Electric resistance welded steel pipe for line pipe

Abstract

This electric resistance welded steel pipe for a line pipe has a base material part and an electric resistance welded part, and the chemical composition of the base material part has a prescribed chemical composition. In the microstructure of the 1 / 2 × tB part of the base material part, the area ratio of ferrite is 40-80% and the average crystal grain size is 35 μm or less. In the microstructure of the 1 / 4 × tS part of the electric resistance welded part, the area ratio of ferrite is 40-70%, the area ratio of island martensite is 0.2-10.0%, the average crystal grain size is 35 μm or less, the yield stress is 360 MPa or more, and the tensile strength is 465 MPa or more.

Description

Electric resistance welded steel pipe for line pipe

[0001] The present invention relates to a steel pipe, and more particularly to an electric resistance welded steel pipe for line pipe.

[0002] Pipelines are one means of transporting crude oil and natural gas, but in recent years, as natural resource extraction areas have become more severe, the environment in which line pipes are laid has also become more severe. One consequence of this is an increase in the number of submarine pipelines laid on the seabed. There are various methods for laying submarine pipelines. One method, known as the reel method, involves first manufacturing the pipe on land, welding it together, and then winding the resulting long pipe onto a spool on a barge. The pipe is then unwound at sea and laid on the seabed. This method allows for efficient laying of submarine line pipes.

[0003] However, when steel pipes are wound and unwound, they are subjected to plastic bending. At this time, compressive force is applied to the inside of the bend, which can cause buckling on the inside of the bend, resulting in the steel pipe buckling. If steel pipe buckling occurs, installation work must be halted, resulting in enormous damage. It has been found that steel pipe buckling can be prevented by lowering the yield ratio (calculated as yield strength (YS) / tensile strength (TS), denoted as YR) in the longitudinal direction of the steel pipe in the base material and electric resistance welded parts of the steel pipe. It has also been found that steel pipe buckling can be prevented by increasing the wall thickness of the steel pipe. Given this background, there is a growing demand for steel pipes that have a low axial YR and a thick wall thickness, in addition to steel pipe strength sufficient to withstand high internal pressure.

[0004] Furthermore, if brittle fracture occurs in a line pipe, oil or natural gas will leak, leading to enormous damage, so excellent toughness is required. Specifically, it is essential to ensure the characteristics in a Charpy test for the base material and in a CTOD test for the electric resistance weld. In particular, the steel pipe for line pipe used in the submarine pipeline of the present application has a thick wall thickness to prevent buckling, so it is not easy to ensure excellent toughness in the base material and the electric resistance weld. Specifically, if the wall thickness of the base material is thick, the finishing reduction rate in hot rolling is insufficient, and further, the cooling rate at the center of the wall thickness cannot be increased, resulting in coarsening of the microstructure, making it difficult to ensure toughness. Furthermore, if the wall thickness of the electric resistance weld is thick, heating from the outer surface after electric resistance welding reaches the required temperature all the way to the inner surface, so the reheating temperature of the outer surface increases, resulting in coarsening of the microstructure, making it difficult to ensure toughness.

[0005] Therefore, electric resistance welded steel pipes used in the reel method are required to have high strength, low YR and excellent toughness in the base material and electric resistance welded parts.

[0006] For example, Patent Document 1 describes an electric resistance welded steel pipe having excellent toughness in the base material and electric resistance welded portion. In the electric resistance welded steel pipe described in Patent Document 1, the chemical composition of the base material contains, in mass %, C: 0.04 to 0.12%, Si: 0.01 to 0.50%, Mn: 0.5 to 2.0%, Ti: 0.005 to 0.030%, Nb: 0.005 to 0.050%, and N: 0.001 to 0.008%, with the balance containing Fe and impurities. When the thickness of the base material is tB and the thickness of the electric resistance weld is tS, the hardness of an outer surface layer B located at a depth of 1 mm from the outer surface of the base material minus the hardness of a ½ tB portion is 30 HV10 or less, and the hardness of an outer surface layer S located at a depth of 1 mm from the outer surface of the electric resistance weld minus the hardness of a ½ tS portion is 0 HV10 or more and 30 HV10 or less. Patent Document 1 describes that this provides excellent low-temperature toughness.

[0007] Patent Document 2 describes an electric resistance welded steel pipe that has excellent toughness in the base metal and electric resistance welded portion. The electric resistance welded steel pipe described in Patent Document 2 contains, by mass%, C: 0.02 to 0.10%, Si: 0.05 to 0.30%, Mn: 0.80 to 2.00%, and Nb: 0.010 to 0.100%, has a composition that satisfies a carbon equivalent Ceq of 0.25 to 0.50, and has a structure consisting of a bainitic ferrite phase and / or a bainite phase, and has high strength of a yield strength of 52 ksi or more and high toughness with a fracture appearance transition temperature vTrs of -45°C or less. The material is a thick hot-rolled steel plate, and the electric resistance welded portion is subjected to heat treatment in which the electric resistance welded portion is induction heated to a minimum temperature of 830°C or higher and a maximum temperature of 1150°C or lower, and cooled at an average cooling rate of 10 to 70°C / s at each position in the thickness direction and at a cooling stop temperature of 550°C or lower, resulting in a structure consisting of a bainitic ferrite phase and / or a bainite phase, and in which the ratio of the average grain size at the coarsest grain position to the average grain size at the finest grain position at each position in the thickness direction is 2.0 or less. Patent Document 2 describes that this results in excellent low-temperature toughness.

[0008] International Publication No. 2020 / 170333 International Publication No. 2015 / 004901

[0009] The inventors' investigations have revealed that if the chemical composition or manufacturing method is inappropriate, the area ratio of island martensite (MA), which is a hard phase, in the electric resistance welded joint may fall outside the appropriate range, which may result in the electric resistance welded joint being unable to achieve both toughness and a low YR.

[0010] In Patent Documents 1 and 2, no consideration is given to controlling the above-mentioned MA in the electric resistance welded portion. Therefore, there is a risk that the low-temperature toughness and low YR of the electric resistance welded portion of the electric resistance welded steel pipes described in Patent Documents 1 and 2 may not necessarily be sufficient.

[0011] The present invention has been made in view of the above circumstances, and has as its object to provide an electric resistance welded steel pipe for linepipe use which has high strength, low YR and excellent low temperature toughness in the base metal and electric resistance welded joints.

[0012] The gist of the present invention is as follows. (1) An electric resistance welded steel pipe for line pipe according to one aspect of the present invention has a base metal portion and an electric resistance welded portion, and the base metal portion has a chemical composition, in mass%, of C: 0.060 to 0.120%, Si: 0.01 to 0.50%, Mn: 0.5 to 2.0%, P: 0.030% or less, S: 0.0050% or less, Al: 0.080% or less, Ti: 0.003 to 0.030%, Nb: 0.003 to 0.046%, N: 0.0010 to 0.0080%, O: 0.005% or less, Cu: 0 to 0.500%, Ni: 0 to 0.500%, Cr: 0 to 0.500%, Mo: 0 to 0.500%, V : 0 to 0.100%, W: 0 to 0.500%, Ca: 0 to 0.0040%, REM: 0 to 0.0050%, and the balance: Fe and impurities, wherein Ceq represented by the following formula (i) is 0.16 to 0.53 mass%, and M represented by the following formula (ii) is 0.06 to 0.25 mass%, wherein, when the thickness of the base material portion is tB and the thickness of the electric resistance weld portion is tS, the tB and tS are 15.0 to 25.4 mm, and the outer diameter is 304.8 to 660.4 mm, in the microstructure of a 1 / 2 x tB portion of the base material, the area ratio of ferrite is 40 to 80% and the average crystal grain size is 35 μm or less, and in the microstructure of a 1 / 4 x tS portion of the electric resistance weld portion, The area fraction of ferrite is 40 to 70%, the area fraction of island martensite is 0.2 to 10.0%, the average grain size is 35 μm or less, the yield stress is 360 MPa or more, and the tensile strength is 465 MPa or more. Ceq=C+Mn / 6+(Ni+Cu) / 15+(Cr+Mo+V) / 5 ... (i) M=C / 3+5×Nb ... (ii) Here, the element symbols in formulas (i) and (ii) are substituted with the content (mass%) of the corresponding element, or 0 is substituted if the element is not contained.(2) In the electric resistance welded steel pipe for line pipe described in (1) above, the base metal may have a chemical composition, in mass%, of one or more elements selected from the group consisting of Cu: more than 0% and not more than 0.500%, Ni: more than 0% and not more than 0.500%, Cr: more than 0% and not more than 0.500%, Mo: more than 0% and not more than 0.500%, V: more than 0% and not more than 0.100%, W: more than 0% and not more than 0.500%, Ca: more than 0% and not more than 0.0040%, and REM: more than 0% and not more than 0.0050%.

