Electric resistance welded steel pipes for steel pipe pile joints, steel pipe pile joints and steel pipe piles

Abstract

To provide an electric resistance welded steel pipe for a steel pipe pile joint excellent in machinability.SOLUTION: Provided is an electric resistance welded steel pipe for a steel pipe pile joint which has a base material part and an electric-resistance-welding part. When a Vickers test with a load 10 gf is carried out at each of a position of a depth of 1 mm from an inner surface of the electric-resistance-welding part, a position of a depth of 1 mm from an outer surface of the electric-resistance-welding part, a position of a depth of 1 mm from an inner surface of the base material part, and a position of a depth of 1 mm from an outer surface of the base material part, a number ratio of indentations having an average hardness of +40 HV or more is 0.30 or less, and a number ratio of indentations with an eccentricity E of 0.20 or more is 0.40 or less at each of the positions. However, the eccentricity E is a value obtained by formula (1) when the maximum value, the minimum value, and an average value of lengths of a line segment OA, a line segment OB, a line segment OC, and a line segment OD are respectively represented as Lmax, Lmin, and Lave, with an intersection of diagonal lines of the indentations represented as O, and respective apexes in a counterclockwise order represented as A, B, C, and D. E=(Lmax-Lmin) / Lave (1).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an electric resistance welded steel pipe for a steel pipe pile joint, a steel pipe pile joint, and a steel pipe pile. [Background technology]

[0002] Mechanical joints (steel pipe pile joints) for mechanically connecting steel pipe piles at construction sites are manufactured by cutting, so they are thinner than the pile itself. Therefore, steel pipe pile joints are manufactured separately from the pile itself using steel pipes that are stronger than the pile itself.

[0003] On the other hand, steel pipes used as materials for steel pipe pile joints are required to have a uniform hardness distribution to prevent deterioration of machinability and tool damage caused by variations in hardness during cutting processes such as threading.

[0004] Patent Document 1 proposes a steel plate for line pipe in which the variation in surface hardness is controlled, and Patent Document 2 proposes an electric resistance welded steel pipe for line pipe in which the absolute value of the surface hardness is controlled. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6521197 [Patent Document 2] Japanese Patent Application Publication No. 2017-179482 Summary of the Invention [Problem to be solved by the invention]

[0006] However, even if the hardness of the surface of the steel plate or steel pipe is controlled as in Patent Document 1 or Patent Document 2, localized hard regions may occur in some crystal grains or near grain boundaries, resulting in reduced machinability.

[0007] The above-mentioned "locally hard regions" are extremely small regions, so they are averaged out by the surrounding low-stress regions in a normal Vickers hardness test, making them difficult to evaluate.

[0008] The present invention has been made in consideration of the above circumstances, and has an object to provide an electric resistance welded steel pipe for steel pipe pile joints that has excellent machinability.

[0009] In the present invention, "excellent cutting performance" means that the wear width of the flank of the tool tip after cutting 1000 m in the cutting test described below is 0.10 mm or less. [Means for solving the problem]

[0010] As a result of intensive research, the inventors have found that if the shape of the indentation in a specified Vickers test (the indentation made on the test piece by conducting a Vickers test, also called a Vickers mark) is uneven, there will be large variations in local hardness, and machinability will be reduced.

[0011] In addition, it was found that the local variation in hardness can be reduced by reducing the proportion of high-dislocation density structures, which have high hardness, and by reducing their connectivity.

