Electric resistance welded steel pipes and steel plates that are the raw material for electric resistance welded steel pipes

Abstract

To provide an electric resistance welded pipe that is excellent in strength, low-temperature toughness, HIC resistance and deformability.SOLUTION: An electric resistance welded pipe contains, in mass%, C: 0.030-0.100%, Si: 0.03-0.60%, Mn: 0.30-1.60%, P: 0-0.030%, S: 0-0.0020%, Al: 0.005-0.500%, N: 0.0005-0.0100%, Nb: 0.005-0.080%, Ti: 0.005-0.200%, Ca: 0.0001-0.0100%, O: 0-0.0050% and satisfies formulae (1)-(3) defined in the embodiments. The microstructure comprises polygonal ferrite of 90-98% in area ratio, and a hard phase and satisfies formula (4) defined in the embodiments.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present disclosure relates to electric resistance welded steel pipes and steel plates, and more particularly to electric resistance welded steel pipes used in pipelines and steel plates that are the raw materials for the electric resistance welded steel pipes. [Background technology]

[0002] A pipeline laid on the seabed is composed of multiple steel pipes (line pipes). A pipeline laid on the seabed is subjected to high pressure from production fluids, such as natural gas and crude oil, passing through the inside of the pipeline. The pipeline is further subjected to repeated strain from waves and seawater pressure from the outside. Therefore, the steel pipes (line pipes) that make up the pipeline are required to have strength and low-temperature toughness. Therefore, when electric resistance welded steel pipes are used as line pipes, the electric resistance welded steel pipes are required to have excellent strength and excellent low-temperature toughness.

[0003] Furthermore, the production fluid flowing inside the pipeline may contain corrosive gases such as hydrogen sulfide, etc. Therefore, electric resistance welded steel pipes for pipeline use are required to have not only excellent strength and excellent low-temperature toughness, but also excellent HIC resistance.

[0004] Techniques for improving the strength, low-temperature toughness, and HIC resistance of electric resistance welded steel pipes have been proposed in WO 2016 / 047023 (Patent Document 1) and JP 2005-290546 A (Patent Document 2).

[0005] The steel strip for electric resistance welded steel pipes disclosed in Patent Document 1 has a composition consisting of, in mass%, C: 0.02-0.06%, Si: 0.1-0.3%, Mn: 0.8-1.3%, P: 0.01% or less, S: 0.001% or less, V: 0.04-0.07%, Nb: 0.04-0.07%, Ti: 0.01-0.04%, Cu: 0.1-0.3%, Ni: 0.1-0.3%, Ca: 0.001-0.005%, Al: 0.01-0.07%, N: 0.007% or less, the balance being Fe and unavoidable impurities, and the contents of C, Nb, V, and Ti satisfy the condition of formula (1). Here, formula (1) is [C]-12([Nb] / 92.9+[V] / 50.9+[Ti] / 47.9)≦0.03%. Furthermore, the steel strip disclosed in Patent Document 1 has a ferrite area ratio of 90% or more. In this steel strip, the steel structure is made into a ferrite single phase, thereby improving HIC resistance. Furthermore, Patent Document 1 describes that the high concentration of Nb, V, and Ti causes fine precipitates to be precipitated, thereby improving the strength of the steel strip.

[0006] The hot rolled steel sheet for electric resistance welded steel pipe disclosed in Patent Document 2 contains, by mass%, C: 0.01-0.12%, Si: 0.01-0.8%, Mn: 0.6-1.8%, P: 0.02% or less, S: 0.01% or less, Ti: 0.001-0.05%, Nb: 0.01-0.10%, Ca: 0.0001-0.005%, Al: 0.01-0.06%, N: 0.006% or less, O: 0.006% or less, the balance being Fe and unavoidable impurities, and has a chemical composition in which the A value defined by formula (1) is 20 to 80. Here, formula (1) is A = {C - (12 / 93 × Nb + 12 / 48 × Ti)} / C × 100. Furthermore, the ratio (SR) of the tensile strength in the same direction to the stress at which a longitudinal elongation of 4% is obtained is 89% or less. Patent Document 2 describes that by limiting the components of this steel plate so that the A value is 20 or more, coarsening of grains in the weld HAZ is suppressed and low-temperature toughness is improved. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2016 / 047023 [Patent Document 2] JP 2005-290546 A Summary of the Invention [Problem to be solved by the invention]

[0008] By the way, the reeling method is one of the methods for laying pipelines on the seabed. In the reeling method, a long pipeline consisting of multiple connected steel pipes (line pipes) is manufactured on land in advance and wound onto a spool on a reel barge. The wound pipeline is unwound from the spool at sea and laid on the seabed.

[0009] In the above-mentioned reeling method, tensile stress and compressive stress are applied to the electric resistance welded steel pipe used in the pipeline due to winding and unwinding. In this case, if the electric resistance welded steel pipe has low deformability, local buckling may occur. Local buckling may be the starting point for fracture of the electric resistance welded steel pipe. Therefore, it is preferable to suppress the occurrence of local buckling. Therefore, electric resistance welded steel pipe for pipeline use is required to have not only strength, low-temperature toughness, and HIC resistance, but also excellent deformability.

[0010] The steel strips and hot-rolled steel sheets disclosed in Patent Documents 1 and 2 mention the strength, low-temperature toughness, and HIC resistance of electric resistance welded steel pipes, but do not study how to improve deformability in addition to the strength, low-temperature toughness, and HIC resistance.

[0011] An object of the present disclosure is to provide an electric resistance welded steel pipe having excellent strength, excellent low-temperature toughness, excellent HIC resistance and excellent deformability, and a steel plate that is a material for the electric resistance welded steel pipe. [Means for solving the problem]

[0012] The electric resistance welded steel pipe according to the present disclosure is The base material is, by mass%, C: 0.030~0.100%, Si: 0.03 to 0.60%, Mn: 0.30-1.60%, P: 0~0.030%, S: 0~0.0020%, Al: 0.005 to 0.500%, N: 0.0005~0.0100%, Nb: 0.005 to 0.080%, Ti: 0.005 to 0.200%, Ca: 0.0001 to 0.0100%, O: 0 to 0.0050%, Ni: 0-1.00%, Mo: 0 to 0.50%, V: 0 to 0.200%, Cr: 0-1.00%, Cu: 0-1.00%, Mg: 0 to 0.0020%, Rare earth elements: 0 to 0.0200%, and The balance: Fe and impurities, and satisfying formulas (1) to (3), The microstructure of the base material portion is Polygonal ferrite with an area ratio of 90 to 98% A hard phase consisting of one or more of pearlite, bainite, and martensite, with an area ratio of 2 to 10%, It consists of: In a rectangular observation region including a wall thickness central portion of the base material, the length of a side extending in the T direction, which is the wall thickness direction of the base material, is 200 μm, and the length of a side extending in the L direction, which is the tube axis direction of the base material, is 200 μm, Nine line segments extending in the T direction, arranged at equal intervals in the L direction, and dividing the rectangular observation area into 10 equal parts in the L direction are defined as T1 to T9; Nine line segments extending in the L direction, arranged at equal intervals in the T direction, and dividing the rectangular observation area into 10 equal parts in the T direction are defined as L1 to L9; The number A1 of intersections between the line segments T1 to T9 and the hard phase interfaces which are interfaces between the polygonal ferrite and the hard phase, and the number A2 of intersections between the line segments L1 to L9 and the hard phase interfaces satisfy formula (4). 0.20≦C+Si / 24+Mn / 6+Ni / 40+Cr / 5+Mo / 4+V / 3+Nb / 3≦0.35 (1) 0.020≦Nb+V+Ti≦0.085 (2) Ca / S≧2.0 (3) A1 / A2≦1.8 (4) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol.

[0013] The steel sheet according to the present disclosure has In mass%, C: 0.030~0.100%, Si: 0.03 to 0.60%, Mn: 0.30-1.60%, P: 0~0.030%, S: 0~0.0020%, Al: 0.005 to 0.500%, N: 0.0005~0.0100%, Nb: 0.005 to 0.080%, Ti: 0.005 to 0.200%, Ca: 0.0001 to 0.0100%, O: 0 to 0.0050%, Ni: 0-1.00%, Mo: 0 to 0.50%, V: 0 to 0.200%, Cr: 0-1.00%, Cu: 0-1.00%, Mg: 0 to 0.0020%, Rare earth elements: 0 to 0.0200%, and The balance: Fe and impurities, and satisfying formulas (1) to (3), The microstructure is Polygonal ferrite with an area ratio of 90 to 98% A hard phase consisting of one or more of pearlite, bainite, and martensite, with an area ratio of 2 to 10%, It consists of: In a rectangular observation area including the center of the thickness of the steel plate, the length of a side extending in the T direction, which is the thickness direction of the steel plate, is 200 μm, and the length of a side extending in the L direction, which is the rolling direction of the steel plate, is 200 μm. Nine line segments extending in the T direction, arranged at equal intervals in the L direction, and dividing the rectangular observation area into 10 equal parts in the L direction are defined as T1 to T9; Nine line segments extending in the L direction, arranged at equal intervals in the T direction, and dividing the rectangular observation area into 10 equal parts in the T direction are defined as L1 to L9; The number A1 of intersections between the line segments T1 to T9 and the hard phase interfaces which are interfaces between the polygonal ferrite and the hard phase, and the number A2 of intersections between the line segments L1 to L9 and the hard phase interfaces satisfy formula (4). 0.20≦C+Si / 24+Mn / 6+Ni / 40+Cr / 5+Mo / 4+V / 3+Nb / 3≦0.35 (1) 0.020≦Nb+V+Ti≦0.085 (2) Ca / S≧2.0 (3) A1 / A2≦1.8 (4) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol. Effect of the Invention

[0014] The electric resistance welded steel pipe according to the present disclosure has excellent strength, excellent low-temperature toughness, excellent HIC resistance and excellent deformability. The steel plate according to the present disclosure is suitable as a material for the above-mentioned electric resistance welded steel pipe. [Brief description of the drawings]

[0015] [Figure 1] FIG. 1 is a continuous cooling transformation curve diagram of the electric resistance welded steel pipe and steel plate according to the present embodiment. [Diagram 2] FIG. 2 is a schematic diagram of the microstructure of an electric resistance welded steel pipe having the same chemical composition as the electric resistance welded steel pipe of this embodiment, and the contents of each element in the chemical composition satisfy formulas (1) to (3), but having a different microstructure. [Diagram 3] FIG. 3 is a schematic diagram of the microstructure of the electric resistance welded steel pipe according to this embodiment. [Figure 4] FIG. 4 is a schematic diagram for explaining how to find F4 (=A1 / A2) according to this embodiment, with reference to FIG. [Diagram 5] FIG. 5 is a schematic diagram for explaining how to find F4 (=A1 / A2) according to this embodiment, with reference to FIG. [Figure 6] FIG. 6 is a flow diagram showing an example of a manufacturing process for an electric resistance welded steel pipe according to this embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The present inventors have investigated and examined the strength, low-temperature toughness, HIC resistance and deformability of electric resistance welded steel pipes for pipeline use.

[0017] First, the present inventors conducted a study from the viewpoint of chemical composition on electric resistance welded steel pipes that have improved strength, low-temperature toughness, and HIC resistance while also providing sufficient deformability. As a result, it was found that the base metal of the electric resistance welded steel pipes contains, in mass%, C: 0.030-0.100%, Si: 0.03-0.60%, Mn: 0.30-1.60%, P: 0-0.030%, S: 0-0.0020%, Al: 0.005-0.500%, N: 0.0005-0.0100%, Nb: 0.005-0.080%, Ti: 0.005-0.200%, Ca: 0.0001-0.0100%, It was believed that a chemical composition consisting of O: 0-0.0050%, Ni: 0-1.00%, Mo: 0-0.50%, V: 0-0.200%, Cr: 0-1.00%, Cu: 0-1.00%, Mg: 0-0.0020%, rare earth elements: 0-0.0200%, and the balance: Fe and impurities could potentially provide excellent strength, excellent low-temperature toughness, excellent HIC resistance, and excellent deformability.

