrosion to progress as well as general corrosion. Once localized corrosion occurs, the pH of the solution becomes very low, and the Si dissolved in the free water on the surface of the steel plate reacts with the O that has also dissolved to form SiO2. ２ It is thermodynamically more stable for Si to exist as elemental Si than to form SiO. Therefore, if the steel sheet does not contain one or more elements selected from Cu, Ni, Cr, Mo, and W, the alloying element Si will form SiO. ２ The formation of a substance and the corrosion-preventive effect due to dehydration do not occur.

[0036] However, if the steel sheet contains one or more elements selected from Cu, Ni, Cr, Mo, and W, then at least one of these elemental metals rapidly concentrates on the surface of the steel sheet as the anodic dissolution of the steel sheet occurs in a low-pH free water environment where localized corrosion takes place. As a result of the concentration of these poorly soluble elemental metals on the surface of the steel sheet, the susceptibility to anodic dissolution at the site of localized corrosion decreases. To obtain this effect, the steel sheet needs to contain one or more elements selected from Cu: 0.05% or more, Ni: 0.05% or more, Cr: 0.05% or more, Mo: 0.03% or more, and W: 0.03% or more.

[0037] On the other hand, excessive amounts of Cu, Ni, Cr, Mo, or W can degrade weldability and toughness, and are also disadvantageous from a cost standpoint. Therefore, the content of Cu, Ni, and Cr should each be 4.00% or less, preferably 3.50% or less, and more preferably 3.00% or less. The content of Mo and W should each be 2.00% or less, preferably 1.50% or less, and more preferably 1.00% or less.

[0038] In a preferred embodiment, the component composition further includes one or more groups selected from groups A to D below.

[0039] Group A: One or more substances selected by mass%, such as Sn: 0.50% or less, Sb: 0.50% or less, and Co: 0.50% or less. Sn, Sb, and Co refer to liquid and / or supercritical CO ２These elements significantly suppress localized corrosion in the environment, and one or more of them can be optionally included in the steel sheet. All of these elements enhance the acid resistance of the steel sheet and have the function of suppressing the corrosion reaction that progresses at an accelerating rate when the pH drops excessively in the area where localized corrosion occurs. For this reason, when optionally included, the content of Sn, Sb, and Co should be greater than 0.00%, preferably 0.02% or more, and more preferably 0.03% or more. On the other hand, if the content of Sn, Sb, or Co is excessive, it will deteriorate weldability and toughness, and will also be disadvantageous from a cost standpoint. For this reason, when optionally included, the content of Sn, Sb, and Co should all be 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less.

[0040] Group B: One or more selected from Ca: 0.0100% or less, Mg: 0.0200% or less, and REM: 0.200% or less, in mass percent. Ca, Mg, and REM (rare earth metals) can be optionally included in the steel plate in one or more forms for the purpose of ensuring the toughness of the welded joint. Therefore, when optionally included, the content of Ca, Mg, and REM should all be greater than 0.00%, preferably 0.0010% or more, and more preferably 0.0020% or more. On the other hand, if the content of Ca, Mg, or REM is excessive, it will lead to deterioration of the toughness of the welded joint and an increase in cost. Therefore, when optionally included, the Ca content should be 0.0100% or less, the Mg content should be 0.0200% or less, and the REM content should be 0.200% or less. Preferably, the Ca content is 0.0080% or less, the Mg content is 0.0080% or less, and the REM content is 0.150% or less. More preferably, the Ca content is 0.0050% or less, the Mg content is 0.0050% or less, and the REM content is 0.100% or less.

[0041] Group C: One or more selected from Ti: 0.100% or less, Zr: 0.100% or less, Nb: 0.100% or less, and V: 0.100% or less, in mass percent. Ti, Zr, Nb, and V can be optionally included in the steel sheet in one or more quantities to ensure the desired strength. Therefore, when optionally included, the content of Ti, Zr, Nb, and V should all be greater than 0.000%, preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, if the content of Ti, Zr, Nb, or V is excessive, it will degrade toughness and weldability. Therefore, when optionally included, the content of Ti, Zr, Nb, and V should all be 0.100% or less, preferably 0.080% or less, and more preferably 0.050% or less.