[0012] The present invention has been completed based on the above findings and comprises the following gist. [1] An electric resistance welded steel pipe for a steel pipe pile joint having a base material portion and an electric resistance welded portion, When a Vickers test was carried out with a load of 10 gf at a position 1 mm deep from the inner surface of the electric resistance weld, a position 1 mm deep from the outer surface of the electric resistance weld, a position 1 mm deep from the inner surface of the base material, and a position 1 mm deep from the outer surface of the base material, At each of the positions, The number ratio of indentations with an average hardness of +40HV or more is 0.30 or less, An electric-resistance welded steel pipe for steel pipe pile joints, in which the number ratio of indentations with an eccentricity E of 0.20 or more is 0.40 or less. The eccentricity E is expressed as follows: the intersection of the diagonal lines of the indentation is O, and the vertices are A, B, C, and D in counterclockwise order. The maximum, minimum, and average lengths of the line segments OA, OB, OC, and OD are L, respectively. max , L min , L ave When this is the case, the value is calculated using equation (1). E=(L max -L min ) / L ave ···(1) [2] The composition of the base material is in mass%: C: 0.020% or more and 0.200% or less, Si: 0.50% or less, Mn: 0.30% or more and 2.00% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% or more and 0.100% or less, and N: Contains 0.0100% or less, Further optionally, Nb: 0.080% or less, V: 0.080% or less, Ti: 0.080% or less, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less Ca: 0.0050% or less, B: 0.0050% or less, Mg: 0.020% or less, Zr: 0.020% or less, REM: Contains one or more selected from 0.020% or less, The electric resistance welded steel pipe for steel pipe pile joints according to [1], wherein the balance consists of Fe and unavoidable impurities. [3] The steel structure at the center of the base material is The average crystal grain size is 15.0 μm or less, The steel structure at a depth of 1 mm from the inner surface of the electric resistance weld, a depth of 1 mm from the outer surface of the electric resistance weld, a depth of 1 mm from the inner surface of the base material, and a depth of 1 mm from the outer surface of the base material are respectively: The number density in the high KAM value region, where the KAM value is between 3.0 and 5.0, is 1000 particles / mm 2 or more, and the area ratio of high KAM value regions having a circle equivalent diameter of 5.0 μm or more is 0.050 or less. [4] A tubular joint that has mechanical means for connecting steel pipes to each other and is attached to the end of a steel pipe, and is a steel pipe pile joint that uses an electric resistance welded steel pipe for steel pipe pile joints described in any of [1] to [3]. [5] A steel pipe pile having the steel pipe pile joint described in [4] at the end of the steel pipe. [Effects of the Invention]

[0013] According to the present invention, an electric resistance welded steel pipe for a steel pipe pile joint having excellent machinability can be provided. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic diagram showing each point in a Vickers mark. [Figure 2] FIG. 2 is a schematic diagram showing the position of the Vickers test on the electric resistance weld. [Figure 3] FIG. 3 is a schematic diagram showing the position of the Vickers test in the base material. [Figure 4] FIG. 4 is a schematic diagram of a steel pipe pile according to one embodiment of the present invention. [Figure 5] FIG. 5 is a schematic diagram showing an electric resistance welded portion in a cross section perpendicular to the pipe axial direction. DETAILED DESCRIPTION OF THE INVENTION

[0015] The electric resistance welded steel pipe for a steel pipe pile joint (hereinafter simply referred to as electric resistance welded steel pipe) of the present invention will be described below.

[0016] First, the reasons for limiting the mechanical properties of the electric resistance welded steel pipe of the present invention will be explained.

[0017] The electric resistance welded steel pipe of the present invention has a base material and an electric resistance weld, and is characterized in that when a Vickers test is conducted with a load of 10 gf at a position 1 mm deep from the inner surface of the electric resistance weld, a position 1 mm deep from the outer surface of the electric resistance weld, a position 1 mm deep from the inner surface of the base material, and a position 1 mm deep from the outer surface of the base material, the percentage of the number of indentations with an average hardness of +40 HV or more is 0.30 or less, and the percentage of the number of indentations with an eccentricity E of 0.20 or more is 0.40 or less at each of the above positions. Note that hereinafter the percentage of the number of indentations will also be simply referred to as the percentage of indentations. As shown in Figure 1, the intersection of the diagonal lines of the Vickers marks is O, and the vertices are A, B, C, and D in counterclockwise order. The maximum, minimum, and average lengths of the line segments OA, OB, OC, and OD are L, respectively. max , L min , L ave When this is the case, the value is calculated using equation (1). E=(L max -L min ) / L ave ···(1)

[0018] When a Vickers test is performed at multiple points on an electric resistance welded steel pipe at each of the predetermined depth positions under a load of 10 gf, if the proportion of indentations with an average hardness of +40 HV or more is high, the proportion of high-hardness regions in the electric resistance welded steel pipe increases, resulting in reduced machinability. In the present invention, the proportion of indentations with an average hardness of +40 HV or more is set to 0.30 or less. While a lower proportion of indentations with an average hardness of +40 HV or more is preferable, an excessive reduction increases manufacturing costs and manufacturing load, so the proportion of indentations is preferably 0.01 or more. The proportion of indentations is more preferably 0.02 or more.

[0019] Furthermore, in the above test, if the proportion of indentations with an eccentricity E of 0.20 or more is high, the local hardness variation in the electric resistance welded steel pipe increases, resulting in reduced machinability. In the present invention, the proportion of indentations with an eccentricity E of 0.20 or more is set to 0.40 or less. The lower the proportion of indentations with an eccentricity E of 0.20 or more, the better, but an excessive reduction leads to increased manufacturing costs and manufacturing load, so the proportion of indentations is preferably 0.01 or more. The proportion of indentations is more preferably 0.02 or more.