[0018] However, it was found that even if the content of each element in the chemical composition of the base material is within the above-mentioned range, there are cases where all of the strength, low-temperature toughness, HIC resistance, and deformability are not necessarily sufficiently improved. Therefore, the present inventors conducted further studies. As a result, it was found that, on the premise that the content of each element in the chemical composition of the base material is within the above-mentioned range, the strength, low-temperature toughness, HIC resistance, and deformability can be further improved by further satisfying the following formulas (1) to (3). 0.20≦C+Si / 24+Mn / 6+Ni / 40+Cr / 5+Mo / 4+V / 3+Nb / 3≦0.35 (1) 0.020≦Nb+V+Ti≦0.085 (2) Ca / S≧2.0 (3) Here, the content (mass %) of the corresponding element is substituted for each element symbol in formulas (1) to (3). Formulas (1) to (3) will be explained below.

[0019] [Regarding formula (1)] In order to improve the deformability of an electric resistance welded steel pipe, it is effective to increase the area ratio of polygonal ferrite in the microstructure of the base material of the electric resistance welded steel pipe. If the microstructure of the base material is mainly polygonal ferrite, the deformability of the electric resistance welded steel pipe is improved. Furthermore, if the polygonal ferrite is fine, the strength of the electric resistance welded steel pipe is improved.

[0020] F1 is defined as C+Si / 24+Mn / 6+Ni / 40+Cr / 5+Mo / 4+V / 3+Nb / 3. F1 is an index related to the microstructure of the base metal of an electric resistance welded steel pipe. Of the elements in the chemical composition of the base metal of an electric resistance welded steel pipe, C, Si, Mn, Ni, Cr, Mo, V, and Nb all affect the S curve (ferrite region, pearlite region, and bainite region) of the Continuous Cooling Transformation Diagram (CCT diagram) of an electric resistance welded steel pipe whose base metal has the above-mentioned chemical composition.

[0021] Fig. 1 is a continuous cooling transformation curve (CCT diagram) of an electric resistance welded steel pipe according to this embodiment. That is, Fig. 1 is a CCT diagram of an electric resistance welded steel pipe in which the content of each element in the chemical composition of the base metal is within the above-mentioned range. In Fig. 1, F indicates ferrite nose, P indicates pearlite nose, and B indicates bainite nose.

[0022] Referring to Fig. 1, when F1, which is composed of C, Si, Mn, Ni, Cr, Mo, V, and Nb, exceeds 0.35, even if the content of each element in the chemical composition of the base metal is within the above-mentioned range and formulas (2) and (3) are satisfied, the S curve in Fig. 1 shifts too far to the right. In this case, the deformability of the electric resistance welded steel pipe decreases. The reason for this is as follows.

[0023] If F1 is too high and the S curve shifts too far to the right, the temperature at which the cooling curve C1 enters the ferrite nose (ferrite region) will be low. In this case, the amount of hard phases such as pearlite, bainite, and martensite produced will increase. Therefore, the area ratio of polygonal ferrite in the microstructure of the base material will decrease. As a result, the deformability of the electric resistance welded steel pipe will decrease.

[0024] On the other hand, if F1 is less than 0.20, even if the content of each element in the chemical composition of the base material is within the above-mentioned range and formulas (2) and (3) are satisfied, the S curve shifts too far to the left in FIG. 1. In this case, the temperature when the cooling curve C1 enters the ferrite nose (ferrite region) becomes high. Therefore, the steel material transforms from austenite to ferrite while the temperature of the steel material is high. The driving force for the phase transformation at this time is small. Therefore, ferrite transformation nuclei are not easily generated. On the other hand, because the temperature of the steel material is high, the growth of ferrite grains is fast. As a result, the generated polygonal ferrite grains become coarse. As a result, the strength of the electric resistance welded steel pipe decreases.

[0025] If F1 is 0.20 to 0.35, the content of each element in the chemical composition of the base material is within the above-mentioned range, and on the premise that formulas (2) and (3) are satisfied, the S curves of each phase in FIG. 1 (ferrite, pearlite, bainite) are positioned at appropriate positions in the CCT diagram. In this case, as shown in FIG. 1, the cooling curve C1 enters the ferrite region at an appropriate temperature, and the steel material can be cooled while passing mainly through the ferrite region. Therefore, a structure mainly composed of fine polygonal ferrite is generated in the base material of the electric resistance welded steel pipe. As a result, excellent strength and excellent deformability can be obtained.

[0026] [Regarding formula (2)] In order to further increase the strength of electric resistance welded steel pipes, it is effective to utilize precipitation strengthening by carbides, nitrides, carbonitrides, and the like of Nb, V, and Ti.

[0027] F2 is defined as Nb + V + Ti. Among the elements in the chemical composition of the base metal of an electric resistance welded steel pipe, Nb, V and Ti all form carbides, nitrides and carbonitrides (hereinafter, carbides, nitrides and carbonitrides are collectively referred to as "carbides, etc."). Therefore, the strength of the electric resistance welded steel pipe is increased by precipitation strengthening of carbides, etc. of Nb, V and Ti. Therefore, F2 is an index of the strength of the electric resistance welded steel pipe.

[0028] If F2 is less than 0.020, even if the content of each element in the chemical composition of the base material is within the above-mentioned range and satisfies formula (1) and formula (3), the precipitation of carbides of Nb, V, and Ti is insufficient. As a result, the strength of the electric resistance welded steel pipe is not sufficiently increased. On the other hand, if F2 exceeds 0.085, even if the content of each element in the chemical composition of the base material is within the above-mentioned range and satisfies formula (1) and formula (3), the strength of the electric resistance welded steel pipe is excessively increased. In this case, the low temperature toughness and HIC resistance of the electric resistance welded steel pipe are reduced.

[0029] If F2 is 0.020 to 0.085, the strength, low-temperature toughness, and HIC resistance of the electric resistance welded steel pipe can be improved, provided that the content of each element in the chemical composition of the base material is within the above-mentioned range and formulas (1) and (3) are satisfied.

[0030] [Regarding formula (3)] In order to improve the HIC resistance of electric resistance welded steel pipes, it is effective to add Ca and control the morphology of sulfides such as MnS in the electric resistance welded steel pipes. Among the inclusions in electric resistance welded steel pipes, sulfides tend to elongate during rolling in the manufacturing process. Hydrogen tends to accumulate in the elongated sulfides. Sulfides that have accumulated hydrogen tend to become the starting point for cracks. Therefore, it is preferable to suppress the formation and elongation of sulfides as much as possible.

[0031] F3 is defined as F3=Ca / S. F3 is an index of HIC resistance of electric resistance welded steel pipe. Among the elements in the chemical composition of the base metal of electric resistance welded steel pipe, Ca and S affect the control of the morphology of sulfides. Specifically, by reducing the S content, it is possible to suppress the formation of sulfides. Ca also bonds with S to form CaS. Therefore, by increasing the Ca content, the amount of S that bonds with Mn is reduced. This suppresses the formation of sulfides such as MnS, and the elongation of sulfides. This suppresses the accumulation of hydrogen in sulfides, and the occurrence of cracks. As a result, the HIC resistance of electric resistance welded steel pipe is improved.

[0032] If F3 is less than 2.0, even if the content of each element in the chemical composition of the base metal is within the above-mentioned range and formulas (1) and (2) are satisfied, the morphology of sulfides is insufficiently controlled, and therefore the HIC resistance of the electric resistance welded steel pipe is not sufficiently improved.

[0033] If F3 is 2.0 or more, the HIC resistance of the electric resistance welded steel pipe can be improved, provided that the content of each element in the chemical composition of the base material is within the above-mentioned range and formulas (1) and (2) are satisfied.

[0034] As described above, if the content of each element in the chemical composition of the base metal is within the above-mentioned range and formulas (1) to (3) are satisfied, the strength, low-temperature toughness, HIC resistance, and deformability of the electric resistance welded steel pipe can be improved to a certain degree. However, it was found that there are still cases where HIC resistance is not sufficiently improved. Therefore, the present inventors investigated the cause of the insufficient improvement of HIC resistance in the electric resistance welded steel pipe having the above-mentioned configuration. As a result, the present inventors obtained the following findings.

[0035] In the electric resistance welded steel pipe having the above-mentioned configuration, the microstructure of the base material is mainly polygonal ferrite. More specifically, the microstructure of the base material is mainly polygonal ferrite, and the remainder is a hard phase. Here, the hard phase is one or more selected from the group consisting of pearlite, bainite, and martensite.

[0036] In the base material of the electric resistance welded steel pipe, the hard phase is likely to be generated in a band-like shape extending in the pipe axis direction of the base material of the electric resistance welded steel pipe. FIG. 2 is a schematic diagram of the microstructure in a cross section including the wall thickness direction (T direction) and the pipe axis direction (L direction) of the electric resistance welded steel pipe. Referring to FIG. 2, the microstructure of the electric resistance welded steel pipe includes polygonal ferrite 20 and hard phase 10. Here, even in an electric resistance welded steel pipe in which the content of each element in the chemical composition is within the above-mentioned range and satisfies formulas (1) to (3), as shown in FIG. 2, multiple hard phases 10 may extend in the L direction. Such a band-like extending hard phase is likely to become the starting point of cracks. Therefore, if there are many extending hard phases, the HIC resistance of the electric resistance welded steel pipe decreases.

[0037] Based on the above findings, the present inventors focused on the shape of the hard phase in the microstructure. As a result of their investigation, they obtained the following findings. Referring to Fig. 3, in a microstructure of an electric resistance welded steel pipe in a cross section including the T-direction and L-direction, in which the content of each element in the chemical composition is within the above-mentioned range and satisfies formulas (1) to (3), even if the area ratio of polygonal ferrite and the area ratio of the hard phase are similar to those in Fig. 2, if the length of the hard phase 10 in the L-direction is short, that is, if the hard phase does not extend in a band shape, the hard phase 10 is unlikely to become a starting point of cracks. Therefore, HIC resistance is increased.

[0038] Based on the above findings, the present inventors further investigated the relationship between the shape of the hard phase in the microstructure and HIC resistance, and obtained the following findings.

[0039] 4, a rectangular observation region SQ is defined as a rectangular observation region that includes the wall thickness center of the base metal portion of the electric resistance welded steel pipe, has sides extending in the T direction of 200 μm long, and has sides extending in the L direction of 200 μm long. In the rectangular observation region SQ, line segments extending in the T direction that divide the rectangular observation region SQ into 10 equal parts in the L direction are defined as T1 to T9. Similarly, line segments extending in the L direction that divide the rectangular observation region SQ into 10 equal parts in the T direction are defined as L1 to L9. In other words, the rectangular observation region SQ is divided into 10 x 10 minute rectangular regions by the line segments T1 to T9 and the line segments L1 to L9.

[0040] The total number of intersections a1 (indicated by "◇" in Figs. 4 and 5) between the line segments T1 to T9 and the interface (hereinafter also referred to as hard phase interface) between the hard phase 10 and the parent phase (polygonal ferrite) 20 in the rectangular observation region SQ is defined as the intersection number A1 (pieces). The total number of intersections a2 (indicated by "●" in Figs. 4 and 5) between the line segments L1 to L9 and the hard phase interface in the rectangular observation region SQ is defined as the intersection number A2 (pieces).

[0041] In this case, if the ratio of the number of intersections A1 to the number of intersections A2 (=A1 / A2) satisfies the following formula (4), the L-direction length of the hard phase 10 is sufficiently short. Therefore, sufficient HIC resistance can be obtained, provided that the content of each element in the chemical composition is within the above-mentioned range, formulas (1) to (3) are satisfied, and the area ratios of polygonal ferrite and hard phase are appropriate. A1 / A2≦1.8 (4)

[0042] In the case of the microstructure example in Fig. 3, A1 / A2 is 1.4, as shown in Fig. 4. On the other hand, in the case of the microstructure example in Fig. 2, A1 / A2 is 6.0, as shown in Fig. 5.

[0043] Based on the above findings, an electric resistance welded steel pipe and a steel plate according to the present embodiment have been completed. The electric resistance welded steel pipe and the steel plate according to the present embodiment have the following configuration.