[0042] Group D: in mass%, B: 0.0300% or less. B is an element that improves the hardenability of steel plates and can be optionally included in steel plates to ensure their strength. Therefore, when B is optionally included, the B content should be greater than 0.0000%, preferably 0.0005% or more, and more preferably 0.0030% or more. On the other hand, if the B content is too high, it will lead to a significant deterioration of toughness. Therefore, when B is optionally included, the B content should be 0.0300% or less, preferably 0.0200% or less, and more preferably 0.0100% or less.

[0043] The remainder of the steel sheet's composition, other than the above, consists of Fe and unavoidable impurities. Unavoidable impurities are impurities that are inevitably mixed in from raw materials, manufacturing processes, or manufacturing equipment, etc., and are permissible to be included in a range that does not hinder the purpose of the present invention. Examples of raw materials used in this embodiment include iron ore, reduced iron, or scrap.

[0044] [Microstructure] Liquid and / or supercritical CO2 according to this embodiment ２ The steel plate for transport line pipes has a microstructure in which, at the 1 / 2 position of the plate thickness, the area ratio of bainite is 90% or more and the area ratio of island martensite is less than 3%. By controlling the microstructure in addition to the component composition, the CO of this embodiment can be achieved. ２Steel plates for transport line pipes can be made to possess both high strength and high toughness. In this specification, "high strength" of a steel plate means that its tensile strength is 625 MPa or higher. Furthermore, "high toughness" of a steel plate means that its Charpy impact absorption energy at -40°C is 320 J or higher.

[0045] Bainite area ratio: 90% or more. Bainite refers to a lath-like bainitic ferrite structure in which cementite particles are precipitated within. The bainite phase is a hard phase and is effective in increasing the strength of steel sheets through transformation structure strengthening. By making the microstructure mainly composed of bainite, it is possible to increase the strength of the steel sheet while stabilizing its toughness at a high level. However, if the bainite area ratio is less than 90%, the total area ratio of the remaining structures such as ferrite, pearlite, martensite, and island martensite will exceed 10%. In such a composite structure, the interphase interfaces become the starting points for ductile and brittle cracks, so the target toughness may not be achieved. Therefore, the bainite area ratio should be 90% or more, preferably 95% or more. The bainite area ratio may be 100% or less.

[0046] The remaining tissue other than bainite may be one or more selected from the group consisting of ferrite, pearlite, martensite, and island martensite. The area ratio of the remaining tissue is 10% or less, preferably 5% or less. The area ratio of the remaining tissue may be 0% or more.

[0047] Area ratio of island martensite: Less than 3% Among the remaining structures, island martensite (Martensite-Austenite Constituent) has high hardness and serves as an initiation point for ductile and brittle cracks. Therefore, if the area ratio of island martensite is 3% or more, the toughness of the steel sheet decreases significantly. On the other hand, if the area ratio of island martensite is less than 3%, the toughness of the steel sheet does not decrease significantly. Therefore, the area ratio of island martensite should be less than 3%, preferably 2% or less. The area ratio of island martensite may be 0% or more.

[0048] As described later, the CO according to the present invention ２The steel plate for a transportation line pipe is supplied in the form of a thick steel plate manufactured by hot rolling. In this embodiment, the evaluation of the area ratio of bainite and island-like martensite is performed at the position of 1 / 2 of the plate thickness, that is, the position where the distances from both surfaces are equal in the center in the thickness direction. Note that CO according to this embodiment ２ The thickness of the steel plate for a transportation line pipe is not particularly limited, but it can be 6 mm or more and 45 mm or less.

[0049] [Strength] The liquid and / or supercritical CO according to this embodiment ２ The steel plate for a transportation line pipe has a strength with a tensile strength of 625 MPa or more. Thereby, high-pressure operation of the pipeline becomes possible, and the transportation cost can be reduced. The tensile strength can be 825 MPa or less.

[0050] [Toughness] The liquid and / or supercritical CO according to this embodiment ２ The steel plate for a transportation line pipe has a toughness with a Charpy impact absorption energy of 320 J or more at -40°C. Thereby, breakage due to unstable ductile fracture of the pipeline can be suppressed. The upper limit of the Charpy impact absorption energy at -40°C is not particularly limited, and the higher it is, the more preferable. Note that as a reference value for evaluating the toughness of the steel plate, if possible, in addition to the measurement of the Charpy impact absorption energy at -40°C, the measurement of the ductile fracture surface ratio (SA value) obtained by a drop weight tear test (hereinafter referred to as "DWTT test") at -40°C may be performed. In this case, the ductile fracture surface ratio in the steel plate according to the present invention is preferably 85% or more.