[0020] The Vickers test is performed at a load of 10 gf for both the electric resistance weld and the base material, using a cross section perpendicular to the pipe axis as the measurement surface, according to the method described in JIS Z 2244 (2020). For the electric resistance weld, as shown in Figure 2, 201 points are tested at 0.2 mm intervals over a 20 mm range on each side of the circumferential direction from the electric resistance weld, at a depth of 1 mm from the inner surface and a depth of 1 mm from the outer surface. For the base material, as shown in Figure 3, 201 points are tested at 0.2 mm intervals over a 40 mm range circumferentially, at a depth of 1 mm from the inner surface and a depth of 1 mm from the outer surface, at positions 90 degrees circumferentially from the electric resistance weld. The Vickers hardness is then calculated for each indentation, and the lengths of the lines OA, OB, OC, and OD shown in Figure 1 are measured to obtain the L. max , L min , L ave Calculate the percentage of indentations with an average hardness of +40 HV or more and an eccentricity E of 0.20 or more at a depth of 1 mm from the inner surface of the electric resistance weld, a depth of 1 mm from the outer surface of the electric resistance weld, a depth of 1 mm from the inner surface of the base material, and a depth of 1 mm from the outer surface of the base material. The average hardness is the average (arithmetic mean) of the Vickers hardness calculated from each indentation at each depth.

[0021] The composition of the base metal of the electric resistance welded steel pipe of the present invention is, in mass%, C: 0.020% or more and 0.200% or less, Si: 0.50% or less, Mn: 0.30% or more and 2.00% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% or more and 0.100% or less, and N: 0.0100% or less, and as optional components, Nb: 0.080% or less, V: 0.080% or less It is preferable that the steel sheet contains one or more elements selected from the group consisting of Ti: 0.080% or less, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less, Ca: 0.0050% or less, B: 0.0050% or less, Mg: 0.020% or less, Zr: 0.020% or less, and REM: 0.020% or less, with the remainder consisting of Fe and unavoidable impurities.

[0022] The preferred chemical composition of the base material of the electric resistance welded steel pipe of the present invention will be described below. In this specification, unless otherwise specified, "%" indicating the content of an element in the chemical composition means "% by mass."

[0023] C: 0.020% or more and 0.200% or less C is an element that increases the strength of steel through solid solution strengthening. Furthermore, C improves the hardenability of steel, thereby refining crystal grains and increasing the strength of steel, as well as the number density of the high KAM value region described below. To achieve these effects, it is preferable to contain 0.020% or more of C. However, excessive C content results in the excessive formation of hard pearlite, martensite, and austenite, resulting in a high proportion of indentations (Vickers indentations) with an average hardness of +40 HV or more. It also increases the proportion of indentations with an eccentricity E of 0.20 or more. It also increases the area proportion of high KAM value regions with an equivalent circle diameter of 5.0 μm or more. Therefore, the C content is preferably 0.200% or less. The C content is more preferably 0.025% or more and 0.180% or less. The C content is even more preferably 0.030% or more and 0.170% or less.

[0024] Si:0.50% or less Silicon is an element that increases the strength of steel through solid solution strengthening. To achieve this effect, it is preferable to contain 0.02% or more of silicon. However, excessive silicon content increases the proportion of indentations with an average hardness of +40 HV or more. Also, the proportion of indentations with an eccentricity E of 0.20 or more increases. Therefore, the silicon content is preferably 0.50% or less. The silicon content is more preferably 0.05% or more and 0.40% or less. The silicon content is even more preferably 0.08% or more and 0.30% or less.

[0025] Mn: 0.30% or more and 2.00% or less Mn is an element that increases the strength of steel through solid solution strengthening. Furthermore, Mn improves the hardenability of steel, thereby refining crystal grains and increasing the strength of steel and the number density of the high KAM value region. To achieve these effects, it is preferable to contain 0.30% or more of Mn. However, excessive Mn content results in the excessive formation of hard martensite and austenite, resulting in a high proportion of indentations with an average hardness of +40 HV or more. It also increases the proportion of indentations with an eccentricity E of 0.20 or more. It also increases the area proportion of high KAM value regions with an equivalent circle diameter of 5.0 μm or more. Therefore, the Mn content is preferably 2.00% or less. The Mn content is more preferably 0.40% or more and 1.90% or less. The Mn content is even more preferably 0.50% or more and 1.80% or less.

[0026] P:0.050% or less Since P segregates at grain boundaries and reduces toughness, it is preferable to reduce P as an inevitable impurity as much as possible, and the P content is preferably in the range of 0.050% or less. The P content is more preferably 0.040% or less, and even more preferably 0.030% or less. Although there is no particular lower limit for P, an excessive reduction leads to an increase in smelting costs, so the P content is preferably 0.001% or more.