[0044] [1] The base material is, by mass%, C: 0.030~0.100%, Si: 0.03 to 0.60%, Mn: 0.30-1.60%, P: 0~0.030%, S: 0~0.0020%, Al: 0.005 to 0.500%, N: 0.0005~0.0100%, Nb: 0.005 to 0.080%, Ti: 0.005 to 0.200%, Ca: 0.0001 to 0.0100%, O: 0 to 0.0050%, Ni: 0-1.00%, Mo: 0 to 0.50%, V: 0 to 0.200%, Cr: 0-1.00%, Cu: 0-1.00%, Mg: 0 to 0.0020%, Rare earth elements: 0 to 0.0200%, and The balance: Fe and impurities, and satisfying formulas (1) to (3), The microstructure of the base material portion is Polygonal ferrite with an area ratio of 90 to 98% A hard phase consisting of one or more of pearlite, bainite, and martensite, with an area ratio of 2 to 10%, It consists of: In a rectangular observation region including a wall thickness central portion of the base material, the length of a side extending in the T direction, which is the wall thickness direction of the base material, is 200 μm, and the length of a side extending in the L direction, which is the tube axis direction of the base material, is 200 μm, Nine line segments extending in the T direction, arranged at equal intervals in the L direction, and dividing the rectangular observation area into 10 equal parts in the L direction are defined as T1 to T9; Nine line segments extending in the L direction, arranged at equal intervals in the T direction, and dividing the rectangular observation area into 10 equal parts in the T direction are defined as L1 to L9; The number A1 of intersections between the line segments T1 to T9 and a hard phase interface which is an interface between the polygonal ferrite and the hard phase, and the number A2 of intersections between the line segments L1 to L9 and the hard phase interface satisfy formula (4). ERW steel pipe. 0.20≦C+Si / 24+Mn / 6+Ni / 40+Cr / 5+Mo / 4+V / 3+Nb / 3≦0.35 (1) 0.020≦Nb+V+Ti≦0.085 (2) Ca / S≧2.0 (3) A1 / A2≦1.8 (4) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol.

[0045] [2] An electric resistance welded steel pipe according to [1], The base material portion comprises, in mass%, Ni: 0.01 to 1.00%, Mo: 0.01 to 0.50%, V: 0.001 to 0.200% Cr: 0.01 to 1.00%, Cu: 0.01 to 1.00%, Mg: 0.0001 to 0.0020%, and Rare earth elements: 0.0001 to 0.0200%, containing one or more selected from the group consisting of ERW steel pipe.

[0046] [3] An electric resistance welded steel pipe according to [1] or [2], The thickness is 10 to 25 mm. The outer diameter is 100~800mm. ERW steel pipe.

[0047] [4] A steel plate, In mass%, C: 0.030~0.100%, Si: 0.03 to 0.60%, Mn: 0.30-1.60%, P: 0~0.030%, S: 0~0.0020%, Al: 0.005 to 0.500%, N: 0.0005~0.0100%, Nb: 0.005 to 0.080%, Ti: 0.005 to 0.200%, Ca: 0.0001 to 0.0100%, O: 0 to 0.0050%, Ni: 0-1.00%, Mo: 0 to 0.50%, V: 0 to 0.200%, Cr: 0-1.00%, Cu: 0-1.00%, Mg: 0 to 0.0020%, Rare earth elements: 0 to 0.0200%, and The balance: Fe and impurities, and satisfying formulas (1) to (3), The microstructure is Polygonal ferrite with an area ratio of 90 to 98% A hard phase consisting of one or more of pearlite, bainite, and martensite, with an area ratio of 2 to 10%, It consists of: In a rectangular observation area including the center of the thickness of the steel plate, the length of a side extending in the T direction, which is the thickness direction of the steel plate, is 200 μm, and the length of a side extending in the L direction, which is the rolling direction of the steel plate, is 200 μm. Nine line segments extending in the T direction, arranged at equal intervals in the L direction, and dividing the rectangular observation area into 10 equal parts in the L direction are defined as T1 to T9; Nine line segments extending in the L direction, arranged at equal intervals in the T direction, and dividing the rectangular observation area into 10 equal parts in the T direction are defined as L1 to L9; The number A1 of intersections between the line segments T1 to T9 and a hard phase interface which is an interface between the polygonal ferrite and the hard phase, and the number A2 of intersections between the line segments L1 to L9 and the hard phase interface satisfy formula (4). steel plate. 0.20≦C+Si / 24+Mn / 6+Ni / 40+Cr / 5+Mo / 4+V / 3+Nb / 3≦0.35 (1) 0.020≦Nb+V+Ti≦0.085 (2) Ca / S≧2.0 (3) A1 / A2≦1.8 (4) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol.

[0048] [5] [4] A steel plate according to the present invention, In mass%, Ni: 0.01 to 1.00%, Mo: 0.01 to 0.50%, V: 0.001 to 0.200%, Cr: 0.01 to 1.00%, Cu: 0.01 to 1.00%, Mg: 0.0001 to 0.0020%, and Rare earth elements: 0.0001 to 0.0200%, containing one or more selected from the group consisting of steel plate.

[0049] The electric resistance welded steel pipe and the steel plate according to the present embodiment will be described in detail below. Here, the steel plate is a steel plate used in the manufacture of the electric resistance welded steel pipe, and corresponds to the material of the electric resistance welded steel pipe. "%" related to elements means mass % unless otherwise specified.

[0050] [Structure of electric resistance welded steel pipe] The electric resistance welded steel pipe according to the present embodiment has a base material and an electric resistance welded portion. The base material is cylindrical. The electric resistance welded portion extends in the axial direction (longitudinal direction) of the electric resistance welded steel pipe.

[0051] [Chemical composition] The chemical composition of the base metal of the electric resistance welded steel pipe according to this embodiment contains the following elements.

[0052] C: 0.030~0.100% Carbon (C) increases the strength of steel material. If the C content is less than 0.030%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the C content exceeds 0.100%, C forms excessive carbides and carbonitrides. Excessive carbides and carbonitrides can become the starting point of cracks. Therefore, in this case, even if the contents of other elements are within the range of this embodiment, the HIC resistance and low-temperature toughness of the electric resistance welded steel pipe are reduced. Therefore, the C content is 0.030 to 0.100%. The preferable lower limit of the C content is 0.032%, more preferably 0.035%, and more preferably 0.038%. The preferable upper limit of the C content is 0.090%, more preferably 0.080%, and more preferably 0.070%.

[0053] Silicon: 0.03 to 0.60% Silicon (Si) deoxidizes steel. If the Si content is less than 0.03%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the Si content exceeds 0.60%, the strength of the electric resistance welded steel pipe increases excessively even if the contents of other elements are within the range of this embodiment. As a result, the HIC resistance and low temperature toughness of the electric resistance welded steel pipe decrease. Therefore, the Si content is 0.03 to 0.60%. The preferred lower limit of the Si content is 0.05%, more preferably 0.08%, more preferably 0.10%, and even more preferably 0.15%. The preferred upper limit of the Si content is 0.50%, more preferably 0.40%, more preferably 0.35%, and even more preferably 0.30%.

[0054] Mn: 0.30-1.60% Manganese (Mn) improves the hardenability of steel material and increases the strength of electric resistance welded steel pipe. If the Mn content is less than 0.30%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the Mn content exceeds 1.60%, the HIC resistance of the electric resistance welded steel pipe decreases even if the contents of other elements are within the range of this embodiment. Therefore, the Mn content is 0.30 to 1.60%. The preferred lower limit of the Mn content is 0.40%, more preferably 0.50%, more preferably 0.60%, more preferably 0.70%, more preferably 0.80%, and more preferably 0.85%. The preferred upper limit of the Mn content is 1.50%, more preferably 1.40%, more preferably 1.35%, more preferably 1.30%, and more preferably 1.25%.

[0055] P: 0~0.030% Phosphorus (P) is an impurity. P segregates at grain boundaries and reduces the HIC resistance and low-temperature toughness of electric resistance welded steel pipes. Therefore, the P content is 0 to 0.030%. A preferred upper limit of the P content is 0.025%, more preferably 0.020%, more preferably 0.015%, and even more preferably 0.010%. The P content is preferably as low as possible. However, excessive reduction of the P content increases the manufacturing cost. Therefore, in consideration of normal industrial production, a preferred lower limit of the P content is more than 0%, more preferably 0.001%, and even more preferably 0.002%.

[0056] S: 0~0.0020% Sulfur (S) is an impurity. S combines with Mn to form MnS. MnS reduces the HIC resistance of electric resistance welded steel pipes. Therefore, the S content is 0 to 0.0020%. A preferred upper limit of the S content is 0.0018%, more preferably 0.0015%, and even more preferably 0.0010%. It is preferable that the S content is as low as possible. However, excessive reduction of the S content increases the manufacturing cost. Therefore, in consideration of normal industrial production, a preferred lower limit of the S content is more than 0%, more preferably 0.0001%, and even more preferably 0.0002%.

[0057] Al: 0.005 to 0.500% Aluminum (Al) deoxidizes steel. If the Al content is less than 0.005%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the Al content exceeds 0.500%, Al nitrides become coarse even if the contents of other elements are within the range of this embodiment. As a result, the HIC resistance and low-temperature toughness of the electric resistance welded steel pipe are reduced. Therefore, the Al content is 0.005 to 0.500%. The preferred lower limit of the Al content is 0.010%, more preferably 0.012%, and more preferably 0.015%. The preferred upper limit of the Al content is 0.400%, more preferably 0.300%, more preferably 0.200%, more preferably 0.100%, more preferably 0.080%, more preferably 0.050%, and more preferably 0.030%.

[0058] N: 0.0005~0.0100% Nitrogen (N) forms nitrides and carbonitrides to suppress the coarsening of austenite grains during the heating process. In other words, it refines polygonal ferrite. As a result, the HIC resistance and low-temperature toughness of the electric resistance welded steel pipe are improved. N also increases the strength of the electric resistance welded steel pipe by solid solution strengthening. If the N content is less than 0.0005%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the N content exceeds 0.0100%, the nitrides and carbonitrides become coarse even if the contents of other elements are within the range of this embodiment. In this case, the HIC resistance and low-temperature toughness of the electric resistance welded steel pipe are reduced. Therefore, the N content is 0.0005 to 0.0100%. The preferable lower limit of the N content is 0.0010%, more preferably 0.0015%, and further preferably 0.0020%. The upper limit of the N content is preferably 0.0090%, more preferably 0.0080%, more preferably 0.0070%, more preferably 0.0060%, and even more preferably 0.0050%.

[0059] Nb: 0.005 to 0.080% Niobium (Nb) combines with C and / or N in the steel material to form fine Nb carbides, etc. Fine Nb carbides, etc. suppress the coarsening of austenite grains. Therefore, polygonal ferrite is refined. As a result, the HIC resistance and low-temperature toughness of the electric resistance welded steel pipe are improved. Fine Nb carbides, etc. further increase the strength of the electric resistance welded steel pipe by precipitation strengthening. If the Nb content is less than 0.005%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the Nb content exceeds 0.080%, the Nb carbides, etc. become coarse even if the contents of other elements are within the range of this embodiment. As a result, the HIC resistance and low-temperature toughness of the electric resistance welded steel pipe are reduced. Therefore, the Nb content is 0.005 to 0.080%. The lower limit of the Nb content is preferably 0.008%, more preferably 0.010%, more preferably 0.013%, more preferably 0.020%, and more preferably 0.025%. The upper limit of the Nb content is preferably 0.075%, more preferably 0.070%, more preferably 0.065%, more preferably 0.060%, more preferably 0.055%, and more preferably 0.050%.