[0013] According to the above-described aspects of the present invention, an electric resistance welded steel pipe for line pipe can be obtained that has high strength, low YR, and excellent low-temperature toughness in the base metal and electric resistance welded portion.

[0014] 1 is a diagram showing the relationship between the MA area ratio of an electric resistance weld and δc, and FIG. 2 is a diagram showing the relationship between the MA area ratio of an electric resistance weld and YR.

[0015] The present inventors have investigated a method for obtaining an electric resistance welded steel pipe for line pipe use that has high strength, low YR and excellent low temperature toughness in both the base metal and the electric resistance welded portion, and have obtained the following findings.

[0016] (I) In the base material, it is important to control the microstructure by controlling the hot rolling conditions in addition to adjusting the chemical composition of the electric resistance welded steel pipe.

[0017] (II) In electric resistance welded steel pipes, when they are as-electrically welded, the microstructure of the weld becomes a quenched martensite structure, which becomes very hard, significantly deteriorating toughness. Therefore, in order to improve toughness, electric resistance welded steel pipes are subjected to a heat treatment in which the electric resistance weld is reheated from the outer surface side by induction heating or the like and then water-cooled. Because water cooling is performed on the outer surface, the outer surface side is rapidly cooled, which increases the hardness of the outer surface side, and this can result in a deterioration of the toughness of the outer surface side. Therefore, it is important to optimize the water-cooling conditions after reheating to reduce the hardness of the outer surface side. Specifically, it is important to optimize the water-cooling conditions so as to generate ferrite, a soft structure, in the microstructure on the outer surface side.

[0018] (III) When carbon concentrates in untransformed austenite during water cooling after reheating, island martensite (MA) is formed. MA is hard, so if its content is high, toughness deteriorates. On the other hand, MA increases the work hardening ability of the material, so if its content is high, tensile strength increases. In order to achieve both low-temperature toughness and low YR of electric resistance welds, it is important to properly control the amount of MA.

[0019] (IV) From the viewpoint of chemical composition, the amount of MA in electric resistance welds is affected by the C content and Nb content. C concentrates in untransformed austenite during water cooling after reheating, forming MA. Therefore, controlling the C content is important for optimizing the amount of MA. On the other hand, Nb has the effect of arresting transformation. The lower the Nb content, the less carbon concentrates in untransformed austenite, thereby suppressing the generation of MA. For these reasons, controlling the C content and Nb content is important for optimizing the amount of MA in electric resistance welds.

[0020] (V) The amount of MA in an electric resistance weld is affected by the water cooling stop temperature after reheating. When the water cooling stop temperature is high, carbon concentration in the untransformed austenite is promoted, and the MA area ratio increases. On the other hand, when the water cooling stop temperature is low, carbon concentration in the untransformed austenite is suppressed, and the MA area ratio decreases.

[0021] The amount of MA in the weld is also affected by the segregation state of the electric resistance weld. When the alloy concentration in the segregation zone is high, carbon enrichment in the untransformed austenite is promoted, and the MA area ratio increases. On the other hand, when the alloy concentration in the segregation zone is low, carbon enrichment in the untransformed austenite is suppressed, and the MA area ratio decreases. The electric resistance weld corresponds to the end of the slab (coil), and the segregation zone present in the electric resistance weld is not a central (macro) segregation present in the center of the plate thickness at the center of the width of the slab (coil), but a microsegregation, so it is significantly affected by the heating conditions during the production of hot-rolled steel sheet. Therefore, to optimize the segregation state in the electric resistance weld, it is important to optimize the slab heating conditions during the production of hot-rolled steel sheet, which is the raw material for electric resistance welded steel pipes.

[0022] Hereinafter, an electric resistance welded steel pipe for line pipe according to one embodiment of the present invention (electric resistance welded steel pipe for line pipe according to this embodiment (sometimes simply referred to as electric resistance welded steel pipe)) made based on the above findings will be described. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible within the scope of the present invention.

[0023] The electric resistance welded steel pipe for line pipe according to this embodiment has a base material portion and an electric resistance welded portion, and the base material portion has a predetermined chemical composition, and when the thickness of the base material portion is tB and the thickness of the electric resistance welded portion is tS, the tB and tS are 15.0 to 25.4 mm, the outer diameter is 304.8 to 660.4 mm, in the microstructure of a 1 / 2×tB portion of the base material, the area fraction of ferrite is 40 to 80% and the average grain size is 35 μm or less, in the microstructure of a 1 / 4×tS portion of the electric resistance welded portion, the area fraction of ferrite is 40 to 70%, the area fraction of island martensite (MA) is 0.2 to 10.0%, the average grain size is 35 μm or less, the yield stress is 360 MPa or more, and the tensile strength is 465 MPa or more. Each requirement of the electric resistance welded steel pipe for line pipe according to this embodiment will be explained in detail below.

[0024] 1. Chemical composition of the base material The reasons for limiting each element are as follows. The numerical ranges described below with "to" include the lower and upper limits. Numerical values ​​indicated as "less than" or "greater than" are not included in the numerical range. In the following description, "%" in the chemical composition means "mass %" unless otherwise specified.

[0025] The electric resistance welded steel pipe for line pipe according to this embodiment has a steel plate as a base material and a welded portion (electric resistance welded portion) provided at the butt joint of the steel plates and extending in the longitudinal direction of the steel plates. In the electric resistance welded steel pipe according to this embodiment, no welding material is used when electric resistance welding the steel plates to form the electric resistance welded steel pipe, so the base material and the electric resistance welded portion have substantially the same chemical composition.

[0026] The electric resistance welded steel pipe for linepipe according to this embodiment has a base metal chemical composition, in mass%, of C: 0.060 to 0.120%, Si: 0.01 to 0.50%, Mn: 0.5 to 2.0%, P: 0.030% or less, S: 0.0050% or less, Al: 0.080% or less, Ti: 0.003 to 0.030%, Nb: 0.003 to 0.046%, N: 0.0010 to 0.0080%, O: 0.005% or less, and the balance: Fe and impurities, with Ceq represented by formula (i) described below being 0.16 to 0.53 mass%, and M represented by formula (ii) described below being 0.06 to 0.25 mass%. Each element will be described in detail below.

[0027] C: 0.060 to 0.120% C is an essential element for improving hardenability and obtaining high strength. Furthermore, it is an element that affects the YR of steel. If the C content is too low, it is not possible to sufficiently reduce the YR in the base metal of an electric resistance welded steel pipe for line pipe. In order to obtain the desired YR in an electric resistance welded steel pipe for line pipe, the C content is set to 0.060% or more. The C content is preferably 0.070% or more. On the other hand, if the C content is too high, the central segregation hardens and the low-temperature toughness of the base metal deteriorates. In order to ensure the low-temperature toughness of the base metal, the C content is set to 0.120% or less. The C content is preferably 0.100% or less.

[0028] Si: 0.01 to 0.50% Si is an element effective for deoxidizing steel. If the Si content is too low, Si cannot fully achieve its effect of improving the soundness of steel through deoxidation (suppressing the occurrence of defects such as blowholes in the steel). To fully achieve this effect and obtain excellent low-temperature toughness in the base material, the Si content is set to 0.01% or more, preferably 0.10% or more. On the other hand, if the Si content exceeds 0.50%, oxides are formed in the electric resistance weld, deteriorating the low-temperature toughness of the electric resistance weld. Therefore, the Si content is set to 0.50% or less. The Si content is preferably 0.40% or less.