[0027] S: 0.0200% or less S is usually present in steel as MnS, which is thinly drawn during the hot rolling process and has a negative effect on ductility and toughness. For this reason, in the present invention, it is preferable to reduce S as much as possible, and the S content is preferably 0.0200% or less. The S content is more preferably 0.0100% or less, and even more preferably 0.0050% or less. Although there is no particular lower limit for S, excessive reduction leads to an increase in smelting costs, so the S content is preferably 0.0001% or more.

[0028] Al: 0.005% or more and 0.100% or less Al is an element that acts as a strong deoxidizer. To achieve this effect, it is preferable to contain 0.005% or more of Al. However, excessive Al content deteriorates weldability and increases the amount of alumina-based inclusions, deteriorating surface properties. For this reason, the Al content is preferably 0.100% or less. The Al content is more preferably 0.010% or more and 0.080% or less. The Al content is even more preferably 0.015% or more and 0.070% or less.

[0029] N: 0.0100% or less N is an inevitable impurity and an element that acts to reduce ductility and toughness by firmly fixing dislocation motion. In the present invention, it is desirable to reduce N as an impurity as much as possible, but an N content of up to 0.0100% is acceptable. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0080% or less, and more preferably 0.0060% or less. Although there is no particular lower limit for N, an excessive reduction leads to an increase in smelting costs, so the N content is preferably 0.0010% or more.

[0030] The base metal of the electric resistance welded steel pipe of the present invention may contain the above-mentioned components as basic components (essential components), with the balance being Fe and unavoidable impurities.

[0031] Furthermore, the base material of the electric resistance welded steel pipe of the present invention may further optionally contain one or more selected from the following:

[0032] Nb: 0.080% or less Nb contributes to improving the strength of steel by forming fine carbides and nitrides in the steel. Furthermore, Nb contributes to refining the structure by suppressing coarsening of austenite during hot rolling, thereby increasing the strength of the steel and increasing the number density of the high KAM value region. Nb can be added as needed. To achieve the above-mentioned effects, if Nb is added, it is preferable to contain 0.002% or more of Nb. However, excessive Nb content reduces the number density of the high KAM value region. Furthermore, the area ratio of the high KAM value region with a circle equivalent diameter of 5.0 μm or more increases. Therefore, if Nb is added, the Nb content is preferably 0.080% or less. The Nb content is more preferably 0.005% or more and 0.070% or less. The Nb content is even more preferably 0.010% or more and 0.060% or less.

[0033] V:0.080% or less V is an element that contributes to improving the strength of steel by forming fine carbides and nitrides in the steel, and can be contained as needed. To achieve the above-mentioned effects, when V is contained, it is preferable that 0.002% or more of V is contained. However, excessive V content reduces the number density of high KAM value regions. Furthermore, the area ratio of high KAM value regions with a circle-equivalent diameter of 5.0 μm or more increases. Therefore, when V is contained, the V content is preferably 0.080% or less. The V content is more preferably 0.005% or more and 0.070% or less. The V content is even more preferably 0.010% or more and 0.060% or less.

[0034] Ti: 0.080% or less Ti is an element that contributes to improving the strength of steel by forming fine carbides and nitrides in the steel. It also contributes to reducing the amount of dissolved N in the steel due to its high affinity with N. Therefore, Ti can be added as needed. To achieve the above-mentioned effects, if Ti is added, it is preferable that the Ti content be 0.002% or more. However, excessive Ti content reduces the number density of high KAM value regions. Furthermore, the area ratio of high KAM value regions with a circle-equivalent diameter of 5.0 μm or more increases. Therefore, if Ti is added, the Ti content is preferably 0.080% or less. The Ti content is more preferably 0.005% or more and 0.070% or less. The Ti content is even more preferably 0.010% or more and 0.060% or less.

[0035] Cu: 0.50% or less, Ni: 0.50% or less Cu and Ni are elements that increase the strength of steel through solid solution strengthening. They also improve the hardenability of steel and contribute to the refinement of the structure, thereby increasing the strength of steel and increasing the number density of the high KAM value region. These elements can be added as needed. To achieve the above-mentioned effects, when Cu and Ni are added, the Cu and Ni contents are preferably 0.01% or more and 0.01% or more, respectively. However, excessive Cu and Ni content reduces the number density of the high KAM value region. Furthermore, the area ratio of high KAM value regions with a circle equivalent diameter of 5.0 μm or more increases. Therefore, when Cu and Ni are added, the Cu and Ni contents are preferably 0.50% or less and 0.50% or less, respectively. More preferably, Cu is 0.05% or more, Cu is 0.40% or less, Ni is 0.05% or more, and Ni is 0.40% or less. More preferably, Cu is 0.10% or more and Cu is 0.30% or less, and Ni is 0.10% or more and Ni is 0.30% or less.