[0060] Ti: 0.005 to 0.200% Titanium (Ti) combines with C and / or N in the steel material to form fine Ti carbides and the like. Fine Ti carbides and the like suppress the coarsening of austenite grains. Therefore, polygonal ferrite is refined. As a result, the HIC resistance and low-temperature toughness of the electric resistance welded steel pipe are improved. Fine Ti carbides and the like further increase the strength of the electric resistance welded steel pipe by precipitation strengthening. If the Ti content is less than 0.005%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the Ti content exceeds 0.200%, the Ti carbides and the like coarse even if the contents of other elements are within the range of this embodiment. As a result, the HIC resistance and low-temperature toughness of the electric resistance welded steel pipe are reduced. Therefore, the Ti content is 0.005 to 0.200%. The preferable lower limit of the Ti content is 0.007%, more preferably 0.009%, and further preferably 0.010%. The upper limit of the Ti content is preferably 0.180%, more preferably 0.150%, more preferably 0.130%, more preferably 0.100%, more preferably 0.080%, more preferably 0.050%, more preferably 0.040%, more preferably 0.030%, and more preferably 0.020%.

[0061] Ca: 0.0001 to 0.0100% Calcium (Ca) controls the morphology of sulfides such as MnS and spheroidizes the sulfides. As a result, the HIC resistance of the electric resistance welded steel pipe is improved. If the Ca content is less than 0.0001%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the Ca content exceeds 0.0100%, coarse Ca oxides are formed even if the contents of other elements are within the range of this embodiment. As a result, the HIC resistance and low temperature toughness of the electric resistance welded steel pipe are reduced. Therefore, the Ca content is 0.0001 to 0.0100%. The preferred lower limit of the Ca content is 0.0005%, more preferably 0.0010%, and more preferably 0.0015%. The preferred upper limit of the Ca content is 0.0080%, more preferably 0.0070%, more preferably 0.0060%, more preferably 0.0050%, and more preferably 0.0040%.

[0062] O: 0 to 0.0050% Oxygen (O) is an impurity. O forms oxides and reduces the HIC resistance and low-temperature toughness of the electric resistance welded steel pipe. Therefore, the O content is 0 to 0.0050%. A preferred upper limit of the O content is 0.0045%, more preferably 0.0040%, more preferably 0.0035%, more preferably 0.0030%, more preferably 0.0025%, and more preferably 0.0020%. The O content is preferably as low as possible. However, excessive reduction of the O content increases the manufacturing cost. Therefore, in consideration of normal industrial production, a preferred lower limit of the O content is more than 0%, more preferably 0.0001%, more preferably 0.0002%, and more preferably 0.0005%.

[0063] The balance of the chemical composition of the base material of the electric resistance welded steel pipe according to this embodiment is composed of Fe and impurities. Here, the impurities refer to substances that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of the electric resistance welded steel pipe, and are permissible within a range that does not adversely affect the electric resistance welded steel pipe according to this embodiment.

[0064] [About optional elements] The chemical composition of the base metal of the electric resistance welded steel pipe according to this embodiment may further contain one or more elements selected from the group consisting of Ni, Mo, V, Cr and Cu in place of a portion of Fe. These elements are optional elements, and all of them increase the strength of the electric resistance welded steel pipe.

[0065] Ni: 0 to 1.00% Nickel (Ni) is an optional element and may not be contained. In other words, the Ni content may be 0%. When it is contained, that is, when the Ni content is more than 0%, Ni enhances the hardenability of the steel material and enhances the strength of the electric resistance welded steel pipe. If even a small amount of Ni is contained, the above effect can be obtained to a certain extent. However, if the Ni content exceeds 1.00%, the strength of the electric resistance welded steel pipe increases excessively even if the contents of other elements are within the range of this embodiment. As a result, the HIC resistance and low temperature toughness of the electric resistance welded steel pipe decrease. Therefore, the Ni content is 0 to 1.00%. The preferred lower limit of the Ni content is more than 0%, more preferably 0.01%, more preferably 0.03%, more preferably 0.05%, more preferably 0.08%, and more preferably 0.10%. The preferred upper limit of the Ni content is 0.80%, more preferably 0.50%, more preferably 0.40%, more preferably 0.30%, and more preferably 0.20%.

[0066] Mo: 0 to 0.50% Molybdenum (Mo) is an optional element and may not be contained. In other words, the Mo content may be 0%. When it is contained, that is, when the Mo content is more than 0%, Mo improves the hardenability of the steel material and increases the strength of the electric resistance welded steel pipe. Mo further combines with C and / or N in the steel material to form fine Mo carbides, etc. The fine Mo carbides, etc. increase the strength of the electric resistance welded steel pipe by precipitation strengthening. The fine Mo carbides, etc. further suppress the coarsening of austenite grains. Therefore, polygonal ferrite is refined. As a result, the HIC resistance and low temperature toughness of the electric resistance welded steel pipe are improved. If even a small amount of Mo is contained, the above effect can be obtained to some extent. However, if the Mo content exceeds 0.50%, the strength of the electric resistance welded steel pipe is excessively increased even if the contents of other elements are within the range of this embodiment. As a result, the HIC resistance and low temperature toughness of the electric resistance welded steel pipe are reduced. If the Mo content exceeds 0.50%, the Mo carbides, etc. further coarsen. Therefore, the HIC resistance and low temperature toughness of the electric resistance welded steel pipe are reduced. If the Mo content exceeds 0.50%, the HIC resistance of the electric resistance welded steel pipe is further reduced. Therefore, the Mo content is 0 to 0.50%. The preferred lower limit of the Mo content is more than 0%, more preferably 0.01%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the Mo content is 0.40%, more preferably 0.30%, more preferably 0.20%, and even more preferably 0.10%.

[0067] V: 0~0.200% Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When it is contained, that is, when the V content is more than 0%, V bonds with C and / or N in the steel material to form fine V carbides, etc. Fine V carbides, etc. increase the strength of the electric resistance welded steel pipe by precipitation strengthening. Fine V carbides, etc. further suppress the coarsening of austenite grains. Therefore, polygonal ferrite is refined. As a result, the HIC resistance and low temperature toughness of the electric resistance welded steel pipe are improved. If even a small amount of V is contained, the above effect can be obtained to a certain extent. However, if the V content exceeds 0.200%, the V carbides, etc. become coarse even if the contents of other elements are within the range of this embodiment. Therefore, the HIC resistance and low temperature toughness of the electric resistance welded steel pipe are reduced. Therefore, the V content is 0 to 0.200%. The lower limit of the V content is preferably more than 0%, more preferably 0.001%, more preferably 0.005%, more preferably 0.010%, and more preferably 0.015%. The upper limit of the V content is preferably 0.180%, more preferably 0.150%, more preferably 0.130%, more preferably 0.100%, more preferably 0.090%, more preferably 0.070%, more preferably 0.050%, and more preferably 0.030%.

[0068] Cr: 0~1.00% Chromium (Cr) is an optional element and may not be contained. That is, the Cr content may be 0%. When contained, that is, when the Cr content is more than 0%, Cr enhances the hardenability of the steel material and enhances the strength of the electric resistance welded steel pipe. If even a small amount of Cr is contained, the above effect can be obtained to a certain extent. However, if the Cr content exceeds 1.00%, the strength of the electric resistance welded steel pipe will be excessively increased even if the contents of other elements are within the range of this embodiment. As a result, the HIC resistance and low temperature toughness of the electric resistance welded steel pipe will decrease. Therefore, the Cr content is 0 to 1.00%. The preferred lower limit of the Cr content is more than 0%, more preferably 0.01%, more preferably 0.05%, more preferably 0.10%, and more preferably 0.15%. The preferred upper limit of the Cr content is 0.80%, more preferably 0.50%, more preferably 0.40%, more preferably 0.30%, and more preferably 0.25%.

[0069] Cu: 0-1.00% Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When it is contained, that is, when the Cu content is more than 0%, Cu enhances the hardenability of the steel material and enhances the strength of the electric resistance welded steel pipe. If even a small amount of Cu is contained, the above effect can be obtained to a certain degree. However, if the Cu content exceeds 1.00%, the strength of the electric resistance welded steel pipe increases excessively even if the contents of other elements are within the range of this embodiment. As a result, the HIC resistance and low temperature toughness of the electric resistance welded steel pipe decrease. Therefore, the Cu content is 0 to 1.00%. The preferred lower limit of the Cu content is more than 0%, more preferably 0.01%, more preferably 0.05%, more preferably 0.10%, and more preferably 0.15%. The preferred upper limit of the Cu content is 0.80%, more preferably 0.50%, more preferably 0.40%, more preferably 0.30%, and more preferably 0.25%.

[0070] The chemical composition of the base metal of the electric resistance welded steel pipe according to this embodiment may further contain one or more elements selected from the group consisting of Mg and rare earth elements in place of a portion of Fe. These elements are optional elements, and all of them deoxidize and desulfurize the steel.

[0071] Magnesium: 0 to 0.0020% Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When it is contained, that is, when the Mg content is more than 0%, Mg deoxidizes and desulfurizes the steel. If even a small amount of Mg is contained, the above effect can be obtained to some extent. However, when the Mg content exceeds 0.0020%, oxides aggregate or coarsen even if the contents of other elements are within the range of this embodiment. As a result, the HIC resistance and low-temperature toughness of the electric resistance welded steel pipe are reduced. Therefore, the Mg content is 0 to 0.0020%. The preferable lower limit of the Mg content is more than 0%, more preferably 0.0001%, more preferably 0.0003%, more preferably 0.0005%, more preferably 0.0007%, and more preferably 0.0010%. The preferable upper limit of the Mg content is 0.0019%, more preferably 0.0017%, and more preferably 0.0015%.

[0072] Rare earth elements: 0~0.0200% Rare earth elements (REM) are optional elements and may not be contained. That is, the REM content may be 0%. When contained, that is, when the REM content is more than 0%, REM deoxidizes and desulfurizes the steel. However, when the REM content exceeds 0.0200%, coarse oxides are formed even if the contents of other elements are within the range of this embodiment. As a result, the HIC resistance and low-temperature toughness of the electric resistance welded steel pipe are reduced. Therefore, the REM content is 0 to 0.0200%. The preferable lower limit of the REM content is more than 0%, more preferably 0.0001%, more preferably 0.0003%, more preferably 0.0005%, more preferably 0.0007%, and more preferably 0.0010%. The upper limit of the REM content is preferably 0.0180%, more preferably 0.0150%, more preferably 0.0130%, more preferably 0.0100%, more preferably 0.0080%, more preferably 0.0050%, more preferably 0.0030%, and more preferably 0.0020%.

[0073] In this embodiment, REM is a collective term for 17 elements in the periodic table, ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, plus yttrium (Y) and scandium (Sc). The content of REM means the total content of one or more of these elements.

[0074] [About parameter formulas] The base metal of the electric resistance welded steel pipe further satisfies formulas (1) to (3), provided that the content of each element in the chemical composition is within the range of this embodiment. 0.20≦C+Si / 24+Mn / 6+Ni / 40+Cr / 5+Mo / 4+V / 3+Nb / 3≦0.35 (1) 0.020≦Nb+V+Ti≦0.085 (2) Ca / S≧2.0 (3) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol.

[0075] Each of formulas (1) to (3) will be explained below.

[0076] [Regarding formula (1)] F1 (=C+Si / 24+Mn / 6+Ni / 40+Cr / 5+Mo / 4+V / 3+Nb / 3) affects the S curve (ferrite region, pearlite region, and bainite region) of the CCT curve diagram. Specifically, if F1 is too high, even if the content of each element in the chemical composition is within the range of this embodiment and satisfies formulas (2) and (3), the S curves of ferrite, pearlite, and bainite in FIG. 1 shift too far to the right in FIG. 1. In this case, the temperature at which the cooling curve C1 enters the ferrite region becomes lower. As a result, the amount of hard phase generated increases, and the area ratio of polygonal ferrite decreases. As a result, the deformability of the electric resistance welded steel pipe decreases.

[0077] On the other hand, if F1 is less than 0.20, even if the content of each element in the chemical composition is within the range of this embodiment and formulas (2) and (3) are satisfied, the S curve shifts too far to the left in FIG. 1. In this case, the temperature at which the cooling curve C1 enters the ferrite region is too high. Therefore, the polygonal ferrite grains become coarse. As a result, the strength of the electric resistance welded steel pipe decreases.