[0029] Mn: 0.5 to 2.0% Mn is an element that improves hardenability and is effective in ensuring the strength of the base material. To obtain the desired strength in the base material, the Mn content is set to 0.5% or more. The Mn content is preferably 0.7% or more. On the other hand, if the Mn content exceeds 2.0%, a hardened phase is formed in the central segregation, significantly deteriorating the low-temperature toughness of the base material. Therefore, the Mn content is set to 2.0% or less. The Mn content is preferably 1.6% or less.

[0030] P: 0.030% or less P is an impurity element that affects the low-temperature toughness of steel. If the P content exceeds 0.030%, grain boundary embrittlement occurs in the base material and electric resistance welded joints, significantly deteriorating low-temperature toughness. Therefore, the P content is set to 0.030% or less. The lower the P content, the better, and 0% is acceptable. However, since the practical lower limit of the P content in mass-produced steel is 0.002%, the P content may be set to 0.002% or more.

[0031] S: 0.0050% or less S is an impurity element that affects the low-temperature toughness of steel. If the S content exceeds 0.0050%, coarse sulfides are formed, deteriorating the low-temperature toughness of the base material and the electric resistance welded joint. Therefore, the S content is set to 0.0050% or less. The lower the S content, the better, and 0% is acceptable. However, since the practical lower limit of the S content in mass-produced steel is 0.0003%, the S content may be set to 0.0003% or more.

[0032] Al: 0.080% or less Al is an effective element as a deoxidizer. However, if the Al content exceeds 0.080%, a large amount of Al oxide is generated, deteriorating the toughness of the base material and the electric resistance weld. Therefore, the Al content is set to 0.080% or less. The Al content is preferably 0.050% or less. Since deoxidation is also possible with Si or Ti, the Al content may be 0%. However, to obtain a sufficient deoxidation effect, the Al content is preferably 0.010% or more.

[0033] Ti: 0.003 to 0.030% Ti is a nitride-forming element that forms nitrides and contributes to the refinement of crystal grains. In order to obtain this effect and ensure the low-temperature toughness of the base material, the Ti content is set to 0.003% or more. The Ti content is preferably 0.010% or more. On the other hand, if the Ti content exceeds 0.030%, the low-temperature toughness of the base material will be significantly deteriorated due to the formation of coarse carbonitrides. Therefore, the Ti content is set to 0.030% or less. The Ti content is preferably 0.025% or less.

[0034] Nb: 0.003 to 0.046% Nb is an element that forms carbides, nitrides, and / or carbonitrides, contributing to improving the strength of steel. Furthermore, Nb is an element that has the effect of improving the low-temperature toughness of the base material of an electric resistance welded steel pipe for linepipe by expanding the non-recrystallization rolling temperature range. Furthermore, Nb has the effect of arresting transformation, and is an effective element for optimizing MA. To achieve these effects, the Nb content is set to 0.003% or more. The Nb content is preferably 0.010% or more. On the other hand, if the Nb content exceeds 0.046%, a large amount of Nb-based carbonitrides is formed, deteriorating the low-temperature toughness of the base material. Therefore, the Nb content is set to 0.046% or less. The Nb content is preferably 0.030% or less.

[0035] N: 0.0010 to 0.0080% N is an element that forms nitrides, refines the crystal grains of the steel, and improves the low-temperature toughness of the base material. To obtain this effect and ensure the low-temperature toughness of the base material, the N content is set to 0.0010% or more. On the other hand, if the N content exceeds 0.0080%, a large amount of nitrides is formed, deteriorating the low-temperature toughness of the base material. Therefore, the N content is set to 0.0080% or less.

[0036] O: 0.005% or less O is an element that affects the low-temperature toughness of steel. If the O content exceeds 0.005%, a large amount of oxide is generated, significantly deteriorating the low-temperature toughness of the base material and the electric resistance weld. Therefore, the O content is set to 0.005% or less. The lower the O content, the better, and 0% is acceptable. However, since the substantial lower limit of the O content in mass-produced steel is 0.001%, the O content may be set to 0.001% or more.

[0037] The electric resistance welded steel pipe for line pipe according to this embodiment basically has a chemical composition containing the above elements, with the balance being Fe and impurities. However, in order to improve strength, low-temperature toughness, and other properties, the following optional elements may also be contained within the ranges described below. However, since the inclusion of these elements is not essential, the lower limit of each element is 0%.

[0038] In addition, in this embodiment, "impurities" refer to components that are mixed in from raw materials such as ore and scrap when steel is industrially produced, or due to various factors in the manufacturing process, and are acceptable within a range that does not adversely affect the properties of the electric-resistance welded steel pipe for line pipe according to this embodiment.

[0039] Cu: 0 to 0.500% Cu is an effective element for increasing strength without deteriorating low-temperature toughness. Therefore, Cu may be contained as necessary. To obtain the above effect, the Cu content is preferably more than 0%, and more preferably 0.010% or more. On the other hand, if the Cu content exceeds 0.500%, cracks tend to occur during heating and electric resistance welding of the steel billet. Therefore, even when Cu is contained, the Cu content is set to 0.500% or less.

[0040] Ni: 0 to 0.500% Ni is an element effective in improving low-temperature toughness and strength. Therefore, Ni may be contained as necessary. To obtain the above effects, the Ni content is preferably more than 0%, and more preferably 0.010% or more. On the other hand, if the Ni content exceeds 0.500%, electric resistance weldability deteriorates. Therefore, even when Ni is contained, the Ni content is set to 0.500% or less.

[0041] Cr: 0 to 0.500% Cr is an element that improves the strength of steel through precipitation strengthening. Therefore, Cr may be contained as necessary. To obtain this effect, the Cr content is preferably more than 0%, and more preferably 0.010% or more. On the other hand, if the Cr content exceeds 0.500%, the hardenability increases, the proportion of bainite in the structure becomes too high, and low-temperature toughness deteriorates. Therefore, even when Cr is contained, the Cr content is set to 0.500% or less.

[0042] Mo: 0 to 0.500% Mo is an element that improves hardenability and at the same time forms carbonitrides, contributing to improving the strength of steel. Therefore, Mo may be added as needed. To obtain the above effects, the Mo content is preferably more than 0%, and more preferably 0.010% or more. On the other hand, if the Mo content exceeds 0.500%, the strength of the steel becomes higher than necessary and the low-temperature toughness deteriorates. Therefore, even when Mo is added, the Mo content is set to 0.500% or less.

[0043] V: 0 to 0.100% V is an element that forms carbides and / or nitrides and contributes to improving the strength of steel. Therefore, V may be added as needed. To obtain the above effects, the V content is preferably more than 0%, and more preferably 0.001% or more. On the other hand, if the V content exceeds 0.100%, the amount of precipitates increases and low-temperature toughness deteriorates. Therefore, even when V is added, the V content is set to 0.100% or less.

[0044] W: 0 to 0.500% W is an element that forms carbides and contributes to improving the strength of steel. Therefore, W may be added as needed. To obtain the above effects, the W content is preferably more than 0%, and more preferably 0.100% or more. On the other hand, if the W content exceeds 0.500%, the amount of carbides increases and low-temperature toughness deteriorates. Therefore, even when W is added, the W content is set to 0.500% or less.

[0045] Ca: 0 to 0.0040% Ca is an element that suppresses the formation of elongated MnS by forming sulfides, thereby contributing to improving low-temperature toughness and lamellar tear resistance. Therefore, Ca may be added as needed. To obtain the above effects, the Ca content is preferably more than 0%, and more preferably 0.0003% or more. On the other hand, if the Ca content exceeds 0.0040%, a large amount of CaO is formed in the electric resistance weld, deteriorating the low-temperature toughness of the electric resistance weld. Therefore, even when Ca is added, the Ca content is set to 0.0040% or less.

[0046] REM: 0 to 0.0050% Like Ca, REM is an element that suppresses the formation of elongated MnS by forming sulfides, thereby contributing to improving low-temperature toughness and lamellar tear resistance. Therefore, REM may be contained as necessary. To obtain the above effects, the REM content is preferably more than 0%, and preferably 0.0010% or more. On the other hand, if the REM content exceeds 0.0050%, the number of REM oxides increases, deteriorating low-temperature toughness. Therefore, even when REM is contained, the REM content is set to 0.0050% or less. Here, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanides, and the REM content refers to the total content of these elements.