[0036] Cr: 0.50% or less, Mo: 0.50% or less Cr and Mo are elements that improve the hardenability of steel, contribute to the refinement of the structure, increase the strength of steel, and increase the number density of the high KAM value region. They can be added as needed. To achieve the above-mentioned effects, when Cr and Mo are added, the Cr and Mo contents are preferably 0.01% or more and 0.01% or more, respectively. However, excessive Cr and Mo content reduces the number density of the high KAM value region. Furthermore, the area ratio of the high KAM value region with an equivalent circle diameter of 5.0 μm or more increases. Therefore, when Cr and Mo are added, the Cr and Mo contents are preferably 0.50% or less and 0.50% or less, respectively. More preferably, the Cr contents are 0.05% or more, 0.40% or less, 0.05% or more, and 0.40% or less. Even more preferably, the Cr contents are 0.10% or more, 0.30% or less, 0.10% or more, and 0.30% or less.

[0037] Ca: 0.0050% or less Ca is an element that contributes to improving the toughness of steel by spheroidizing sulfides such as MnS that are thinly drawn in the hot rolling process, and can be added as needed. To achieve the above-mentioned effects, if Ca is added, it is preferable that the Ca content be 0.0005% or more. However, excessive Ca content causes the formation of Ca oxide clusters in the steel, deteriorating toughness. Therefore, if Ca is added, the Ca content is preferably 0.0050% or less. The Ca content is more preferably 0.0008% or more and 0.0040% or less. The Ca content is even more preferably 0.0010% or more and 0.0035% or less.

[0038] B: 0.0050% or less B is an element that contributes to the refinement of the structure by lowering the transformation start temperature, increases the strength of the steel, and increases the number density of the high KAM value region, and can be contained as needed. To achieve the above-mentioned effects, if B is contained, it is preferable to contain 0.0002% or more of B. However, excessive B content reduces the number density of the high KAM value region. Furthermore, the area ratio of the high KAM value region with an equivalent circle diameter of 5.0 μm or more increases. Therefore, if B is contained, the B content is preferably 0.0050% or less. The B content is more preferably 0.0005% or more and 0.0040% or less. The B content is even more preferably 0.0008% or more and 0.0030% or less.

[0039] Mg: 0.020% or less, Zr: 0.020% or less, REM: 0.020% or less Mg, Zr, and REM are elements that contribute to the refinement of the microstructure, increase the strength of the steel, and increase the number density of the high KAM value region. These elements can be added as needed. The contents of Mg, Zr, and REM may each be 0%, but when Mg, Zr, and REM are contained, the contents of Mg, Zr, and REM are preferably 0.0005% or more, Zr is 0.0005% or more, and REM is 0.0005% or more, respectively. However, excessive Mg, Zr, and REM content reduces the number density of the high KAM value region. Furthermore, the area ratio of the high KAM value region with a circle equivalent diameter of 5.0 μm or more increases. Therefore, when Mg, Zr, and REM are contained, the contents are preferably 0.020% or less, 0.020% or less, and 0.020% or less, respectively. More preferably, Mg is 0.010% or less, Zr is 0.010% or less, and REM is 0.010% or less. Here, REM is a collective term for 17 elements, including Sc, Y, and lanthanoids. One or more of these 17 elements can be contained in the steel, and the REM content refers to the total content of these elements.

[0040] The balance is Fe and unavoidable impurities. Examples of the unavoidable impurities in the balance include Sn, As, Sb, Bi, Co, Pb, Zn, and O. However, within the range that does not impair the effects of the present invention, the content of Sn is not restricted to 0.1% or less, As, Sb, and Co are each 0.05% or less, and Bi, Pb, Zn, and O are each 0.005% or less.

[0041] In the electric resistance welded steel pipe of the present invention, the steel structure at the center of the wall thickness of the base material has an average crystal grain size of 15.0 μm or less, and the steel structure at a position 1 mm deep from the inner surface of the electric resistance weld, a position 1 mm deep from the outer surface of the electric resistance weld, a position 1 mm deep from the inner surface of the base material, and a position 1 mm deep from the outer surface of the base material each has a number density of 1000 grains / mm 2 or more, and the area ratio of high KAM value regions having a circle equivalent diameter of 5.0 μm or more is preferably 0.050 or less.

[0042] The average grain size and KAM value distribution are measured using the SEM / EBSD method. The measurement area is 400 μm x 400 μm, the measurement step size is 0.1 μm, and measurements from five or more fields are averaged. Based on the obtained EBSD data, the crystal orientation analysis software OIM Analysis™ is used to determine the grain boundary and grain size distribution, with boundaries with a misorientation of 15° or more considered grain boundaries (high-angle grain boundaries). A KAM value distribution image (KAM map) is also obtained. The average grain size is calculated as the diameter (equivalent circle diameter) of a circle with an area equal to the total measured area divided by the number of grains. When calculating the average grain size, grains with a grain size of 1.0 μm or less are excluded as measurement noise.