[0078] Assuming that the content of each element in the chemical composition is within the range of this embodiment and formula (2) and formula (3) are satisfied, if F1 is 0.20 to 0.35, the S curves of each phase (ferrite, pearlite, bainite) in FIG. 1 are positioned at appropriate positions in the CCT diagram. In this case, as shown in FIG. 1, the cooling curve C1 enters the ferrite region at an appropriate temperature, and the steel material can be cooled while passing mainly through the ferrite region. Therefore, a structure mainly composed of fine polygonal ferrite is generated. As a result, excellent strength and excellent deformability can be obtained in the electric resistance welded steel pipe.

[0079] The upper limit of F1 is preferably 0.33, more preferably 0.31, and even more preferably 0.29. The lower limit of F1 is preferably 0.21, more preferably 0.23, and even more preferably 0.25. The numerical value of F1 is a value obtained by rounding off to two decimal places.

[0080] [Regarding formula (2)] F2 (=Nb+V+Ti) is an index of strength based on precipitation strengthening. If F2 is less than 0.020, precipitation strengthening is not fully exerted even if the content of each element in the chemical composition is within the range of this embodiment and satisfies formula (1) and formula (3). Therefore, the strength of the electric resistance welded steel pipe is not sufficiently increased. On the other hand, if F2 exceeds 0.085, the strength of the electric resistance welded steel pipe is excessively increased even if the content of each element in the chemical composition is within the range of this embodiment and satisfies formula (1) and formula (3). Therefore, the HIC resistance and low temperature toughness of the electric resistance welded steel pipe are reduced.

[0081] If F2 is 0.020 to 0.085, the content of each element in the chemical composition is within the range of this embodiment, and on the premise that formulas (1) and (3) are satisfied, it is possible to obtain excellent strength, excellent HIC resistance, and excellent low-temperature toughness in an electric resistance welded steel pipe.

[0082] The lower limit of F2 is preferably 0.025, more preferably 0.030, more preferably 0.035, and more preferably 0.040. The upper limit of F2 is preferably 0.080, more preferably 0.075, and more preferably 0.070. The numerical value of F2 is a value obtained by rounding off the fourth decimal place.

[0083] [Regarding formula (3)] F3 (=Ca / S) is an index of HIC resistance based on morphological control of sulfides such as MnS. If F3 is less than 2.0, even if the contents of each element in the chemical composition are within the range of this embodiment and formulas (1) and (2) are satisfied, the morphological control of sulfides is insufficient. As a result, the HIC resistance of the electric resistance welded steel pipe is not sufficiently improved.

[0084] If F3 is 2.0 or more, the HIC resistance of the electric resistance welded steel pipe is sufficiently improved, provided that the content of each element in the chemical composition is within the range of this embodiment and formulas (1) and (2) are satisfied.

[0085] The lower limit of F3 is preferably 2.5, more preferably 3.0, and even more preferably 3.5. The upper limit of F3 is not particularly limited, but is, for example, 12.0. The numerical value of F3 is a value obtained by rounding off to one decimal place.

[0086] [About the microstructure] The microstructure of the base material of the electric resistance welded steel pipe according to this embodiment is composed of polygonal ferrite with an area ratio of 90 to 98% and a hard phase with an area ratio of 2 to 10%. The hard phase is composed of one or more of pearlite, bainite and martensite. In this specification, the "microstructure of the base material" refers to the microstructure of the base material excluding the heat-affected zone.

[0087] [Regarding the area ratio of polygonal ferrite] The microstructure of the base material of the electric resistance welded steel pipe according to this embodiment is mainly composed of polygonal ferrite, and the area ratio of polygonal ferrite is 90 to 98%.

[0088] If the microstructure is mainly composed of polygonal ferrite, that is, if the area ratio of polygonal ferrite is 90% or more, the deformability of the electric resistance welded steel pipe will be improved, provided that the content of each element in the chemical composition is within the range of this embodiment, the above formulas (1) to (3) are satisfied, and the shape of the hard phase satisfies the below-described formula (4). The preferable lower limit of the area ratio of polygonal ferrite is 91%, more preferably 92%, and even more preferably 93%.

[0089] The upper limit of the area ratio of polygonal ferrite is not particularly limited. However, in the case of the chemical composition of the base material portion of the present embodiment, the upper limit of the area ratio of polygonal ferrite is, for example, 98%. The upper limit of the area ratio of polygonal ferrite may be 97% or 96%.

[0090] [Area ratio of hard phase] As described above, the hard phase is one or more selected from the group consisting of pearlite, bainite, and martensite. If the area ratio of the hard phase is too high, the deformability of the electric resistance welded steel pipe is reduced. Therefore, the area ratio of the hard phase is 10% or less. The less the hard phase, the better. However, excessive reduction in the area ratio of the hard phase increases the manufacturing cost. Therefore, in consideration of normal industrial production, the preferred lower limit of the hard phase is 2%. Therefore, the area ratio of the hard phase is 2 to 10%. In addition, when the hard phase is a plurality of phases among pearlite, bainite, and martensite, the area ratio of the hard phase is the sum of the area ratios of each phase. The preferred upper limit of the area ratio of the hard phase is 9%, more preferably 8%, and more preferably 7%. The lower limit of the area ratio of the hard phase may be 3% or 4%.

[0091] [Method for measuring area ratio of polygonal ferrite and area ratio of hard phase] The area ratio of polygonal ferrite and the area ratio of hard phase are measured by the following method: A sample is taken from the base material of an electric resistance welded steel pipe, from the center of the wall thickness at a position 90° circumferentially shifted from the electric resistance weld (i.e., the base material portion excluding the heat-affected zone).

[0092] The surface of the sample to be observed is polished with colloidal silica abrasive for 30 to 60 minutes. The polished sample is subjected to the KAM (Kernel Average Misorientation) method using EBSP-OIM (trademark) to determine the area ratio (%) of polygonal ferrite and the area ratio (%) of hard phase by the following method. The observation field of view when measuring the area ratio of polygonal ferrite and the area ratio of hard phase by the KAM method is 200 μm × 500 μm. The observation magnification is 400 times, and the measurement step is 0.3 μm.

[0093] In the KAM method, an arbitrary regular hexagonal pixel in the measurement data is set as the center pixel. The orientation difference between each pixel is calculated for the first approximation (total of 7 pixels) using 6 pixels adjacent to this center pixel, or the second approximation (total of 19 pixels) using 12 pixels further outside these 6 pixels, or the third approximation (total of 37 pixels) using 18 pixels further outside these 12 pixels. The calculated orientation differences are averaged, and the obtained arithmetic average value is set as the value of the center pixel. This operation is performed for all pixels. Pixels whose orientation difference between adjacent pixels is 5° or less by the third approximation are displayed on the map. In this embodiment, the area fraction of pixels calculated to have an orientation difference of 1° or less in the third approximation relative to the total area of ​​the field of view is defined as the area fraction of polygonal ferrite (%). Phases other than polygonal ferrite are considered to be hard phases. The area fraction calculated by the following formula is defined as the area fraction of hard phase (%). Area ratio of hard phase (%) = 100 (%) - Area ratio of polygonal ferrite (%)

[0094] [Regarding the hard phase shape definition (Equation (4))] In the electric welded steel pipe according to this embodiment, the content of each element in the chemical composition is within the above-mentioned range, and formulas (1) to (3) are satisfied. The microstructure is presumably composed of 90 to 98% polygonal ferrite and 2 to 10% hard phase by area ratio. Furthermore, the shape of the hard phase in the microstructure is as follows.

[0095] Referring to Figure 4, a rectangular observation region SQ is defined as an area that includes the center of the thickness of the base material of the electric welded steel pipe, has a side length of 200 μm extending in the T direction, which is the thickness direction of the base material, and has a side length of 200 μm extending in the L direction, which is the pipe axial direction of the base material.

[0096] Fig. 4 is a schematic diagram of a rectangular observation region SQ to explain how to find F4 (=A1 / A2) according to this embodiment. With reference to Fig. 4, in the rectangular observation region SQ, line segments extending in the T direction that divide the rectangular observation region SQ into 10 equal parts in the L direction are defined as T1 to T9. Similarly, line segments extending in the L direction that divide the rectangular observation region SQ into 10 equal parts in the T direction are defined as L1 to L9. In other words, the rectangular observation region SQ is divided into minute rectangular regions of 10 x 10 by the line segments T1 to T9 and L1 to L9.

[0097] The number of intersections between the line segments T1 to T9 and the hard phase interfaces, which are interfaces between the polygonal ferrite and the hard phase, is designated as A1, and the number of intersections between the line segments L1 to L9 and the hard phase interfaces is designated as A2. In the electric resistance welded steel pipe according to this embodiment, the above A1 and A2 satisfy the following formula (4). A1 / A2≦1.8 (4)

[0098] F4 is defined as A1 / A2. F4 is an index of the shape of the hard phase in the microstructure of the electric resistance welded steel pipe. If F4 exceeds 1.8, the hard phase extends in the L direction even if the content of each element in the chemical composition of the base material is within the above-mentioned range, formulas (1) to (3) are satisfied, and the area ratio of polygonal ferrite in the microstructure of the base material is 90 to 98%, and the area ratio of the hard phase is 2 to 10%. Therefore, the HIC resistance of the electric resistance welded steel pipe is not sufficiently improved.

[0099] If F4 is 1.8 or less, the content of each element in the chemical composition of the base material is within the above-mentioned range, and formulas (1) to (3) are satisfied. On the premise that the area ratio of polygonal ferrite is 90 to 98% and the area ratio of hard phase is 2 to 10% in the microstructure of the base material, the generation of hard phase extending in the L direction is sufficiently suppressed. Therefore, the HIC resistance of the electric resistance welded steel pipe is sufficiently improved. The upper limit of F4 is preferably 1.7, more preferably 1.6, and even more preferably 1.5. The lower limit of F4 is not particularly limited, but is, for example, 1.0. The numerical value of F4 is a value obtained by rounding off to one decimal place.

[0100] [How to calculate F4] The above F4 is obtained by the following method. The above A1 and A2 are measured in the rectangular observation area SQ. The base material of the electric resistance welded steel pipe is cut parallel to the pipe axis direction (L direction) and the wall thickness direction (T direction) so as to pass through the center of the wall thickness (i.e., the base material part excluding the heat-affected zone) at a position 90° circumferentially shifted from the electric resistance welded part, and a sample including the above rectangular observation area SQ is taken. Of the surfaces of the taken sample, the surface corresponding to the cross section parallel to the pipe axis direction (L direction) of the base material is used as the observation surface.

[0101] After polishing the observation surface, it is etched with 3% nitric acid alcohol (Nital etching solution). Photographic images of five arbitrary rectangular observation areas SQ are generated using an optical microscope at 500x magnification of the etched observation surface. In each rectangular observation area SQ, the polygonal ferrite and the hard phase have different contrasts for each phase. Therefore, each phase is identified based on the contrast. In each rectangular observation area SQ, line segments T1 to T9 and line segments L1 to L9 are drawn according to the above-mentioned procedure. In any of the five rectangular observation areas SQ, the total number of intersections a1 between the line segments T1 to T9 and the hard phase interface, which is the interface between the polygonal ferrite and the hard phase, is calculated and defined as A1. In any of the five rectangular observation areas SQ, the total number of intersections a2 between the line segments L1 to L9 and the hard phase interface is calculated and defined as A2. Based on the obtained A1 and A2, F4 is calculated.

[0102] [Uses of electric resistance welded steel pipes] The electric resistance welded steel pipe according to this embodiment can be used for pipeline applications, and is particularly suitable for pipeline applications in sour environments exposed to corrosive gases such as hydrogen sulfide. The wall thickness of the electric resistance welded steel pipe according to this embodiment is not particularly limited. Even if the electric resistance welded steel pipe according to this embodiment is a thick-walled electric resistance welded steel pipe having a wall thickness of 10 mm or more, it can obtain excellent strength, excellent low-temperature toughness, excellent HIC resistance, and excellent deformability. The lower limit of the wall thickness of the electric resistance welded steel pipe according to this embodiment is, for example, 10 mm. The upper limit of the wall thickness of the electric resistance welded steel pipe according to this embodiment is, for example, 25 mm.