[0051] 2. Liquid and / or supercritical CO ２ In other embodiments of the manufacturing method of the steel plate for a transportation line pipe, the present invention provides a step of heating a steel slab having the same component composition as the component composition of the steel plate for a transportation line pipe according to the present invention to 1000°C or more and 1250°C or less, and then performing rolling on the steel slab in the austenite recrystallization temperature range and rolling in the austenite non-recrystallization temperature range with a cumulative reduction ratio of 60% or more, and the rolling end temperature is (Ar ２ The steel plate for a transportation line pipe has a strength with a tensile strength of 625 MPa or more. Thereby, high-pressure operation of the pipeline becomes possible, and the transportation cost can be reduced. The tensile strength can be 825 MPa or less. ３point + 50 °C) or higher (Ar ３ point + 150 °C) or lower, and then subjecting the steel slab to hot rolling to obtain a steel sheet, and thereafter, for the steel sheet, starting from a cooling start temperature of Ar ３ point or higher (Ar ３ point + 100 °C) or lower, accelerating cooling at an average cooling rate of 10 °C / s or higher and 80 °C / s or lower from the cooling start temperature to a cooling stop temperature of (Ms point - 100 °C) or higher and (Ms point + 100 °C) or lower, and a method for producing a steel sheet for a transportation line pipe, which has ２ In the following description, "temperature" refers to the surface temperature of the steel slab or the steel sheet.

[0052] [Heating and Hot Rolling] In the method for producing a steel sheet for a transportation line pipe according to the present embodiment, in the method for producing a steel sheet for a transportation line pipe using liquid and / or supercritical CO ２ a steel slab having the same component composition as that of the steel sheet for a transportation line pipe according to the present invention is heated to 1000 °C or higher and 1250 °C or lower, and then the steel slab is subjected to rolling in the austenite recrystallization temperature range and rolling in the austenite non-recrystallization temperature range with a cumulative reduction ratio of 60% or higher, and the hot rolling is performed such that the rolling end temperature is (Ar ２ point + 50 °C) or higher (Ar ３ point + 50 °C) or higher (Ar ３ point + 150 °C) or lower to obtain a steel sheet.

[0053] Heating temperature of the steel slab: 1000 °C or higher and 1250 °C or lower In the production method according to the present invention, first, a steel slab having the same component composition as that of the steel sheet for a transportation line pipe according to the present invention is produced. The method for producing the steel slab may be either a continuous casting method or an ingot casting method. From the viewpoint of preventing macro-segregation of components, it is preferable to produce the steel slab by the continuous casting method. ２ The method for producing the steel slab may be either a continuous casting method or an ingot casting method. From the viewpoint of preventing macro-segregation of components, it is preferable to produce the steel slab by the continuous casting method.

[0054] Next, the manufactured steel slab is heated for hot rolling. If the heating temperature is too low, carbides such as Nb and V in the steel slab may not dissolve sufficiently, and the strength-enhancing effect due to precipitation may not be obtained. For this reason, the heating temperature of the steel slab should be 1000°C or higher, preferably 1050°C or higher. On the other hand, if the heating temperature is too high, the initial austenite grains may coarseen, which may reduce toughness. For this reason, the heating temperature of the steel slab should be 1250°C or lower, preferably 1150°C or lower.

[0055] In addition to the conventional method of heating steel slabs by first cooling them to room temperature and then reheating them, energy-saving processes such as the direct rolling method, in which the steel slabs are loaded into the heating furnace as hot pieces without cooling immediately after manufacturing and subjected to hot rolling; the direct rolling method, in which the steel slabs are subjected to hot rolling immediately after manufacturing after a short period of heat retention; and the hot piece loading method, in which the steel slabs are loaded into the heating furnace at a high temperature immediately after manufacturing and some of the reheating is omitted, can all be applied without any problems.

[0056] Following rolling in the austenite recrystallization temperature range, the steel slab is heated and held before being rolled again in the austenite recrystallization temperature range. This process refines the austenite through recrystallization, improving toughness. While there are no specific requirements for the cumulative reduction ratio in the austenite recrystallization temperature range, it is preferable to set it to 50% or more. There are also no specific upper limits for the cumulative reduction ratio in the austenite recrystallization temperature range, but if the cumulative reduction ratio is excessive, the effect of recrystallization will saturate. Therefore, it is preferable to set the cumulative reduction ratio to 75% or less.