[0043] Here, the KAM (Kernel Average Misorientation) value is calculated using the following method. At each measurement point (a regular hexagonal pixel), the misorientation between each pixel is calculated using the center pixel and the three neighboring pixels (37 pixels in total), and the average of the misorientation values calculated is used as the KAM value for the center pixel. This operation is performed for all pixels in the field of view to obtain a KAM map. The higher the KAM value, the higher the dislocation density at that measurement point tends to be, and therefore the higher the hardness tends to be.

[0044] The number density of high KAM value regions and the area percentage of high KAM value regions with a circle-equivalent diameter of 5.0 μm or greater were determined by analyzing the resulting KAM map using the image analysis software ImageJ 1.52p. First, the KAM map was binarized into regions with KAM values between 3.0 and 5.0 (high KAM value regions) and other regions. The "Analyze Particles" function in ImageJ 1.52p was then used to determine the number of high KAM value regions in the KAM map and their area. The circle-equivalent diameter of each high KAM value region was also calculated from its area. The number density of high KAM value regions was calculated by dividing the number of high KAM value regions by the area of the KAM map. However, high KAM value regions with a circle-equivalent diameter of less than 2.0 μm were excluded as measurement noise. The area ratio of high KAM value regions with a circle-equivalent diameter of 5.0 μm or more is calculated by dividing the total area of high KAM value regions with a circle-equivalent diameter of 5.0 μm or more by the area of the KAM map.

[0045] If the average grain size in the steel structure at the center of the wall thickness of the base material becomes large, the strength of the electric resistance welded steel pipe decreases. Therefore, the average grain size is preferably 15.0 μm or less. The average grain size is more preferably 10.0 μm or less. Note that if the average grain size in the steel structure at the center of the wall thickness of the base material becomes small, the hardness near the inner surface and the outer surface of the electric resistance welded steel pipe increases, and the area ratio of high KAM value regions with a circle equivalent diameter of 5.0 μm or more in the steel structure at a specified depth increases. Therefore, the average grain size is preferably 2.0 μm or more. The average grain size is more preferably 3.0 μm or more.

[0046] In the steel structure at a depth of 1 mm from the inner surface of the electric resistance weld, a depth of 1 mm from the outer surface of the electric resistance weld, a depth of 1 mm from the inner surface of the base material, and a depth of 1 mm from the outer surface of the base material, if the number density of high KAM value regions becomes small, the area of high hardness regions in the structure at each of the predetermined depth positions will become large, and the local hardness variation will become large. Therefore, when a Vickers test is performed at multiple points at each of the predetermined depth positions with a load of 10 gf, the proportion of indentations with an average hardness of +40 HV or more will become high. Also, the proportion of indentations with an eccentricity E of 0.20 or more will become high. Therefore, the number density of high KAM value regions in the structure at each of the predetermined depth positions will become 1000 indentations / mm 2 The number density is preferably 2000 particles / mm or more. 2 That is all. If the number density of the high KAM value regions becomes excessively large, the total area ratio of high hardness regions becomes high, and the local hardness variation becomes large. Therefore, when a Vickers test is performed at multiple points at each of the predetermined depth positions with a load of 10 gf, the ratio of indentations with an average hardness of +40 HV or more becomes high. Also, the ratio of indentations with an eccentricity E of 0.20 or more becomes high. Therefore, the number density of the high KAM value regions in the structure at each of the predetermined depth positions becomes 50,000 pieces / mm 2 The number density is preferably 30,000 pieces / mm or less. 2 The following is the result.

[0047] In the steel structure at a depth of 1 mm from the inner surface of the electric resistance weld, a depth of 1 mm from the outer surface of the electric resistance weld, a depth of 1 mm from the inner surface of the base metal, and a depth of 1 mm from the outer surface of the base metal, if the area ratio of high KAM value regions with a circle equivalent diameter of 5.0 μm or more increases, the area of high hardness regions in the structure at each of the predetermined depths increases, resulting in greater local hardness variation. Therefore, when a Vickers test is performed at multiple points at each of the predetermined depths with a load of 10 gf, the ratio of indentations with an average hardness of +40 HV or more increases. Furthermore, the ratio of indentations with an eccentricity E of 0.20 or more increases. Therefore, the area ratio of high KAM value regions with a circle equivalent diameter of 5.0 μm or more in the structure at each of the predetermined depths is preferably 0.050 or less. The area ratio is more preferably 0.030 or less. While a lower area ratio is preferable, an excessive reduction increases manufacturing costs and production load, so the area ratio is preferably 0.001 or more. The area ratio is more preferably 0.002 or more.