[0103] The outer diameter of the electric resistance welded steel pipe according to this embodiment is not particularly limited. The lower limit of the outer diameter of the electric resistance welded steel pipe according to this embodiment may be, for example, 100 mm, 110 mm, or 114 mm. The upper limit of the outer diameter of the electric resistance welded steel pipe according to this embodiment may be, for example, 800 mm, 700 mm, or 660 mm.

[0104] [Strength of electric resistance welded steel pipes] The electric resistance welded steel pipe according to the present embodiment has a preferred yield strength YS of 415 to 550 MPa. A more preferred lower limit of the yield strength YS of the electric resistance welded steel pipe according to the present embodiment is 450 MPa, more preferably 480 MPa, and more preferably 500 MPa. A more preferred upper limit of the yield strength YS of the electric resistance welded steel pipe according to the present embodiment is 550 MPa, more preferably 545 MPa, and even more preferably 540 MPa.

[0105] [Method of measuring yield strength YS] The yield strength YS of the electric resistance welded steel pipe according to this embodiment can be obtained by the following method. A test piece (tensile test piece) is taken from the electric resistance welded steel pipe according to this embodiment. Specifically, a tensile test piece in the pipe axis direction is taken from the center of the wall thickness (i.e., the base material portion excluding the heat-affected zone) at a position 90° circumferentially shifted from the electric resistance welded part of the base material part of the electric resistance welded steel pipe according to this embodiment. The thickness of the tensile test piece corresponds to the wall thickness of the electric resistance welded steel pipe. The cross section of the tensile test piece is arc-shaped, and the longitudinal direction of the tensile test piece is parallel to the pipe axis direction of the steel pipe. The size of the tensile test piece is not particularly limited. A tensile test is performed at room temperature (24°C) in accordance with the 5CT provisions of the API standard. The yield strength YS (MPa) of the electric resistance welded steel pipe is obtained based on the test results.

[0106] [Low temperature toughness of electric resistance welded steel pipes] In the electric resistance welded steel pipe according to this embodiment, the absorbed energy at -20°C is preferably 300 J or more. A preferred lower limit of the absorbed energy at -20°C is 310 J, more preferably 330 J, and even more preferably 350 J. A preferred upper limit of the absorbed energy at -20°C of the electric resistance welded steel pipe according to this embodiment is not particularly limited, but is, for example, 500 J.

[0107] [Low temperature toughness test] The absorbed energy of the electric resistance welded steel pipe according to this embodiment at -20 ° C. can be obtained by the following method. A V-notch test piece conforming to the standard test piece of JIS Z2242 (2018) is prepared from the center of the wall thickness at a position 90 ° circumferentially shifted from the electric resistance weld of the electric resistance welded steel pipe according to this embodiment. The longitudinal direction of the V-notch test piece is a direction perpendicular to the pipe axis direction and the wall thickness direction of the electric resistance welded steel pipe (i.e., the circumferential direction). A V-notch is prepared in the part of the test piece corresponding to the electric resistance weld so that the propagation direction of the crack in the Charpy impact test is the pipe axis direction of the electric resistance welded steel pipe. The cross section of the V-notch test piece is 10 mm × 10 mm, and the depth of the V-notch is 2 mm. Using the V-notch test piece, a Charpy impact test conforming to JIS Z2242 (2018) is performed at -20 ° C. to obtain the absorbed energy at -20 ° C.

[0108] [On the HIC resistance of electric resistance welded steel pipes] In the electric resistance welded steel pipe according to this embodiment, the crack length ratio (CLR) is preferably 15% or less when a HIC resistance evaluation test described later is carried out. A more preferable upper limit of the crack length ratio (CLR) is 14%, more preferably 13%, and even more preferably 10%. A preferable lower limit of the crack length ratio (CLR) of the electric resistance welded steel pipe according to this embodiment is not particularly limited, but is, for example, 0%.

[0109] [HIC resistance evaluation test] The crack length ratio CLR of the electric resistance welded steel pipe according to this embodiment can be obtained by the following method. An HIC test piece is taken from the base metal at a position 90° circumferentially shifted from the electric resistance welded part of the electric resistance welded steel pipe according to this embodiment. The circular arc-shaped member taken in the pipe circumferential direction from the taking position is developed into a flat plate. The size of the HIC test piece is 20 mm wide x 100 mm long x wall thickness (mm). Using the obtained HIC test piece, an HIC test in accordance with NACE-TM0284 is performed. Specifically, the HIC test piece is immersed for 96 hours in a test liquid obtained by saturating 100% H2S gas in Solution A liquid (5 mass% NaCl + 0.5 mass% glacial acetic acid aqueous solution). The test piece after 96 hours of immersion is measured for the presence or absence of HIC occurrence using an ultrasonic flaw detector. Based on the measurement results, the crack length ratio CLR (%) is calculated using the following formula. CLR(%) = (total crack length / test piece length) x 100(%)

[0110] [Deformation ability of electric resistance welded steel pipes] In the electric resistance welded steel pipe according to the present embodiment, the uniform elongation is preferably 5.0% or more. A more preferable lower limit of the uniform elongation is 5.5%, more preferably 6.0%, and even more preferably 7.0%. A preferable upper limit of the uniform elongation of the electric resistance welded steel pipe according to the present embodiment is not particularly limited, but is, for example, 13.0%.

[0111] [Method of measuring uniform elongation] The uniform elongation of the electric resistance welded steel pipe according to this embodiment can be determined by the following method. A test piece (tensile test piece) is taken from the electric resistance welded steel pipe according to this embodiment. A tensile test is performed using the taken tensile test piece in the same manner as the above-mentioned method for measuring the yield strength YS. The uniform elongation of the electric resistance welded steel pipe is determined based on the test results.

[0112] [About steel plates] The steel plate according to this embodiment is a steel plate used in the manufacture of the electric resistance welded steel pipe of this embodiment, and serves as a raw material for the electric resistance welded steel pipe. Therefore, the configuration of the steel plate of this embodiment is the same as the configuration of the base material of the electric resistance welded steel pipe of this embodiment. Specifically, the steel plate of this embodiment contains, in mass%, C: 0.030-0.100%, Si: 0.03-0.60%, Mn: 0.30-1.60%, P: 0-0.030%, S: 0-0.0020%, Al: 0.005-0.500%, N: 0.0005-0.0100%, Nb: 0.005-0.080%, Ti: 0.005-0.20 0%, Ca: 0.0001-0.0100%, O: 0-0.0050%, Ni: 0-1.00%, Mo: 0-0.50%, V: 0-0.200%, Cr: 0-1.00%, Cu: 0-1.00%, Mg: 0-0.0020%, rare earth elements: 0-0.0200%, and the balance: Fe and impurities, and satisfies formulas (1) to (3). The microstructure is composed of polygonal ferrite with an area ratio of 90-98%, and one or more of pearlite, bainite, and martensite, and a hard phase with an area ratio of 2-10%. In a rectangular observation area including the center of the thickness of the steel plate, with a side length of 200 μm extending in the T direction, which is the thickness direction of the steel plate, and a side length of 200 μm extending in the L direction, which is the rolling direction of the steel plate, the number of intersections A1 and the number of intersections A2 satisfy formula (4).

[0113] [Method for measuring area ratio of polygonal ferrite and area ratio of hard phase in steel plate] The area ratio of polygonal ferrite and the area ratio of hard phase in the microstructure of a steel plate can be measured by the following method: A sample is taken from the center of the plate thickness of the steel plate. Using the taken sample, the area ratio of polygonal ferrite and the area ratio of hard phase are measured in the same manner as in the case of electric resistance welded steel pipe.

[0114] [How to calculate F4 for steel plate] F4 in steel plate can be measured by the following method. A rectangular observation area SQ is an area including the center of the plate thickness of the steel plate, with a side length of 200 μm extending in the T direction, which is the plate thickness direction of the steel plate, and a side length of 200 μm extending in the L direction, which is the rolling direction of the steel plate. The other measurement methods are the same as those for F4 in electric resistance welded steel pipes.

[0115] [Manufacturing method] A method for manufacturing the electric resistance welded steel pipe and steel plate according to this embodiment will be described. Note that the manufacturing method described below is one example, and the manufacturing method for the electric resistance welded steel pipe and steel plate according to this embodiment is not limited to this. In other words, as long as the electric resistance welded steel pipe and steel plate having the above-mentioned configuration can be manufactured, the manufacturing method is not limited to the manufacturing method described below. However, the manufacturing method described below is a suitable manufacturing method for the electric resistance welded steel pipe and steel plate according to this embodiment.

[0116] FIG. 6 is a flow diagram showing an example of a manufacturing process of an electric resistance welded steel pipe according to this embodiment. With reference to FIG. 6, in this manufacturing method, a slab, which is a raw material, is manufactured using molten steel satisfying the above-mentioned chemical composition (raw material preparation process: S0). The manufactured slab is heated in a heating furnace (heating process: S1). The heated slab is hot-rolled to manufacture a steel material (hot rolling process: S2). The manufactured steel material is cooled using a run-out table (ROT: Run Out Table) (ROT cooling process: S3). In the ROT cooling process (S3), first, the steel material is cooled in a water cooling device (first cooling process: S31). After the first cooling process, the steel material is continuously cooled at a cooling rate slower than the cooling rate in the first cooling process (second cooling process: S32). The steel material after the ROT cooling process (S3) is coiled (cooling process: S4). Through the above manufacturing processes, a steel plate, which is a raw material for an electric resistance welded steel pipe, is manufactured.

[0117] Furthermore, an electric resistance welded steel pipe is manufactured using the steel plate (pipe making process: S5). In the pipe making process, the steel plate is formed into a cylindrical mother pipe (open pipe) using forming rolls. In the formed mother pipe, the plate width direction of the steel plate is shaped to be the circumferential direction of the mother pipe. The butt joints extending in the longitudinal direction of the mother pipe are electric resistance welded (welding process). An electric resistance welded steel pipe is manufactured by the above pipe making process. Each process will be described in detail below.

[0118] [Material preparation process (S0)] A material having the above-mentioned chemical composition is prepared. Specifically, molten steel having the above-mentioned chemical composition is produced. A material (slab) is produced using the molten steel. A cast piece may be produced by a continuous casting method. An ingot may be produced using the molten steel, and the material (slab) may be produced by blooming the ingot.

[0119] [Heating process (S1)] In the heating step (S1), the manufactured slab is heated in a heating furnace. The heating temperature T (°C) of the slab in the heating furnace may be a known temperature, for example, 1200 to 1300°C. The residence time t (min) is not particularly limited, but is, for example, 150 to 400 minutes. In this specification, the residence time t (min) is the time from when the slab is charged into the heating furnace to when the slab is removed from the heating furnace.

[0120] Furthermore, in the heating step (S1), F5 defined by the following formula (5) is set to 2300 or more. F5=(T+273.15)×log(t) / (Mn+5×Mo) (5) In formula (5), T is the heating temperature (°C) in the heating step (S1), and t is the time spent in the furnace (min). Each element symbol in formula (5) is substituted with the content (mass%) of the corresponding element in the chemical composition of the slab, i.e., the chemical composition of the base material of the electric resistance welded steel pipe, and the chemical composition of the steel plate.

[0121] The band-like extending hard phase is present locally and is likely to be generated along the segregation zone extending in the tube axial direction. In the chemical composition of this embodiment, the elements that are likely to segregate are Mn and Mo. Mn and Mo are elements that enhance hardenability. Therefore, in the hot working process described below, if a Mn and / or Mo segregation zone extending locally in the tube axial direction exists, the structure near the segregation zone is likely to become a hard phase. Therefore, the hard phase is generated along the segregation zone and extends in the tube axial direction.

[0122] In order to suppress such local segregation of Mn and Mo, it is effective to reduce the Mn content and Mo content in the chemical composition of the electric resistance welded steel pipe and the chemical composition of the steel plate. However, in the electric resistance welded steel pipe and steel plate according to this embodiment, as described above, the Mn content and Mo content specified above are necessary to improve the strength, HIC resistance and low temperature toughness of the electric resistance welded steel pipe.

[0123] Therefore, in this embodiment, the slab is heated under heating conditions that take into account the Mn and Mo contents in the slab, thereby uniformly diffusing Mn and Mo in the slab before hot rolling and suppressing the segregation bands of Mn and Mo. Specifically, the heating temperature and the residence time are set so that the heating condition formula F5, which takes into account the Mn and Mo contents in the slab, is 2300 or more.