[0057] In this specification, the "austenite recrystallization temperature range" refers to the temperature range that is equal to or exceeds the temperature at which recrystallization to austenite begins during heating. The temperature at which austenite recrystallization begins can be calculated by the following formula (1). The symbol C enclosed in parentheses in the formula represents the carbon content (mass%) in the steel sheet. Within the composition range of the steel sheet according to the present invention, the temperature at which austenite recrystallization begins is approximately 950°C.

[0058]

[0059] Cumulative reduction ratio in the austenite pre-recrystallization temperature range: 60% or more Next, by performing rolling with a cumulative reduction ratio of 60% or more in the austenite pre-recrystallization temperature range, the austenite grains elongate, becoming finer in the thickness direction of the plate, and the toughness of the steel obtained by accelerated cooling in this state is good. If the cumulative reduction ratio in the austenite pre-recrystallization temperature range is less than 60%, the grain refining effect is insufficient, and the target toughness may not be obtained. For this reason, it is preferable to set the cumulative reduction ratio in the austenite pre-recrystallization temperature range to 60% or more, and to 70% or more if further improvement in toughness is required. There is no particular upper limit to the cumulative reduction ratio in the austenite pre-recrystallization temperature range, but since efficiency decreases if it becomes excessively high, it is preferable to set it to 80% or less.

[0060] In this specification, the term "austenite non-recrystallization temperature range" refers to a temperature range lower than the temperature at which austenite recrystallization begins, as calculated by equation (1) above.

[0061] Rolling end temperature: (Ar ３ point +50℃) or more (Ar ３ A steel slab heated to below (Ar 150°C) is subjected to hot rolling, which includes rolling in the austenite recrystallization temperature range and rolling in the austenite non-recrystallization temperature range, to produce a steel sheet. The rolling completion temperature at this time is (Ar 150°C). ３ point +50℃) or more (Ar ３ The temperature shall be below (point + 150°C). In this specification, "Ar ３ The "point" refers to the temperature at which the transformation from austenite to ferrite begins during the cooling of a steel sheet. High cumulative reduction ratios in the austenite-preserved recrystallization temperature range are effective in improving toughness, and this effect is further increased by reducing the temperature at lower temperatures. However, (Ar ３ When rolling is performed at low temperatures below (Ar + 50°C), a texture develops in the austenite grains, and if accelerated cooling is then performed to create a bainite-dominant structure, the texture is partially inherited by the transformed structure. As a result, separation is more likely to occur, and toughness is significantly reduced. On the other hand, when the rolling is completed at a temperature below (Ar ３If the temperature exceeds (Ar + 150°C), the grain refinement effect, which is effective in improving toughness, may not be sufficiently obtained. Therefore, the temperature at which rolling is terminated in the austenite non-recrystallization temperature range is (Ar ３ point +50℃) or more (Ar ３ The temperature should be below (150°C + 150°C).

[0062] Note that Ar in this embodiment ３ The points shall be calculated using the following formula (2) based on the content of each element in the steel sheet. The element symbols enclosed in parentheses in the formula represent the content (mass %) of each element in the steel sheet. Elements that are not present shall be treated as 0.

[0063]

[0064] [Accelerated Cooling] Liquid and / or supercritical CO2 according to this embodiment ２ The manufacturing method for steel plates for transport line pipes involves hot rolling the steel slab as described above, and then applying Ar to the resulting steel plate. ３ points or more (Ar ３ The process includes a step of performing accelerated cooling at an average cooling rate of 10°C / s to 80°C / s from a cooling start temperature of (Ms point - 100°C) or less to a cooling stop temperature of (Ms point + 100°C) or more to (Ms point + 100°C).