[0048] Furthermore, the steel structure at the center of the thickness of the base material preferably has a total volume fraction of ferrite and bainite of 80% or more, with the remainder comprising one or more selected from pearlite, martensite, and austenite. More preferably, the remainder comprises one or more selected from pearlite, martensite, and austenite.

[0049] Ferrite is a soft structure. Bainite is harder than ferrite and softer than pearlite, martensite, and austenite.

[0050] If the volume fraction of bainite is low, the proportion of soft ferrite increases, resulting in a decrease in strength. Therefore, the volume fraction of bainite is preferably 10% or more, and more preferably 20% or more.

[0051] As the total volume fraction of ferrite and bainite decreases, the proportions of hard pearlite, martensite, and austenite increase near the inner surface and the outer surface of the electric resistance welded steel pipe. As a result, the number density of high KAM value regions increases in the structure at each of the predetermined depth positions, and the area proportion of high KAM value regions with a circle equivalent diameter of 5.0 μm or more increases. Therefore, the total volume fraction of ferrite and bainite is preferably 0.80% or more. The total volume fraction is more preferably 85% or more. On the other hand, as the proportions of hard pearlite, martensite, and austenite approach 0%, ductility decreases, so the total volume fraction of ferrite and bainite is preferably 99% or less. The total volume fraction is more preferably 98% or less.

[0052] The nucleation sites of the above-mentioned various structures, excluding austenite, are austenite grain boundaries or deformation bands within austenite grains. In hot rolling, by increasing the reduction at low temperatures where austenite recrystallization is difficult to occur, a large number of dislocations can be introduced into the austenite, refining the austenite, and introducing a large number of deformation bands within the grains. This increases the area of nucleation sites, increasing the frequency of nucleation and enabling the steel structure to be refined.

[0053] Here, the steel structure can be observed by the method described below. First, a test piece for structure observation is prepared by taking a cross-section parallel to both the axial direction and the thickness direction of the electric resistance welded steel pipe and at the center of the wall thickness, polishing it, and then etching it with nital. For structure observation, an optical microscope (magnification: 1000x) or a scanning electron microscope (SEM, magnification: 1000x) is used to observe and photograph the structure at the center of the wall thickness. Next, the area fractions of bainite and the remainder (ferrite, pearlite, martensite, austenite) are determined from the obtained optical microscope images and SEM images. The area fraction of each structure is calculated as the average value of the values obtained in five or more fields of view. In the present invention, the area fraction obtained by structure observation is taken as the volume fraction of each structure.

[0054] Ferrite is a product of diffusion transformation and exhibits a nearly recovered structure with low dislocation density. This includes polygonal ferrite and pseudo-polygonal ferrite.

[0055] Bainite is a complex phase structure of lath-shaped ferrite and cementite with a high dislocation density.

[0056] Pearlite is a eutectoid structure of iron and iron carbide (ferrite + cementite), and exhibits a lamellar structure in which linear ferrite and cementite are arranged alternately.

[0057] Martensite is a lath-like structure transformed at low temperatures with a very high dislocation density. In SEM images, it exhibits brighter contrast than ferrite and bainite.

[0058] It is difficult to distinguish between martensite and austenite in optical microscope images and SEM images. Therefore, the area fraction of the structure observed as martensite or austenite in the obtained SEM image is measured, and the volume fraction of austenite, measured using the method described below, is subtracted from this measurement to determine the volume fraction of martensite.

[0059] Austenite is an fcc phase, and the volume fraction of austenite is measured by X-ray diffraction using a test piece prepared in the same manner as the test piece described above. The volume fraction of austenite is calculated from the integrated intensities of the (200), (220), and (311) planes of the obtained fcc iron and the (200) and (211) planes of the obtained bcc iron.

[0060] Furthermore, the electric resistance welded steel pipe of the present invention preferably has a yield strength of 400 MPa or more in order to withstand internal pressure and axial load. The yield strength is more preferably 450 MPa or more. If the yield strength is excessively high, the machinability of the electric resistance welded steel pipe decreases. Therefore, the yield strength is preferably 1100 MPa or less. The yield strength is more preferably 1000 MPa or less. The test specimens used for strength measurement are JIS No. 5 full-thickness tensile test specimens taken from the base material of the electric resistance welded steel pipe so that the tensile direction is parallel to the pipe axis. The tensile test is carried out in accordance with the provisions of JIS Z 2241. The yield strength (MPa) is the flow stress at a nominal strain of 0.5%.