[0047] As described above, the electric resistance welded steel pipe for line pipe according to this embodiment has a chemical composition in the base material and the electric resistance welded portion that contains essential elements, optionally contains optional elements as needed, and the balance is Fe and impurities.

[0048] In the electric resistance welded steel pipe according to this embodiment, the content of each element must be controlled as described above, and furthermore, Ceq and M, which are determined by the content of each element, must be within predetermined ranges as described below.

[0049] Ceq: 0.16 to 0.53 mass% Ceq is a value that serves as an index of hardenability and is expressed by the following formula (i). If Ceq is less than 0.16 mass%, the desired strength cannot be obtained in the base material and the electric resistance weld. Therefore, Ceq is set to 0.16 mass% or more. Ceq is preferably 0.25 mass% or more, and more preferably 0.30 mass% or more. On the other hand, if Ceq exceeds 0.53 mass%, the low-temperature toughness of the base material and the electric resistance weld will deteriorate. Therefore, Ceq is set to 0.53 mass% or less. Ceq is preferably 0.45 mass% or less, and more preferably 0.40 mass% or less.

[0050] Ceq=C+Mn / 6+(Ni+Cu) / 15+(Cr+Mo+V) / 5 (i) In the above formula (i), the content (mass%) of the corresponding element is substituted for each element symbol, and 0 is substituted when the element is not contained.

[0051] M: 0.06 to 0.25 mass% M is a value that serves as an index of carbon concentration in untransformed austenite and is expressed by the following (ii). If M is less than 0.06 mass%, the desired amount of MA cannot be obtained in the electric resistance weld, and the YR cannot be reduced. Therefore, M is set to 0.06 mass% or more. M is preferably 0.10 mass% or more, and more preferably 0.12 mass% or more. On the other hand, if M exceeds 0.25 mass%, the MA area ratio increases and the low-temperature toughness of the electric resistance weld deteriorates. Therefore, M is set to 0.25 mass% or less. M is preferably 0.20 mass% or less.

[0052] M=C / 3+5×Nb (ii) In the above formula (ii), the content (mass%) of the corresponding element is substituted for each element symbol, and 0 is substituted when the element is not contained.

[0053] 2. Microstructure As described above, in order to improve the strength and low-temperature toughness of an electric resistance welded steel pipe for linepipe, it is important to control the microstructure of the base material and the electric resistance weld. The microstructures of the base material and the electric resistance weld are described in detail below. In this embodiment, the thickness of the base material is denoted as tB, and the thickness of the electric resistance weld is denoted as tS. In this embodiment, the electric resistance weld refers to the range from the butt surface of the electric resistance weld to a position 600 μm away from the base material in the circumferential direction (i.e., a range of 1200 μm in total, centered on the butt surface).

[0054] <Base Material> [Area Fraction of Ferrite in the Microstructure of the 1 / 2×tB Portion of the Base Material is 40 to 80%] Controlling the microstructure of the base material is important to ensure the strength and low-temperature toughness of an electric resistance welded steel pipe for linepipe. Specifically, the microstructure of the base material needs to contain 40 to 80% ferrite in area fraction. If the area fraction of ferrite contained in the base material is less than 40%, the low-temperature toughness of the base material will deteriorate. Therefore, the area fraction of ferrite in the base material is set to 40% or more. The area fraction of ferrite is preferably 45% or more, and more preferably 50% or more. On the other hand, if the area fraction of ferrite in the base material is more than 80%, sufficient strength cannot be obtained in the base material. Therefore, the area fraction of ferrite in the base material is set to 80% or less. The area fraction of ferrite is preferably 75% or less, and more preferably 70% or less.

[0055] In this embodiment, the concept of "ferrite" includes polygonal ferrite and pseudo-polygonal ferrite. Furthermore, the microstructure of the base material may include one or more of pearlite (P), bainite (B), and retained austenite (γ) as residual structures. The concept of "bainite" includes granular bainitic ferrite and bainitic ferrite. Furthermore, the concept of "pearlite" includes pseudo-pearlite, which is an incomplete lamellar cementite structure. The area ratio of these residual structures may be 20 to 60% in relation to the area ratio of ferrite.

[0056] In this embodiment, the "½×tB portion of the base material" refers to a position (½)×tB from the outer surface of the base material in the thickness direction. The reason for limiting the microstructure at the position (½)×tB from the outer surface of the base material is that the structure at this position affects the low-temperature toughness of the base material. In this embodiment, when simply referring to the surface of an electric resistance welded steel pipe for linepipe, it means the outer surface, not the inner surface.

[0057] The ferrite ratio (area fraction) of the microstructure of the base material is measured using the following method. For the base material of an electric resistance welded steel pipe for line pipe use, a sample for microstructure observation is taken from a position 90° circumferentially from the electric resistance weld so that the cross section parallel to the pipe axis direction (longitudinal direction) and thickness direction serves as the observation surface. The electric resistance weld can be easily distinguished from the base material because the bead formed by electric resistance welding is machined. The sample for microstructure observation taken is polished for 30 to 60 minutes using a colloidal silica abrasive. The polished sample is analyzed using EBSP-OIM (trademark) (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy) to determine the ferrite area fraction. The field of view range in the thickness direction is a 200 μm range centered on (½)×tB from the outer surface in the thickness direction, and a 500 μm range at any position in the tube axis direction. The observation magnification is 400x, the acceleration voltage is 20 kV, and the measurement step is 0.3 μm. The measurement device used is an EBSD device consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL high-speed Hikari detector).

[0058] Specifically, the ferrite area ratio is determined using the KAM (Kernel Average Misorientation) method implemented in the EBSP-OIM (trademark). In the KAM method, any one regular hexagonal pixel in the measurement data is taken as the center pixel. The misorientation between each pixel is determined for a first approximation (7 pixels in total) using six pixels adjacent to this center pixel, a second approximation (19 pixels in total) using 12 pixels further outside these six pixels, or a third approximation (37 pixels in total) using 18 pixels further outside these 12 pixels. The determined misorientation is averaged, and the obtained average value is taken as the value of the center pixel. This operation is performed for all pixels.

[0059] In this embodiment, the area ratio of pixels calculated to have a misorientation of 1° or less to the third approximation relative to the total area of ​​the field of view is defined as the area ratio of ferrite. Misorientations exceeding 1° to the third approximation are defined as structures other than ferrite, such as bainite. Retained austenite is defined as the fcc phase measured by EBSP-OIM, and pearlite is identified by observation with an optical microscope at 400x magnification after nital etching.

[0060] [Average grain size of 35 μm or less in the microstructure of the 1 / 2 × tB portion of the base material] In the electric resistance welded steel pipe for line pipe according to this embodiment, in order to ensure good toughness of the base material, the average grain size in the microstructure of the 1 / 2 × tB portion of the base material is set to 35 μm or less. If the average grain size exceeds 35 μm, sufficient toughness in the base material is deteriorated. The average grain size is preferably 30 μm or less, and more preferably 20 μm or less. There is no particular restriction on the lower limit of the average grain size in the microstructure of the 1 / 2 × tB portion of the base material, but it may be 1 μm or more, 3 μm or more, or 5 μm or more.

[0061] The average grain size in the microstructure of the 1 / 2 × tB portion of the base material is measured using the following method. Using the same sample as the sample used to measure the ferrite area ratio, the microstructure in the 1 / 2 × tB portion of the base material is analyzed using EBSP-OIM to determine the average grain size. The field of view is a 200 μm range centered at (1 / 2) × tB from the outer surface in the thickness direction, and a 500 μm range at any position in the tube axis direction. The observation magnification is 400x, and the measurement step is 0.3 μm. From the data obtained by the measurement, regions surrounded by high-angle grain boundaries with a tilt angle of 15° or more are considered to be grains, and the circle-equivalent diameter of these grains is considered to be the grain size. The average grain size is calculated from the obtained grain size using the AREA FRACTION method. However, regions with a circle-equivalent diameter of 0.25 μm or less are excluded from the calculation of the average grain size. This is because an area of ​​0.25 μm or less in equivalent circle diameter cannot be evaluated correctly due to the measurement limit.