[0065] Cooling start temperature for accelerated cooling: Ar ３ points or more (Ar ３ The starting temperature for accelerated cooling is below point + 100°C. ３ Below a certain temperature, protereminate ferrite may form from austenite grain boundaries during the air cooling process after hot rolling until accelerated cooling begins, potentially lowering the strength of the base material. Furthermore, an increase in the amount of protereminate ferrite can increase the number of ferrite-bainite interfaces, which are the starting points for ductile and brittle cracks, thus potentially reducing toughness. On the other hand, if the cooling start temperature is (Ar ３If the temperature exceeds Ar (100°C), the rolling termination temperature will also be high, which may prevent the acquisition of a refined microstructure that is effective in improving toughness. Furthermore, even if the air cooling time before accelerated cooling begins is short after the end of rolling, austenite recovery and grain growth may progress, which may reduce the toughness of the base material. Therefore, the starting temperature for accelerated cooling should be Ar ３ points or more (Ar ３ The temperature shall be no more than (point + 100°C). Note that the cooling start temperature is lower than the rolling end temperature, and the difference between the two may be between 20°C and 80°C.

[0066] Average cooling rate of accelerated cooling: 10°C / s or more and 80°C / s or less. In this specification, "average cooling rate" refers to the value obtained by dividing the difference between the cooling start temperature and the cooling stop temperature by the cooling time. If the average cooling rate of accelerated cooling is less than 10°C / s, ferrite transformation may occur during cooling, and the strength of the base material may decrease. For this reason, the average cooling rate of accelerated cooling should be 10°C / s or more, preferably 20°C / s or more. On the other hand, if the average cooling rate of accelerated cooling exceeds 80°C / s, martensitic transformation may occur, especially near the surface of the steel sheet, and although the strength of the base material increases, the toughness of the base material decreases significantly. For this reason, the average cooling rate of accelerated cooling should be 80°C / s or less, preferably 60°C / s or less.

[0067] The cooling stop temperature for accelerated cooling is between (Ms point - 100°C) and (Ms point + 100°C). In this specification, "Ms point" refers to the temperature at which the transformation from austenite to martensite begins. If the cooling stop temperature for accelerated cooling is below (Ms point - 100°C), the martensitic transformation becomes more pronounced, and although the strength of the base material increases, the toughness of the base material may decrease significantly. This tendency is particularly pronounced near the surface of the steel sheet. Therefore, the cooling stop temperature for accelerated cooling should be above (Ms point - 100°C), preferably above the Ms point. On the other hand, if the cooling stop temperature exceeds (Ms point + 100°C), large cementite and island-like martensite associated with bainite transformation may be generated during the air cooling process after the cooling stop, which may reduce toughness. Therefore, the cooling stop temperature for accelerated cooling should be below (Ms point + 100°C), preferably below (Ms point + 60°C).

[0068] In this invention, the Ms point shall be a value obtained by calculating it using the following formula (3) based on the content of each element in each steel material. Elements that are not present shall be treated as 0.

[0069]

[0070] The steel sheet according to the present invention, manufactured by the aforementioned manufacturing method, is a liquid and / or supercritical CO ２ Suitable for use as steel plates for transport line pipes. Liquid and / or supercritical CO2 according to this embodiment. ２ Using steel plates for transport line pipes, liquids and / or supercritical CO2 ２ To manufacture transport line pipes, they are formed into a roughly cylindrical shape using a U-press or O-press, or by a press bending method involving repeated three-point bending. These pipes are then welded using submerged arc welding or similar methods, and expanded to achieve the desired shape. Line pipes manufactured in this way may be painted on the surface as needed. Heat treatment may also be performed to improve toughness or other properties.

[0071] Next, embodiments of the present invention will be described. However, the present invention is not limited to these embodiments.

[0072] Steel containing the elements shown in Table 1, with the remainder being Fe and unavoidable impurities, was melted in a converter and continuously cast to produce steel slabs with a thickness of 220 mm.

[0073]

[0074] Next, the obtained steel slabs were subjected to hot rolling and accelerated cooling under the conditions shown in Table 2 to produce steel plates with a thickness of 15 mm to 35 mm. The cumulative reduction ratio in the austenite recrystallization temperature range is shown in Table 2. Here, based on the elemental content shown in Table 1, Ar was calculated using equation (2). ３ The temperature at each point and the temperature at the Ms point calculated using equation (3) are shown in Table 2. The thickness of the resulting steel plate is also shown in Table 2.

[0075]

[0076] Next, the obtained steel plates were evaluated as described below.