[0124] Assuming that the content of each element in the chemical composition of the slab is within the range of this embodiment and formulas (1) to (3) are satisfied, when F5 is less than 2300, a hard phase extending locally in a band shape is generated in the steel sheet immediately after hot working even if other manufacturing conditions are satisfied. Therefore, a microstructure is generated in which the shape of the hard phase does not satisfy formula (4).

[0125] If F5 is 2300 or more, the hard phase extending in a band shape is sufficiently suppressed, provided that other manufacturing conditions are satisfied, and therefore the shape of the hard phase satisfies formula (4).

[0126] The preferred lower limit of F5 is 2400, more preferably 2500, more preferably 2600, more preferably 2700, and more preferably 2800. The upper limit of F5 is not particularly limited. However, the upper limit of F5 may be 13700, 10000, or 5000.

[0127] [Hot rolling process (S2)] In the hot rolling step (S2), hot rolling is performed on the slab after the heating step (S1) to produce a steel material. The rolling step may include a rough rolling step and a finish rolling step. Rolling may be performed using a tandem rolling mill including a plurality of rolling stands arranged in a row (each rolling stand having a pair of work rolls). Rolling may also be performed using a reversing rolling mill having a pair of work rolls. The finish rolling temperature (°C) may be a known temperature, for example, 950°C or less.

[0128] [ROT cooling process (S3)] In the ROT (run-out table) cooling process (S3), the steel material produced in the hot rolling process (S2) is cooled. The ROT cooling process (S3) includes a first cooling process (S31) and a second cooling process (S32).

[0129] [First cooling step (S31)] First, the first cooling is performed on the steel material after the hot rolling process until the surface temperature of the steel material reaches 725°C. That is, the first cooling stop temperature ST1 is 725°C. The cooling method of the first cooling process is, for example, water cooling using a water cooling device. In the first cooling process (S31), the average cooling rate V1 (°C / s) at the center of the plate thickness of the steel material is set to exceed 8°C / s.

[0130] Even if the chemical composition of the base material and the content of each element in the chemical composition of the steel sheet are within the range of this embodiment and satisfy formulas (1) to (3), when the average cooling rate V1 is 8°C / s or less, the timing of ferrite transformation on the surface of the steel material and the timing of ferrite transformation in the center of the sheet thickness become more variable. In the ROT cooling process (S3), the generation of ferrite transformation nuclei progresses from the surface of the steel material toward the center of the sheet thickness. Therefore, as the ferrite transformation progresses, Mn and Mo are discharged from the ferrite to the untransformed austenite. In this case, even if Mn and Mo in the slab are sufficiently diffused in the heating process (S1), Mn and Mo segregation bands are generated locally, and the generation of a hard phase extending in a band shape is promoted.

[0131] If the average cooling rate V1 exceeds 8°C / s, the variation in timing of ferrite transformation between the surface layer of the steel sheet and the center of the sheet thickness is sufficiently suppressed. In other words, the steel sheet is easily cooled uniformly. Therefore, the formation of localized segregation bands of Mn and Mo is suppressed, and as a result, the formation of a hard phase extending in a band shape is suppressed. As a result, the shape of the hard phase in the microstructure of the steel sheet satisfies formula (4).

[0132] [Second cooling process (S32)] In the second cooling step (S32), the steel material cooled in the first cooling step (S31) is cooled until the surface temperature of the steel material reaches the coiling temperature ST2 described below. In the second cooling step (S32), the average cooling rate V2 (°C / s) at the center of the plate thickness of the steel material is set to 8°C / s or less. When the average cooling rate V2 exceeds 8°C / s, the temperature of the steel material passes through the ferrite region in the CCT diagram and reaches the pearlite region, the bainite region and / or the martensite region. In this case, the area ratio of polygonal ferrite decreases. As a result, the deformability of the electric resistance welded steel pipe is not sufficiently increased.

[0133] When the average cooling rate V2 is 8°C / s or less, the temperature of the steel passes through the ferrite region in the CCT diagram. Therefore, the area ratio of polygonal ferrite is sufficiently increased. As a result, the deformability of the electric resistance welded steel pipe is sufficiently increased.

[0134] [Winding process (S4)] In the coiling process (S4), the steel material cooled in the ROT cooling process (S3) is coiled to produce a coiled steel sheet.

[0135] After the ROT cooling step (S3) is completed, the steel material is coiled. The surface temperature ST2 of the steel material during coiling (hereinafter referred to as coiling temperature) is, for example, 450 to 650°C.

[0136] The steel plate (used as a material for electric resistance welded steel pipe) according to this embodiment is manufactured by the above manufacturing process. The microstructure of the steel plate manufactured by the above manufacturing process is composed of polygonal ferrite with an area ratio of 90 to 98% and a hard phase with an area ratio of 2 to 10%. The hard phase is composed of one or more of pearlite, bainite and martensite. Furthermore, the shape of the hard phase satisfies formula (4). Therefore, when the steel plate according to this embodiment is manufactured into an electric resistance welded steel pipe, it has excellent strength, excellent low-temperature toughness, excellent HIC resistance and excellent deformability.

[0137] [Pipe making process (S5)] In the pipe making process (S5), electric resistance welded steel pipe is manufactured while unwinding the coiled steel plate. Specifically, the steel plate is bent into a cylindrical shape (open pipe) by a series of forming rolls. Next, the widthwise ends (butt joints) of the open pipe are brought into contact with each other, and electric resistance welding is performed by high-frequency induction heating while applying pressure (welding process). Using the steel pipe after the welding process, well-known seam heat treatment is performed on the electric resistance welded portion as necessary.

[0138] The electric resistance welded steel pipe according to this embodiment is manufactured by the above manufacturing process. The microstructure of the base material of the electric resistance welded steel pipe manufactured by the above manufacturing process is composed of polygonal ferrite with an area ratio of 90 to 98% and a hard phase with an area ratio of 2 to 10%. The hard phase is composed of one or more of pearlite, bainite and martensite. Furthermore, the shape of the hard phase satisfies formula (4). Therefore, the electric resistance welded steel pipe according to this embodiment has excellent strength, excellent low-temperature toughness, excellent HIC resistance and excellent deformability. EXAMPLES

[0139] Molten steels having steel grade numbers A to X shown in Table 1 were continuously cast to produce slabs.

[0140] [Table 1]

[0141] In Table 1, "-" indicates that the content of the corresponding element was below the detection limit. In other words, it means that the corresponding element was not contained. For example, the Ni content of steel grade number A was "0"% when rounded off to three decimal places. The balance other than the elements listed in Table 1 was Fe and impurities. Electric resistance welded steel pipes with test numbers 1 to 27 shown in Table 2 were manufactured using multiple slabs with steel grade numbers A to X.

[0142] [Table 2]

[0143] Specifically, the slab was heated in a heating furnace. The heating temperature T (°C), the time in the furnace t (min), and F5 were as shown in Table 2. The heated slab was subjected to hot rolling. In the hot rolling process, the finish rolling temperature was 950°C or less. The steel material after hot rolling was subjected to ROT cooling. In the ROT cooling process, the steel material was cooled to a first cooling stop temperature ST1: 725°C at an average cooling rate V1 (°C / s) shown in Table 2. After the first cooling process, the steel material was cooled to a coiling temperature ST2 (°C) shown in Table 2 at an average cooling rate V2 (°C / s) shown in Table 2. The obtained steel material was coiled at the coiling temperature ST2 (°C) shown in Table 2 to produce a steel sheet.

[0144] The steel plates were used to make pipes by the above-mentioned well-known method, and electric resistance welded steel pipes having a wall thickness of 10 to 25 mm and an outside diameter of 100 to 800 mm were produced.

[0145] [Evaluation test] For the electric resistance welded steel pipes of test numbers 1 to 27, microstructural observation of the base material (area ratio of polygonal ferrite, area ratio of hard phase, shape of hard phase), tensile test, low temperature toughness test, and HIC resistance evaluation test were performed. For the steel plates of test numbers 1 to 27, microstructural observation (area ratio of polygonal ferrite, area ratio of hard phase, shape of hard phase) was performed.

[0146] [Microstructure observation] The area ratio of polygonal ferrite and the area ratio of hard phase in the base material of the electric resistance welded steel pipe of each test number and the steel plate of each test number were measured using EBSP-OIM. Furthermore, the shape of the hard phase was observed in the base material of the electric resistance welded steel pipe of each test number and the steel plate.

[0147] [Regarding the area ratio of polygonal ferrite and the area ratio of hard phase] A sample was taken from the base metal of the electric resistance welded steel pipe of each test number, from the center of the wall thickness at a position circumferentially shifted by 90° from the electric resistance weld (i.e., the base metal portion excluding the heat-affected zone).

[0148] The observation surface of the collected sample was polished with colloidal silica abrasive for 30 to 60 minutes. The area ratio (%) of polygonal ferrite and the area ratio (%) of hard phase were determined for the polished sample by the above-mentioned method using the KAM method with EBSP-OIM. The observation field of view when measuring the area ratio of polygonal ferrite and the area ratio of hard phase by the KAM method was 200 μm × 500 μm. The observation magnification was 400 times, and the measurement step was 0.3 μm.

[0149] A sample was taken from the center of the thickness of the steel plate of each test number. The area ratio of polygonal ferrite and the area ratio of hard phase were measured for each sample. As a result, the area ratio of polygonal ferrite and the area ratio of hard phase of the steel plate of each test number were the same as the area ratio of polygonal ferrite and the area ratio of hard phase of the electric resistance welded steel pipe of the corresponding test number.

[0150] [Observation of the shape of the hard phase] The measurements of A1 and A2 were carried out in the rectangular observation area SQ described above. The base metal of the electric resistance welded steel pipe of each test number was cut parallel to the pipe axis direction (L direction) and the wall thickness direction (T direction) of the base metal so as to pass through the center of the wall thickness (i.e., the base metal part excluding the heat-affected zone) at a position shifted 90° in the circumferential direction from the electric resistance welded part, and a sample including the rectangular observation area SQ described above was taken. Of the surfaces of the taken samples, the surface corresponding to the cross section parallel to the pipe axis direction (L direction) of the base metal was used as the observation surface.

[0151] After polishing the observation surface, it was etched with 3% nitric acid alcohol (Nital etching solution). Photographic images of five arbitrary rectangular observation areas SQ were generated using an optical microscope at 500x magnification of the etched observation surface. In each rectangular observation area SQ, the polygonal ferrite and the hard phase have different contrasts for each phase. Therefore, each phase was identified based on the contrast. In each rectangular observation area SQ, line segments T1 to T9 and line segments L1 to L9 were drawn according to the above-mentioned procedure. The total number of intersections a1 between the line segments T1 to T9 and the hard phase interface in any of the five rectangular observation areas SQ was calculated and defined as A1. The total number of intersections a2 between the line segments L1 to L9 and the hard phase interface in any of the five rectangular observation areas SQ was calculated and defined as A2. F4 was calculated based on the obtained A1 and A2.

[0152] The rectangular observation area SQ was defined as an area including the center of the thickness of the steel plate of each test number, with the side length extending in the T direction, which is the thickness direction of the steel plate, being 200 μm, and the side length extending in the L direction, which is the rolling direction of the steel plate, being 200 μm. F4 was determined for the rectangular observation area SQ in the same manner as for the electric resistance welded steel pipe of each test number. As a result, the F4 value of the steel plate of each test number was the same as the F4 value of the electric resistance welded steel pipe of the corresponding test number.

[0153] [Tensile test] Tensile test pieces were taken from the electric resistance welded steel pipes of each test number. Specifically, tensile test pieces in the pipe axis direction were taken from the wall thickness center part (i.e., the base metal part excluding the heat-affected zone) at a position 90° circumferentially shifted from the electric resistance welded part in the base metal part of the electric resistance welded steel pipe. The thickness of the tensile test piece corresponded to the wall thickness of the electric resistance welded steel pipe. The cross section of the tensile test piece was arc-shaped, and the longitudinal direction of the tensile test piece was parallel to the pipe axis direction of the steel pipe. Using the tensile test piece, a tensile test was performed at room temperature (24°C) in accordance with the 5CT provisions of the API standard. Based on the test results, the yield strength YS (MPa) and uniform elongation (%) of the electric resistance welded steel pipe were obtained. If the yield strength YS was 415 MPa or more, the strength of the electric resistance welded steel pipe was judged to be high. If the uniform elongation was 5.0% or more, the deformability of the electric resistance welded steel pipe was judged to be high.