[0062] <Earth Resistance Welded Zone> [Area fraction of ferrite in the microstructure at the ¼×tS portion of the electric resistance welded zone is 40 to 70%] The microstructure of the electric resistance welded zone can be controlled by reheating the electric resistance welded zone and then water cooling it from the outer surface side. In the electric resistance welded zone, the low-temperature toughness deteriorates due to an increase in hardness on the outer surface side. Therefore, in the electric resistance welded steel pipe for linepipe according to this embodiment, it is necessary to include soft ferrite in the microstructure of the ¼×tS portion of the electric resistance welded zone in order to ensure low-temperature toughness in the electric resistance welded zone.

[0063] Specifically, the ferrite area ratio in the microstructure of the 1 / 4×tS portion of the electric resistance weld needs to be 40 to 70%. If the ferrite area ratio is less than 40%, the hardness of the outer surface of the electric resistance weld increases, resulting in a deterioration in low-temperature toughness. The ferrite area ratio is preferably 45% or more, and more preferably 50% or more. Furthermore, if the ferrite area ratio in the microstructure of the 1 / 4×tS portion of the electric resistance weld exceeds 70%, the strength of the electric resistance weld decreases. The ferrite area ratio is preferably 65% ​​or less, and more preferably 60% or less.

[0064] [Insular martensite (MA) area fraction is 0.2 to 10.0% in the microstructure of the 1 / 4×tS portion of the electric resistance weld] The area fraction of island martensite (MA) also affects the low-temperature toughness of the electric resistance weld. Because MA increases the fracture initiation point and hardness, the lower the MA area fraction, the better the low-temperature toughness of the electric resistance weld. Figure 1 shows the relationship between the MA area fraction of the electric resistance weld and δc. δc represents the critical opening displacement in a CTOD test at -20°C conducted using the method described below. As shown in Figure 1, the critical opening displacement δc in a CTOD test at -20°C of the electric resistance weld increases as the MA area fraction of the electric resistance weld decreases. It can be seen that if the MA area fraction is 10.0% or less, δc is 0.15 mm or greater.

[0065] Furthermore, MA increases the work hardening capacity, and therefore also affects the yield ratio (YR). The higher the area ratio of MA, the better the work hardening capacity and the lower the YR. Figure 2 shows the relationship between the MA area ratio of an electric resistance weld and the YR obtained by conducting a tensile test using the method described below. As shown in Figure 2, the YR of an electric resistance weld decreases as the MA area ratio of the electric resistance weld increases, and it can be seen that if the MA area ratio is 0.2% or more, the YR of the electric resistance weld is 90% or less. Unlike the base material, an electric resistance weld does not contain a hard structure caused by center segregation, so controlling the MA area ratio is essential to reduce the YR of the electric resistance weld.

[0066] Therefore, in the electric resistance welded steel pipe for line pipe according to this embodiment, in order to achieve both excellent low-temperature toughness and low YR in the electric resistance weld, the area fraction of MA in the microstructure of the 1 / 4×tS portion of the electric resistance weld is set to 0.2 to 10.0%. If the area fraction of MA is less than 0.2%, the work hardening capacity decreases, resulting in an increased YR. The area fraction of MA is preferably 1.0% or more, more preferably 2.0% or more, and even more preferably 3.0% or more. Furthermore, if the area fraction of MA in the microstructure of the 1 / 4×tS portion of the electric resistance weld exceeds 10.0%, the low-temperature toughness of the electric resistance weld deteriorates. The area fraction of MA is preferably 8.0% or less, more preferably 7.0% or less, and even more preferably 6.0% or less.

[0067] In this embodiment, the "¼×tS portion of the electric resistance weld" refers to a position (¼)×tS from the outer surface of the electric resistance weld in the thickness direction. The reason for limiting the microstructure at the position (¼)×tS from the outer surface of the electric resistance weld in the thickness direction is that the structure at this position affects the low-temperature toughness and YR of the electric resistance weld.

[0068] The microstructure of the electric resistance weld may contain one or more of pearlite (P), bainite (B), and retained austenite (γ) as residual structures. The concept of "bainite" includes granular bainitic ferrite and bainitic ferrite. The area ratio of these residual structures may be 20.0 to 59.8% in relation to the area ratios of ferrite and MA.

[0069] The ferrite area fraction in the microstructure of an electric resistance weld is determined by the following method. A sample for microstructure observation is taken from an electric resistance welded steel pipe for linepipe use, so that the observation surface is a cross section perpendicular to the pipe axis direction, including the electric resistance weld. As described above, in this embodiment, the electric resistance weld refers to the area from the butt surface of the electric resistance weld to a position 600 μm away from the base metal in the circumferential direction (i.e., a total of 1200 μm from the butt surface). The observation surface is mirror-finished by wet polishing, and the ferrite area fraction is measured using EBSD in the same manner as for the base metal. The measurement positions are a 200 μm range from the outer surface in the thickness direction, centered at (¼) × tS, and a 200 μm range in the circumferential direction, centered at a position 400 μm away from the butt surface of the electric resistance weld. The butt surface of the electric resistance weld can be identified and distinguished from the base metal by etching with nital.

[0070] The area ratio of MA in the microstructure of the electric resistance weld is determined by the following method. A sample is collected using the same method as when measuring the area ratio of ferrite in the electric resistance weld. After LePera etching the observation surface, a microstructure photograph is taken using an optical microscope at 400x magnification. White areas observed in the obtained microstructure photograph can be identified as island martensite, and the area ratio of island martensite (MA) is calculated by image analysis. The field of view is a 200 μm range centered at (¼) × tS from the outer surface in the thickness direction, and a 200 μm range centered at a position 400 μm away from the butt surface of the electric resistance weld in the circumferential direction. However, areas with a circle equivalent diameter of 1 μm or less are excluded from the area ratio of MA. This is because MA with a circle equivalent diameter of 1 μm or less does not affect low-temperature toughness and YR.

[0071] [The average grain size in the microstructure at the ¼ × tS portion of the electric resistance weld is 35 μm or less] In order to ensure good low-temperature toughness in the electric resistance weld, it is important to control the area ratio of ferrite and MA as described above, as well as to refine the microstructure. In the electric resistance weld steel pipe for line pipe according to this embodiment, the average grain size in the microstructure at the ¼ × tS portion of the electric resistance weld is controlled to 35 μm or less in order to ensure low-temperature toughness in the electric resistance weld. If the average grain size exceeds 35 μm, the low-temperature toughness of the electric resistance weld deteriorates. The average grain size is preferably 30 μm or less, more preferably 20 μm or less, and even more preferably 15 μm or less. There is no particular lower limit for the average grain size, but it may be 1 μm or more, 3 μm or more, or 5 μm or more.

[0072] The average grain size in the electric resistance weld is determined in the same manner as for the base material. The field of view is a 200 μm range centered on (1 / 4) × tS from the outer surface in the thickness direction, and a 200 μm range centered 400 μm away from the butt surface of the electric resistance weld in the circumferential direction. Regions with an equivalent circle diameter of 0.25 μm or less are excluded from the calculation of the average grain size. This is because regions with an equivalent circle diameter of 0.25 μm or less do not adversely affect the low-temperature toughness of the electric resistance weld.

[0073] 3. Mechanical Properties (Base Material and Electric Resistance Welded Part) Yield stress (YS): 360 MPa or more Tensile strength (TS): 465 MPa or more Yield ratio (YR): 90% or less Since the electric resistance welded steel pipe for line pipe according to this embodiment is intended to be used as a line pipe, the yield stress (YS) measured in both the base material and the electric resistance welded part is 360 MPa or more, and the tensile strength (TS) is 465 MPa or more. The yield stress is preferably 400 MPa or more or 450 MPa or more. The yield stress may be 600 MPa or less or 550 MPa or less. The tensile strength is preferably 500 MPa or more or 550 MPa or more. The tensile strength may also be 700 MPa or less or 650 MPa or less. The yield ratio (YR) is preferably 90% or less. The yield ratio may also be 80% or more or 85% or more. The yield ratio can be determined by dividing the yield ratio by the tensile strength (YS / TS).