[0077] <Microstructure> A specimen for microstructure observation was taken from the 1 / 2 position of the obtained steel plate thickness. The cross section parallel to the rolling direction and the thickness direction was mirror-polished, and after etching with Nital, microscopic images of the specimen were taken using a scanning electron microscope (SEM). Microstructure images were taken from five randomly selected fields of view from the mirror-polished surface, with a magnification of 2000x. The size of the observation field was 60 μm × 45 μm. Based on the characteristics of the microstructure captured in the microstructure images, each phase such as bainite, martensite, ferrite, and pearlite was identified, and their area percentages were determined by image analysis.

[0078] Next, the specimens used to evaluate the area ratio of each phase were etched by electrolytic etching to reveal only island-like martensite. The electrolyte was prepared by mixing 25 g of sodium hydroxide and 5 g of picric acid in 100 ml of distilled water and dissolving them. Microscopic images were taken of the electrolytically etched specimens using a scanning electron microscope. Microscopic images were taken from five randomly selected fields of view from the electrolytically etched surface, at a magnification of 2000x. The area ratio of island-like martensite at the 1 / 2 thickness position in the captured microscopic images was determined by image analysis. For the area ratios of the five phases obtained, the average value of the five fields of view was calculated. In the calculation, the average value of the area ratio was rounded to one decimal place. The results are shown in Table 3.

[0079] <Corrosion Resistance> From the obtained steel plate, a steel billet measuring 25 mm in length, 10 mm in width, and 3 mm in thickness was cut out and the entire surface was polished with #600 grit sandpaper to prepare a test specimen. The obtained test specimen was subjected to supercritical CO2 treatment using the method described below. ２ Corrosion tests were conducted in an environmental setting to evaluate resistance to both general and localized corrosion.

[0080] First, all test specimens were ultrasonically degreased in acetone for 5 minutes and their mass was measured. Next, the test specimens were suspended from a jig using Teflon strings and placed inside a 2-liter pressure vessel, which was then sealed. After that, the pressure vessel was cooled with ice water and vacuumed to -0.1 MPa, and then 1500 ppmH was injected using a glass syringe. ２ O, 100 ppmv O ２ , 500ppmv SO ２ , and 500 ppmv NO ２ Inject it into the pressure vessel, and finally CO ２ The pressure vessel was then filled with [the substance]. Subsequently, the temperature of the atmospheric gas inside the pressure vessel was raised to 50°C, and the pressure was maintained at 10 MPa to begin the test.

[0081] After 168 hours from the start of the test, the test specimen was removed from the pressure vessel, and the corrosion products adhering to the surface of the specimen were washed off with a sponge or the like. Then, the corrosion products were completely removed with a rust removal solution to which an inhibitor had been added. This rust removal solution was prepared by mixing 500 ml of 37% hydrochloric acid, 3.5 g of hexamethylenetetramine, and 3 ml of Hibilon (an inhibitor manufactured by Aiko Chemical Co., Ltd.), and then adding distilled water to make a total volume of 1 liter. Next, the test specimen was washed with pure water, then washed in ethanol, air-dried, and the mass of the test specimen after the test was measured.

[0082] The resistance of the test specimen to overall corrosion was evaluated by the average corrosion rate. The average corrosion rate was calculated by determining the difference in mass of the test specimen before and after the test, and converting this to the value of the thickness of the test specimen reduced over one year. The resistance to overall corrosion was then evaluated according to the following criteria: [Criteria for evaluation of resistance to overall corrosion] A: Average corrosion rate less than 0.10 mm / y B: Average corrosion rate of 0.10 mm / y or more and less than 0.30 mm / y C: Average corrosion rate of 0.30 mm / y or more

[0083] The resistance of the test specimens to localized corrosion was evaluated by the maximum corrosion depth. The maximum corrosion depth was determined by measuring the corrosion depth on the surface of the test specimen after testing using a three-dimensional laser microscope and finding the maximum value. The laser wavelength of the three-dimensional laser microscope was 658 nm, and the measurement pitch was 0.5 μm. The resistance to overall corrosion was then evaluated according to the following criteria: [Criteria for evaluation of resistance to localized corrosion] A: Maximum corrosion depth is less than 50 μm B: Maximum corrosion depth is 50 μm or more and less than 100 μm C: Maximum corrosion depth is 100 μm or more

[0084] The results are shown in Table 3. Note that, in the above evaluation criteria, A means excellent, B means good, and C means poor. If the evaluation result is A or B, the test specimen is judged to have sufficient resistance to corrosion.