[0154] [Low temperature toughness test] V-notch test pieces conforming to the standard test piece of JIS Z2242 (2018) were prepared from the center of the wall thickness at a position 90° circumferentially shifted from the electric weld of the electric welded steel pipe of each test number. The longitudinal direction of the V-notch test piece was perpendicular to the axial direction and wall thickness direction of the electric welded steel pipe (i.e., the circumferential direction). A V-notch was prepared in the part of the test piece corresponding to the electric weld so that the propagation direction of the crack in the Charpy impact test was the axial direction of the electric welded steel pipe. The cross section of the V-notch test piece was 10 mm × 10 mm, and the depth of the V-notch was 2 mm. Using the V-notch test piece, a Charpy impact test conforming to JIS Z2242 (2018) was performed at -20 ° C, and the absorbed energy at -20 ° C was obtained. If the absorbed energy at -20 ° C was 300 J or more, the electric welded steel pipe was judged to have high low-temperature toughness.

[0155] [HIC resistance evaluation test] HIC test pieces were taken from the base metal at a position 90° circumferentially shifted from the electric resistance welded part of the electric resistance welded steel pipe of each test number. The circular arc-shaped member taken from the taking position in the pipe circumferential direction was unfolded to a flat plate. The size of the HIC test piece was 20 mm wide x 100 mm long x wall thickness (mm). Using the obtained HIC test piece, an HIC test was carried out in accordance with NACE-TM0284. Specifically, the HIC test piece was immersed for 96 hours in a test liquid in which 100% H2S gas was saturated in Solution A liquid (5 mass% NaCl + 0.5 mass% glacial acetic acid aqueous solution). The test piece after 96 hours of immersion was measured for the occurrence of HIC using an ultrasonic flaw detector. Based on the measurement results, the crack length ratio CLR (%) was calculated using the following formula. If the CLR was 15% or less, it was determined that the HIC resistance was excellent. CLR(%) = (total crack length / test piece length) x 100(%)

[0156] [Test Results] The test results are shown in Table 3.

[0157] [Table 3]

[0158] With reference to Tables 1 to 3, the electric resistance welded steel pipes and steel plates of test numbers 1 to 17 had appropriate chemical compositions, satisfied formulas (1) to (3), and were also manufactured under appropriate conditions. Therefore, the microstructure of the base metal of the electric resistance welded steel pipe of each test number was a structure consisting of polygonal ferrite with an area ratio of 90 to 98% and a hard phase with an area ratio of 2 to 10%. The hard phase consisted of one or more of pearlite, bainite, and martensite. Furthermore, in the electric resistance welded steel pipe of each test number, F4 satisfied formula (4). Therefore, the yield strength YS was 415 to 550 MPa, and the strength of the electric resistance welded steel pipe was high. The uniform elongation was 5.0% or more, and the deformability of the electric resistance welded steel pipe was high. The absorbed energy at -20°C was 300 J or more, and the low-temperature toughness of the electric resistance welded steel pipe was high. Furthermore, the CLR of the base metal of the electric resistance welded steel pipe was 15% or less, and the HIC resistance of the electric resistance welded steel pipe was high.

[0159] On the other hand, in test number 18, F1 was less than 0.20. In other words, F1 did not satisfy formula (1). Therefore, the yield strength YS was less than 415 MPa. As a result, the strength of the electric resistance welded steel pipe was low.

[0160] In test number 19, F1 exceeded 0.35. That is, F1 did not satisfy formula (1). Therefore, the area ratio of polygonal ferrite was less than 90%, and the area ratio of the hard phase exceeded 10%. Therefore, the uniform elongation of the electric resistance welded steel pipe was less than 5.0%, and the deformability of the electric resistance welded steel pipe was low. Furthermore, the yield strength YS exceeded 550 MPa, and the strength of the electric resistance welded steel pipe was too high. Furthermore, in the heating process, F5 was less than 2300. Therefore, F4 exceeded 1.8, and the shape of the hard phase did not satisfy formula (4). Therefore, the CLR of the base metal part of the electric resistance welded steel pipe exceeded 15%, and the HIC resistance of the electric resistance welded steel pipe was low.

[0161] In test number 20, F3 was less than 2.0. In other words, F3 did not satisfy formula (3). Therefore, the CLR of the base metal of the electric resistance welded steel pipe was more than 15%, and the HIC resistance of the electric resistance welded steel pipe was low.

[0162] In test numbers 21 and 27, F2 was less than 0.020. In other words, F2 did not satisfy formula (2). Therefore, the yield strength YS was less than 415 MPa.

[0163] In test number 22, F2 exceeded 0.085. In other words, F2 did not satisfy formula (2). Therefore, the yield strength YS exceeded 550 MPa, and the strength of the electric resistance welded steel pipe was too high. Therefore, the absorbed energy at -20°C was less than 300 J, and the low-temperature toughness of the electric resistance welded steel pipe was low. Furthermore, the CLR of the base metal of the electric resistance welded steel pipe exceeded 15%, and the HIC resistance of the electric resistance welded steel pipe was low.

[0164] In the test No. 23, in the ROT cooling process, the average cooling rate V1 in the first cooling step was 8°C / s or less. Therefore, F4 exceeded 1.8. In other words, the shape of the hard phase did not satisfy formula (4). Therefore, the CLR of the base metal of the electric resistance welded steel pipe exceeded 15%, and the HIC resistance of the electric resistance welded steel pipe was low.

[0165] In the test No. 24, the average cooling rate V2 in the second cooling step in the ROT cooling process exceeded 8°C / s. Therefore, the area ratio of polygonal ferrite was less than 90%, and the area ratio of the hard phase exceeded 10%. As a result, the uniform elongation of the electric resistance welded steel pipe was less than 5.0%, and the deformability of the electric resistance welded steel pipe was low.

[0166] In the case of test number 25, F5 was less than 2300 in the heating process. Therefore, F4 exceeded 1.8. In other words, the shape of the hard phase did not satisfy formula (4). Therefore, the CLR of the base metal of the electric resistance welded steel pipe exceeded 15%, and the HIC resistance of the electric resistance welded steel pipe was low.

[0167] In test number 26, F1 exceeded 0.35. In other words, F1 did not satisfy formula (1). Therefore, the area ratio of polygonal ferrite was less than 90%, and the area ratio of the hard phase exceeded 10%. Therefore, the uniform elongation of the electric resistance welded steel pipe was less than 5.0%, and the deformability of the electric resistance welded steel pipe was low. Furthermore, the yield strength YS exceeded 550 MPa, and the strength of the electric resistance welded steel pipe was too high.

[0168] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments without departing from the spirit of the present disclosure.

Claims

1. The base material is, in mass%, C: 0.030-0.100%, Si: 0.03 to 0.60%, Mn: 0.30-1.60%, P: 0 to 0.030%, S: 0-0.0020%, Al: 0.005-0.500%, N: 0.0005-0.0100%, Nb: 0.005-0.080%, Ti: 0.005-0.200%, Ca: 0.0001-0.0100%, O: 0 to 0.0050%, Ni: 0 to 1.00%, Mo: 0 to 0.50%, V: 0-0.200%, Cr: 0-1.00%, Cu: 0 to 1.00%, Mg: 0 to 0.0020%, Rare earth elements: 0 to 0.0200%, and The balance: Fe and impurities, and satisfying formula (1) to formula (3), The microstructure of the base material portion is Polygonal ferrite with an area ratio of 90 to 98%; A hard phase consisting of one or more of pearlite, bainite, and martensite, with an area ratio of 2 to 10%; It consists of: In a rectangular observation region including a thickness center portion of the base material, the length of a side extending in the T direction, which is the thickness direction of the base material, is 200 μm, and the length of a side extending in the L direction, which is the tube axis direction of the base material, is 200 μm, Nine line segments extending in the T direction, arranged at equal intervals in the L direction, and dividing the rectangular observation area into 10 equal parts in the L direction are defined as T1 to T9; Nine line segments extending in the L direction, arranged at equal intervals in the T direction, and dividing the rectangular observation area into 10 equal parts in the T direction are defined as L1 to L9; The number A1 of intersections between the line segments T1 to T9 and the hard phase interface which is the interface between the polygonal ferrite and the hard phase, and the number A2 of intersections between the line segments L1 to L9 and the hard phase interface satisfy formula (4). Electric resistance welded steel pipe for pipelines. 0.20≦C+Si / 24+Mn / 6+Ni / 40+Cr / 5+Mo / 4+V / 3+Nb / 3≦0.35 (1) 0.020≦Nb+V+Ti≦0.085 (2) Ca / S≧2.0 (3) A1 / A2≦1.8 (4) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol.

2. The electric resistance welded steel pipe for pipeline according to claim 1, The base material portion comprises, in mass%, Ni: 0.01-1.00%, Mo: 0.01-0.50%, V: 0.001-0.200%, Cr: 0.01-1.00%, Cu: 0.01 to 1.00%, Mg: 0.0001 to 0.0020%, and Rare earth elements: 0.0001 to 0.0200%, containing one or more selected from the group consisting of Electric resistance welded steel pipe for pipelines.

3. The electric resistance welded steel pipe for pipeline according to claim 1 or 2, The thickness is 10 to 25 mm, The outer diameter is 100 to 800 mm. Electric resistance welded steel pipe for pipelines.

4. A steel plate used as a material for electric resistance welded steel pipe for pipelines, In mass percent, C: 0.030-0.100%, Si: 0.03 to 0.60%, Mn: 0.30-1.60%, P: 0 to 0.030%, S: 0-0.0020%, Al: 0.005-0.500%, N: 0.0005-0.0100%, Nb: 0.005-0.080%, Ti: 0.005-0.200%, Ca: 0.0001-0.0100%, O: 0 to 0.0050%, Ni: 0 to 1.00%, Mo: 0 to 0.50%, V: 0-0.200%, Cr: 0-1.00%, Cu: 0 to 1.00%, Mg: 0 to 0.0020%, Rare earth elements: 0 to 0.0200%, and The balance: Fe and impurities, and satisfying formula (1) to formula (3), The microstructure is Polygonal ferrite with an area ratio of 90 to 98%; A hard phase consisting of one or more of pearlite, bainite, and martensite, with an area ratio of 2 to 10%; It consists of: In a rectangular observation area including the thickness center of the steel plate, the length of a side extending in the T direction, which is the thickness direction of the steel plate, is 200 μm, and the length of a side extending in the L direction, which is the rolling direction of the steel plate, is 200 μm. Nine line segments extending in the T direction, arranged at equal intervals in the L direction, and dividing the rectangular observation area into 10 equal parts in the L direction are defined as T1 to T9; Nine line segments extending in the L direction, arranged at equal intervals in the T direction, and dividing the rectangular observation area into 10 equal parts in the T direction are defined as L1 to L9; The number A1 of intersections between the line segments T1 to T9 and the hard phase interface which is the interface between the polygonal ferrite and the hard phase, and the number A2 of intersections between the line segments L1 to L9 and the hard phase interface satisfy formula (4). steel plate. 0.20≦C+Si / 24+Mn / 6+Ni / 40+Cr / 5+Mo / 4+V / 3+Nb / 3≦0.35 (1) 0.020≦Nb+V+Ti≦0.085 (2) Ca / S≧2.0 (3) A1 / A2≦1.8 (4) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol.

5. The steel plate according to claim 4, In mass percent, Ni: 0.01-1.00%, Mo: 0.01-0.50%, V: 0.001-0.200%, Cr: 0.01-1.00%, Cu: 0.01 to 1.00%, Mg: 0.0001 to 0.0020%, and Rare earth elements: 0.0001 to 0.0200%, containing one or more selected from the group consisting of steel plate.

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