[0074] (Base material portion) Charpy impact absorption energy at -20°C: 150 J or more In the electric resistance welded steel pipe according to this embodiment, the base material portion preferably has a Charpy impact absorption energy at -20°C of 150 J or more. If the base material portion has a Charpy impact absorption energy at -20°C of 150 J or more, sufficient toughness can be ensured even when used in cold climates, etc. The Charpy impact absorption energy at -20°C may be 400 J or less or 350 J or less.

[0075] (Electricity welded portion) Critical opening displacement δc in CTOD test at -20°C: 0.15 mm or more In the electric resistance welded portion of the electric resistance welded steel pipe for line pipe according to this embodiment, the critical opening displacement δc in a CTOD test at -20°C is preferably 0.15 mm or more. If the critical opening displacement δc in a CTOD test at -20°C is 0.15 mm or more, it can be determined that the electric resistance welded portion has excellent low-temperature toughness. The critical opening displacement δc in a CTOD test at -20°C may be 1.00 mm or less or 0.80 mm or less.

[0076] The tensile test of the base material is performed using a full-thickness test piece in the longitudinal direction of the electric resistance welded steel pipe for line pipe. The yield strength and tensile strength are measured based on the tensile test results. The base material tensile test piece is taken from a portion corresponding to a position 90° circumferentially from the seam of the electric resistance welded steel pipe. The tensile test is performed in accordance with DNV-ST-F101 (2021 edition).

[0077] For the tensile test of the electric resistance welded portion, a round bar test piece in the longitudinal direction of the electric resistance welded steel pipe for line pipe is used as the tensile test piece, and the tensile test is performed by taking it from the 1 / 2 × tS portion of the weld of the electric resistance welded steel pipe. The round bar tensile test piece is an ISO 6892 (2019 edition) proportional test piece with a parallel section diameter of φ12.7 mm and a gauge length of 65 mm. Yield strength and tensile strength are measured based on the tensile test results. The tensile test is performed in accordance with ISO 6892.

[0078] In a Charpy test to evaluate the low-temperature toughness of the base material, a V-notch Charpy test specimen is taken from the center of the wall thickness of the base material of an electric resistance welded steel pipe for line pipe (the portion corresponding to a position 90° circumferentially from the butt surface of the electric resistance weld) so that the longitudinal direction of the test specimen is the circumferential direction of the electric resistance welded steel pipe for line pipe. In this case, the depth direction of the V-notch is the longitudinal direction of the steel pipe. The V-notch Charpy test is conducted at a test temperature of -20°C, and the impact absorption energy at -20°C is measured. The Charpy test is conducted in accordance with DNV-ST-F101 (2021 edition).

[0079] In a CTOD (Crack Tip Opening Displacement) test for evaluating the low-temperature toughness of an electric resistance weld, an electric resistance welded steel pipe for line pipe is cut into a length of 300 mm in the longitudinal direction and 300 mm in the circumferential direction, including the electric resistance weld, to obtain a CTOD test specimen including the electric resistance weld. The CTOD test specimen is fabricated with a fatigue pre-crack on the butt surface of the electric resistance weld, with the depth direction of the fatigue pre-crack aligned in the longitudinal direction of the steel pipe. A CTOD test was conducted on this CTOD test specimen at a test temperature of -20°C in accordance with the provisions of BS7448-1:1991, and the critical tip opening displacement δc (mm) at -20°C was measured. If a CTOD test specimen cannot be obtained due to its circular arc shape, the sleeve of the test specimen may be welded together and the test may be conducted.

[0080] 4. Wall Thickness The wall thickness tB of the base metal portion and the wall thickness tS of the electric resistance welded portion of the electric resistance welded steel pipe for line pipe according to this embodiment are set to 15.0 mm or more from the viewpoint of buckling resistance performance when used as a line pipe. The wall thickness tB and the wall thickness tS are preferably 17.0 mm or more. On the other hand, the wall thickness tB and the wall thickness tS of the electric resistance welded steel pipe for line pipe generally have an upper limit of 25.4 mm.

[0081] 5. Outer Diameter The outer diameter of the electric resistance welded steel pipe for line pipe according to this embodiment is set to 304.8 mm or more from the viewpoint of improving the transport efficiency of the fluid passing through the pipe when used as a line pipe. On the other hand, the upper limit of the outer diameter of the electric resistance welded steel pipe for line pipe is generally 660.4 mm.

[0082] 6. Manufacturing Method The electric resistance welded steel pipe for line pipe according to this embodiment can achieve the effects as long as it has the above-mentioned characteristics, regardless of the manufacturing method. The electric resistance welded steel pipe for line pipe according to this embodiment can be manufactured by a manufacturing method including, for example, the following steps: (a) a casting step of manufacturing a slab having a predetermined chemical composition; (b) a heating step of heating the slab; (c) a hot rolling step of hot rolling the heated slab to form a hot rolled steel sheet; (d) a winding step of cooling and coiling the hot rolled steel sheet after the hot rolling step; (e) an electric resistance welding step of uncoiling the hot rolled steel sheet after the winding step, roll-forming it into a tubular shape, and electric resistance welding it to form an electric resistance welded steel pipe; (f) a heat treatment step of heat treating the electric resistance welded portion of the electric resistance welded steel pipe; and (g) sizing may be performed, if necessary, to improve roundness. Preferred conditions for each step are described below.

[0083] <Casting Step> In the casting step, steel having the above-described chemical composition is melted in a furnace and then cast into a slab. The casting method is not particularly limited, and may be any of ordinary continuous casting, casting by the ingot method, thin slab casting, etc.

[0084] <Heating Step> In the heating step, the manufactured slab is heated in a heating furnace. The heating temperature T (°C) of the slab in the heating furnace is preferably 1100 to 1170°C. The residence time t (minutes) in the furnace is preferably 100 to 450 minutes. In this embodiment, the residence time t (minutes) is the time from when the slab is charged into the heating furnace until when the slab is removed from the heating furnace.

[0085] In the heating step, it is further preferable that F1 defined by the following formula (iii) be 2800 to 3700.

[0086] F1=(T+273.15)×log(t) (iii) In the formula (iii), T is the heating temperature (° C.) in the heating step, and t is the time (minutes) the material remains in the furnace.

[0087] If the heating conditions are inappropriate, the austenite grain size during heating will coarsen, which will in turn coarsen the average crystal grain size in the 1 / 2 × tB portion of the base material, potentially degrading the low-temperature toughness of the base material. On the other hand, the segregation state at the slab (coil) end corresponding to the welded portion of an electric resistance welded steel pipe is affected by the slab heating conditions, ultimately affecting the generation of MA in the electric resistance weld. A segregation zone caused by the slab (coil) exists in the electric resistance weld. Because the segregation zone has a higher alloy concentration of C, Mn, etc. than the non-segregation zone, transformation is relatively more difficult to initiate compared to the non-segregation zone. The alloy concentrates in the non-segregation zone that transformed first, and MA is generated along the segregation zone. If the alloy concentration of the segregation zone is high, the area fraction of MA in the electric resistance weld increases, which may degrade the low-temperature toughness of the electric resistance weld. If the alloy concentration of the segregation zone of the electric resistance weld is low, the area fraction of MA in the electric resistance weld decreases, thereby increasing the YR. The electric resistance welded zone corresponds to the end of the slab (coil), and the segregation zone in this zone is not a central (macro) segregation that exists in the center of the plate thickness at the center of the width of the slab (coil), but a microsegregation, and is therefore significantly affected by the heating conditions during the production of the hot-rolled steel sheet.

[0088] Therefore, in this embodiment, by appropriately heating the slab under heating conditions that take into account the heating temperature and residence time in the furnace, coarsening of the heated austenite grain size in the slab before hot rolling is suppressed, and atoms are uniformly diffused to control the segregation zone at the width edge of the hot-rolled steel sheet, i.e., the electric resistance weld after electric resistance welding. Specifically, it is preferable to control the heating temperature and residence time in the furnace so that F1 expressed by formula (iii) is 2800 to 3700.

[0089] Assuming that the content of each element in the chemical composition of the slab is within the range of this embodiment and that formulas (i) and (ii) are satisfied, if F1 is less than 2800, even if other manufacturing conditions are met, the MA area ratio of the electric resistance weld will increase, and the low-temperature toughness of the electric resistance weld may deteriorate. Furthermore, if F1 exceeds 3700, the austenite grain size will coarsen during heating, and the average crystal grain size of the base material will coarsen, which may deteriorate the low-temperature toughness of the base material. Furthermore, if F1 exceeds 3700, the MA area ratio of the electric resistance weld will decrease, and the YR of the electric resistance weld may increase.