[0085] <Tensile Strength> From the obtained steel plate, full-thickness tensile test specimens were taken with the tensile direction in the C direction, in accordance with the API 5L standard for carbon steel pipes applied to oil and gas transportation systems. Next, tensile tests were performed using the taken full-thickness tensile test specimens to determine the yield strength (YS) and tensile strength (TS). The results are shown in Table 3.

[0086] <Toughness> The toughness of the test specimens was evaluated by a Charpy impact test. In the Charpy impact test, a Charpy impact specimen with a V-notch with a depth of 2 mm and a longitudinal direction of C was taken from the 1 / 2 position of the thickness of the obtained steel plate. Next, a Charpy impact test in accordance with ASTM A370 was performed at -40°C using the taken Charpy impact specimen, and the Charpy impact absorption energy (vE-40°C) was determined.

[0087] In addition to the Charpy impact test, a DWTT test was conducted to obtain other reference values ​​for evaluating the toughness of the test specimens. In the DWTT test, a full-thickness DWTT specimen with a press notch in accordance with the API 5L standard and with the longitudinal direction as the C direction was taken from the obtained steel plate. Next, an impact bending load by dropping a weight was applied to the taken full-thickness DWTT specimen at -40°C, and the ductile fracture ratio (SA-40°C) of the fractured fracture surface was determined. The obtained results are shown in Table 3.

[0088]

[0089] As shown in Table 3, all of the test specimens of the inventive example exhibited excellent resistance to both general and localized corrosion. In addition, the base material had high strength with a tensile strength of 625 MPa or higher and high toughness with a Charpy impact absorption energy of 320 J or higher at -40°C. In contrast, the test specimens of the comparative example were insufficient in any of the above characteristics. The improvement effect of the present invention is clear from the comparison between the inventive example and the comparative example.

Claims

1. Contains, by mass%, C: 0.03% to 0.08%, Mn: 1.50% to 2.50%, P: 0.030%, S: 0.0100%, N: 0.0100%, Al: 0.010% to 0.200%, Si: 0.10% to 1.00%, and O: 0.0005% to 0.0050%, and further contains one or more selected from Cu: 0.05% to 4.00%, Ni: 0.05% to 4.00%, Cr: 0.05% to 4.00%, Mo: 0.03% to 2.00%, and W: 0.03% to 2.00%. A liquid and / or supercritical CO2 having the following characteristics: [Si] / [O], where [Si] is the mass percent content of Si and [O] is the mass percent content of O, is between 20 and 900, and the remainder consists of Fe and unavoidable impurities; a microstructure in which, at the 1 / 2 position of the plate thickness, the area ratio of bainite is 90% or more and the area ratio of island martensite is less than 3%; a strength of tensile strength of 625 MPa or more; and toughness of Charpy impact absorption energy of 320 J or more at -40°C. ２ Steel plates for transport line pipes.

2. The liquid and / or supercritical CO2 according to claim 1, wherein the component composition further contains one or more groups selected from groups A to D below. ２ Steel plates for transport line pipes. Group A: One or more types selected from Sn: 0.50% or less, Sb: 0.50% or less, and Co: 0.50% or less by mass%. Group B: One or more types selected from Ca: 0.0100% or less, Mg: 0.0200% or less, and REM: 0.200% or less by mass%. Group C: One or more types selected from Ti: 0.100% or less, Zr: 0.100% or less, Nb: 0.100% or less, and V: 0.100% or less by mass%. Group D: B: 0.0300% or less by mass%.

3. A step of heating a steel slab having the component composition described in claim 1 or 2 to 1000°C or more and 1250°C or less; and thereafter, rolling the steel slab in the austenite recrystallization temperature range and rolling in the austenite non-recrystallization temperature range with a cumulative reduction ratio of 60% or more, wherein the rolling completion temperature is (Ar ３ point +50℃) or more (Ar ３ A process of hot rolling at a temperature of 150°C or lower to form a steel sheet, and thereafter, Ar ３ points or more (Ar ３ A process for accelerated cooling of liquid and / or supercritical CO2, comprising: a step of performing accelerated cooling at an average cooling rate of 10°C / s to 80°C / s from a cooling start temperature of (Ms point - 100°C) or less to a cooling stop temperature of (Ms point - 100°C) or more to (Ms point + 100°C) or less, wherein the cooling is performed on liquid and / or supercritical CO2. ２ A method for manufacturing steel plates for transport line pipes.

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