[0090] <Hot Rolling Process> In the hot rolling process, it is preferable to set the reduction ratio in the recrystallized region to 2.0 or more and the reduction ratio in the non-recrystallized region to 2.0 or more. In particular, by setting the reduction ratio in the non-recrystallized region to 2.0 or more, it is possible to make the average crystal grain size of the base material 20 μm or less. The boundary between the recrystallized region and the non-recrystallized region is about 900 to 950° C., depending on the composition of the steel.

[0091] The finish rolling start temperature is preferably 900 to 950°C in order to ensure low-temperature toughness by rolling in the non-recrystallized region. The hot rolling end temperature (finish rolling end temperature) is preferably 770°C or higher. If the hot rolling end temperature is less than 770°C, the rolling will be in a two-phase region, and the toughness of the base material will deteriorate.

[0092] <Coiling process> In the coiling process, the steel sheet after the hot rolling process is cooled to a surface temperature in the range of 500 to 650°C so that the average cooling rate at the center of the sheet thickness is in the range of 5 to 80°C / sec, and the steel sheet is coiled in this temperature range. The average cooling rate at the center of the sheet thickness can be calculated by heat transfer calculation from the temperature history of the outer surface.

[0093] In order to control the microstructure of the base material of the electric resistance welded steel pipe for line pipe according to this embodiment so that it has a predetermined structure, it is particularly important to control the cooling rate. If the average cooling rate is less than 5°C / sec, ferrite transformation may proceed, and the ferrite area ratio may exceed 80%. On the other hand, if the average cooling rate exceeds 80°C / sec, the cooling rate may be too fast, preventing ferrite transformation and resulting in a ferrite area ratio of less than 40%.

[0094] Furthermore, if the cooling stop temperature exceeds 650°C, ferrite transformation occurs after coiling, and the ferrite area ratio may exceed 80%. If the cooling stop temperature (coiling temperature) is less than 500°C, temperature variations during cooling increase, resulting in variations in strength, making it impossible to stably produce the electric resistance welded steel pipe for line pipe according to this embodiment.

[0095] <Electric Resistance Welding Process> An electric resistance welded steel pipe for line pipe is manufactured while uncoiling a coiled hot-rolled steel sheet. Specifically, the hot-rolled steel sheet is processed into an open pipe by bending using a series of forming rolls. Next, the joints of the open pipe, i.e., both end faces in the width direction of the hot-rolled steel sheet, are welded by electric resistance welding to manufacture an electric resistance welded steel pipe for line pipe.

[0096] <Heat Treatment Step> In the heat treatment step, the electric resistance welded portion formed in the electric resistance welding step is heated from the outer surface and then water-cooled from the outer surface side. Heating can be performed by, for example, induction heating.

[0097] Specifically, the heating involves heating the electric resistance weld to a temperature range of 870 to 1070°C, and then cooling it with water to a surface temperature of 500°C or less so that the average cooling rate of the 1 / 4×tS portion is in the range of 5 to 30°C / s. The average cooling rate of the 1 / 4×tS portion can be calculated by heat transfer calculations based on the temperature history of the outer surface. This heat treatment (heating and cooling) makes it possible to control the microstructure of the electric resistance weld (fraction of each structure, average crystal grain size) within the above-mentioned ranges.

[0098] If the heating temperature is below 870°C, regions that do not transform into austenite remain during heat treatment, causing the microstructure to coarsen and the average crystal grain size to coarsen, which may result in a deterioration in the low-temperature toughness of the electric resistance weld.If the heating temperature exceeds 1070°C, coarse austenite is generated during heat treatment, causing the microstructure to coarsen after cooling, which may result in a deterioration in the low-temperature toughness of the electric resistance weld.

[0099] If the average cooling rate is less than 5°C / s, the ferrite area fraction increases and the strength of the electric resistance weld may decrease. If the average cooling rate is more than 80°C / s, the ferrite area fraction may fall below 40%, and the low-temperature toughness of the electric resistance weld may deteriorate.

[0100] If the cooling stop temperature exceeds 500°C, the area ratio of MA in the electric resistance welded portion increases, which may result in a deterioration in low temperature toughness.

[0101] The effects of one embodiment of the present invention will be explained in more detail below using examples, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0102] Steel types A1 to A47 having the chemical compositions shown in Tables 1A and 1B (the balance being Fe and impurities) were produced. These steel types A1 to A47 were heated, hot-rolled, cooled, and coiled under the conditions shown in Tables 2A and 2B to obtain hot-rolled steel sheets.

[0103] The obtained hot-rolled steel sheets were subjected to bending using forming rolls and electric resistance welding, and the welded portions were heat treated (heated and water cooled) under the specified conditions as shown in Tables 2A and 2B to produce electric resistance welded steel pipes for line pipes. The wall thicknesses tB and tS of the produced electric resistance welded steel pipes were in the range of 15.0 to 25.4 mm, and the outer diameters were in the range of 304.8 to 660.4 mm.

[0104] For the obtained electric resistance welded steel pipes, the area fraction of ferrite and the average grain size in the microstructure of the 1 / 2×tB portion of the base material, and the area fraction of ferrite, the area fraction of island martensite (MA), and the average grain size in the microstructure of the 1 / 4×tS portion of the electric resistance weld were evaluated using the methods described above. The results are shown in Tables 3A and 3B. In the metal structure of the base material, the remainder other than ferrite was bainite, pearlite, and retained austenite. In the metal structure of the electric resistance weld, the remainder other than ferrite was bainite, pearlite, and retained austenite.

[0105] Furthermore, tensile tests, Charpy tests at -20°C, and CTOD tests at -20°C were performed using the methods described above to evaluate strength (yield stress, tensile strength), yield ratio (YR), and low-temperature toughness (impact absorption energy at -20°C, critical opening displacement δc at -20°C). The results are shown in Tables 4A and 4B.

[0106] When the yield stress (YS) was 360 MPa or more and the tensile strength (TS) was 465 MPa or more in both the base material and the electric resistance welded parts, the electric resistance welded steel pipe for line pipe use was judged to be pass, as it had high strength in the base material and the electric resistance welded parts. On the other hand, when the yield stress (YS) was less than 360 MPa or the tensile strength (TS) was less than 465 MPa, the electric resistance welded steel pipe for line pipe use was judged to be fail, as it did not have high strength in the base material and the electric resistance welded parts.

[0107] When the yield ratio (YR = YS / TS) was 90% or less in both the base material and the electric resistance welded portion, the electric resistance welded steel pipe for line pipe use was judged to be acceptable as it had a low YR in the base material and the electric resistance welded portion. On the other hand, when the yield ratio was more than 90%, the electric resistance welded steel pipe for line pipe use was judged to be unacceptable as it did not have a low YR in the base material and the electric resistance welded portion.

[0108] When the Charpy impact absorption energy at −20° C. was 150 J or more in the base material, the electric resistance welded steel pipe for line pipe had excellent low-temperature toughness in the base material and was judged to pass. On the other hand, when the Charpy impact absorption energy at −20° C. was less than 150 J, the electric resistance welded steel pipe for line pipe did not have excellent low-temperature toughness in the base material and was judged to fail.

[0109] When the critical opening displacement δc in the CTOD test at −20° C. was 0.15 mm or more in the electric resistance welded portion, the electric resistance welded steel pipe for line pipe use was judged to have excellent low-temperature toughness in the electric resistance welded portion and to have passed the test. On the other hand, when the critical opening displacement δc in the CTOD test at −20° C. was less than 0.15 mm, the electric resistance welded steel pipe for line pipe use was judged to have excellent low-temperature toughness in the electric resistance welded portion and to have failed the test.

[0110] As shown in Tables 1A to 4B, for Test Nos. 1 to 34, the chemical composition and microstructure of the base material were within the ranges of the present invention, and the microstructure of the electric resistance weld was also within the ranges of the present invention. As a result, it can be seen that electric resistance welded steel pipes for linepipe use were obtained that had high strength, low YR, and excellent low-temperature toughness. On the other hand, for Test Nos. 35 to 61, which are comparative examples, the properties did not satisfy the pass criteria for the reasons described below.

[0111] In Test No. 35, the C content exceeded the upper limit of the range of the present invention, causing the center segregation to harden, resulting in a deterioration in the low-temperature toughness of the base material.

[0112] In Test No. 36, the C content was below the lower limit of the range of the present invention, resulting in an increase in YR in the base material.

[0113] In Test No. 37, the Si content exceeded the upper limit of the range of the present invention, and oxides increased in the electric resistance welded joint, resulting in a deterioration in the low temperature toughness of the electric resistance welded joint.

[0114] In Test No. 38, the Si content was below the lower limit of the range of the present invention, and deoxidation was insufficient, resulting in deterioration of the toughness of the base material.

[0115] In Test No. 39, the Mn content exceeded the upper limit of the range of the present invention, and the central segregation in the base material hardened, resulting in a deterioration in the low-temperature toughness of the base material.

[0116] In Test No. 40, the Mn content was below the lower limit of the range of the present invention, and as a result, sufficient strength was not obtained in the base material and the electric resistance welded joint.

[0117] In Test No. 41, the Ti content exceeded the upper limit of the range of the present invention, and coarse inclusions were formed, resulting in a deterioration in the low-temperature toughness of the base material.

[0118] In Test No. 42, the Ti content was below the lower limit of the range of the present invention, resulting in coarsening of the average crystal grain size of the base material and deterioration of the low-temperature toughness.

[0119] In Test No. 43, the Nb content and M content exceeded the upper limits of the ranges defined by the present invention, resulting in the formation of coarse inclusions. As a result, the low-temperature toughness of the base material deteriorated. In addition, the MA area ratio in the electric resistance weld increased, resulting in the deterioration of the low-temperature toughness of the electric resistance weld.

[0120] In Test No. 44, the Nb content and M content were below the lower limit of the range of the present invention. As a result, the average crystal grain size of the base material was coarsened, and the low-temperature toughness of the base material was deteriorated. In addition, sufficient strength was not obtained in the base material. In addition, the MA area ratio in the electric resistance welded joint was reduced, and the YR was increased.

[0121] In Test No. 45, the N content exceeded the upper limit of the range of the present invention, and coarse inclusions were formed, resulting in a deterioration in the low-temperature toughness of the base material.

[0122] In Test No. 46, the N content was below the lower limit of the range of the present invention, resulting in coarsening of the average crystal grain size of the base material and deterioration of the low-temperature toughness of the base material.

[0123] In Test No. 47, the Ceq exceeded the upper limit of the range of the present invention, resulting in a decrease in the ferrite fraction, which resulted in a deterioration in the low-temperature toughness of the base material and the electric resistance welded joint.

[0124] In Test No. 48, Ceq was below the lower limit of the range of the present invention, and the ferrite fraction increased, resulting in insufficient strength being obtained in the base material and the electric resistance welded joint.

[0125] In Test No. 49, M exceeded the upper limit of the range of the present invention. As a result, the MA area ratio in the electric resistance weld increased, and the low-temperature toughness of the electric resistance weld deteriorated.

[0126] In Test No. 50, M was below the lower limit of the range of the present invention. As a result, the MA area ratio in the electric resistance welded portion decreased and YR increased.

[0127] In Test No. 51, F1 exceeded the upper limit, resulting in coarsening of the average grain size of the base material. As a result, the low-temperature toughness of the base material deteriorated. In addition, the alloy concentration in the segregation zone of the electric resistance weld became too low, resulting in a decrease in the MA area ratio in the electric resistance weld and an increase in YR.

[0128] In Test No. 52, F1 was below the lower limit, so the alloy concentration in the segregation zone of the electric resistance weld was high and the area ratio of MA increased. As a result, the low-temperature toughness of the electric resistance weld was deteriorated.

[0129] In Test No. 53, the rolling reduction ratio in the non-recrystallized region was below the lower limit, so the average crystal grain size of the base material increased, resulting in a deterioration in the low temperature toughness of the base material.

[0130] In Test No. 54, the cooling rate after hot rolling was high, and the ferrite area ratio of the base material was reduced, resulting in a deterioration in the low-temperature toughness of the base material.

[0131] In Test No. 55, the cooling rate after hot rolling was low, and the ferrite area ratio of the base material increased. As a result, sufficient strength was not obtained in the base material.

[0132] In Test No. 56, the cooling stop temperature after hot rolling and the coiling temperature were high, which increased the ferrite area ratio of the base material. As a result, sufficient strength was not obtained in the base material.

[0133] In Test No. 57, the heating temperature of the electric resistance weld was high, which increased the average grain size of the electric resistance weld, resulting in a deterioration in the low temperature toughness of the weld.

[0134] In Test No. 58, the heating temperature of the electric resistance weld was low, and the average crystal grain size of the electric resistance weld was large. As a result, the low temperature toughness of the electric resistance weld was deteriorated.

[0135] In Test No. 59, the cooling rate of the electric resistance weld was high, resulting in a decrease in the ferrite area ratio, and as a result, the low-temperature toughness of the electric resistance weld was deteriorated.

[0136] In Test No. 60, the cooling rate of the electric resistance weld was low, and the ferrite area ratio increased. As a result, sufficient strength was not obtained in the electric resistance weld.

[0137] In Test No. 61, the cooling stop temperature of the weld was high, and the area ratio of MA increased, resulting in a deterioration in the low-temperature toughness of the electric resistance weld.

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[0146] According to the above-described aspects of the present invention, an electric resistance welded steel pipe for line pipe can be obtained that has high strength, low YR, and excellent low-temperature toughness in the base metal and electric resistance welded joint, and therefore has high industrial applicability.

Claims

1. A steel sheet having a base metal portion and an electric resistance welded portion, wherein the chemical composition of the base metal portion is, in mass%, C: 0.060 to 0.120%, Si: 0.01 to 0.50%, Mn: 0.5 to 2.0%, P: 0.030% or less, S: 0.0050% or less, Al: 0.080% or less, Ti: 0.003 to 0.030%, Nb: 0.003 to 0.046%, N: 0.0010 to 0.0080%, O: 0.005% or less, Cu: 0 to 0.500%, Ni: 0 to 0.500%, Cr: 0 to 0.500%, Mo: 0 to 0.500%, V: 0 to 0.100%, W : 0 to 0.500%, Ca: 0 to 0.0040%, REM: 0 to 0.0050%, and the balance: Fe and impurities, wherein Ceq represented by the following formula (i) is 0.16 to 0.53 mass%, M represented by the following formula (ii) is 0.06 to 0.25 mass%, wherein, when the thickness of the base material portion is tB and the thickness of the electric resistance weld portion is tS, the tB and tS are 15.0 to 25.4 mm, and the outer diameter is 304.8 to 660.4 mm, in the microstructure of a 1 / 2 x tB portion of the base material, the area ratio of ferrite is 40 to 80% and the average crystal grain size is 35 μm or less, and in the microstructure of a 1 / 4 x tS portion of the electric resistance weld portion, An electric resistance welded steel pipe for linepipe, having an area fraction of ferrite of 40 to 70%, an area fraction of island martensite of 0.2 to 10.0%, an average grain size of 35 μm or less, a yield stress of 360 MPa or more, and a tensile strength of 465 MPa or more, wherein Ceq=C+Mn / 6+(Ni+Cu) / 15+(Cr+Mo+V) / 5 ... (i) M=C / 3+5×Nb ... (ii) where the element symbols in formulas (i) and (ii) are substituted with the content (mass%) of the corresponding element, or 0 is substituted if the element is not contained.

2. An electric resistance welded steel pipe for line pipe according to claim 1, wherein the chemical composition of the base material contains, in mass %, one or more elements selected from the group consisting of Cu: more than 0% and not more than 0.500%, Ni: more than 0% and not more than 0.500%, Cr: more than 0% and not more than 0.500%, Mo: more than 0% and not more than 0.500%, V: more than 0% and not more than 0.100%, W: more than 0% and not more than 0.500%, Ca: more than 0% and not more than 0.0040%, and REM: more than 0% and not more than 0.0050%.

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