[0061] Furthermore, in a cutting test, the electric resistance welded steel pipe of the present invention preferably has a wear width on the flank of the tool tip of 0.10 mm or less after cutting 1000 m. The wear width is more preferably 0.08 mm or less. The cutting test is performed by placing the electric resistance welded steel pipe on a lathe and cutting the outer or inner periphery of the electric resistance welded steel pipe using a rectangular tip that is a P10 class cemented carbide tool (JIS B 4053) at a cutting speed of 100 m / min, a feed of 0.1 mm / rev, and a cutting depth of 0.5 mm. Cutting is stopped after cutting 1000 m, and the wear width on the flank of the tool tip is measured.

[0062] The wall thickness of the electric resistance welded steel pipe of the present invention is, for example, 5 mm or more, and also, for example, 40 mm or less.

[0063] Next, a method for producing an electric resistance welded steel pipe according to one embodiment of the present invention will be described.

[0064] The electric resistance welded steel pipe of the present invention is produced, for example, by subjecting a steel material having the above-described chemical composition to the following heating, hot rolling, and cooling processes, then winding it into a coil to form a hot-rolled steel sheet, and then forming the hot-rolled steel sheet into a cylindrical shape by cold rolling, electric resistance welding, and then subjecting it to the following heat treatment processes.

[0065] In the following description of the manufacturing method, unless otherwise specified, the temperature indicated in "°C" refers to the surface temperature of the steel material or steel plate (hot-rolled plate) and the outer surface temperature of the steel pipe. These surface temperatures can be measured using a radiation thermometer or the like. The temperatures inside the steel plate and the inner surface of the steel pipe can be determined by calculating the temperature distribution within the cross section of the steel plate and the steel pipe using heat transfer analysis and correcting the results using the surface temperature of the steel plate or the outer surface temperature of the steel pipe. Furthermore, "hot-rolled steel plate" includes hot-rolled plate and hot-rolled steel strip.

[0066] In the present invention, the method for melting the steel material (steel slab) is not particularly limited, and any of the known melting methods such as converter, electric furnace, and vacuum melting furnace are suitable. The casting method is also not particularly limited, and the steel slab can be produced to the desired dimensions by a known casting method such as continuous casting. However, there is no problem if an ingot-blooming and blooming rolling method is used instead of the continuous casting method. The molten steel may further be subjected to secondary refining such as ladle refining.

[0067] The resulting steel material (steel slab) is then subjected to a heating step, a hot rolling step, a cooling step, and then wound into a coil to form a hot-rolled steel sheet.

[0068] In the heating step, the steel material is heated to a heating temperature of, for example, 1100°C or higher and 1300°C or lower.

[0069] If the heating temperature is low, the deformation resistance of the rolled material increases, making rolling difficult. On the other hand, if the heating temperature is high, the austenite grains become coarse, making it difficult to obtain fine austenite grains in the subsequent rolling (rough rolling, finish rolling), and the average crystal grain size becomes large. For this reason, the heating temperature in the hot rolling process is preferably 1100°C or higher and 1300°C or lower. The heating temperature is more preferably 1120°C or higher and 1280°C or lower.

[0070] In the present invention, the conventional method of producing a steel slab (slab) and then cooling it to room temperature and then reheating it can be adopted. In addition, the present invention can also be applied to energy-saving direct rolling processes such as charging the hot slab into a heating furnace without cooling it to room temperature, or immediately rolling it after a short period of heat retention.

[0071] In the hot rolling step, for example, hot rolling is performed such that the temperature at the 1 / 4t (t: plate thickness) position is in the range of 1000°C to 1100°C, and a reduction rate of 10% to 50% is performed five or more times at intervals of 50 seconds or less, so that the total reduction rate in the finish rolling is 50% or more and the finish rolling end temperature is 750°C to 850°C. The reduction in the range of 1000°C to 1100°C may be performed in rough rolling, in finish rolling, or in both rough rolling and finish rolling.

[0072] If the reduction ratio at each reduction temperature between 1000°C and 1100°C is low, the austenite will coarsen and the average grain size will increase. Furthermore, recrystallization of austenite will be insufficient, resulting in residual coarse austenite with high hardenability, a low number density in the high KAM value region, and a high area ratio of the high KAM value region with a circle equivalent diameter of 5.0 μm or more. On the other hand, if the reduction ratio is high, the deformation resistance of the rolled material will increase, making rolling difficult. Therefore, the reduction ratio is preferably between 10% and 50%. The reduction ratio is more preferably between 15% and 45%.

[0073] If the interval (time interval) between each reduction at 1000°C or higher and 1100°C or lower is long, austenite coarsens and the average grain size increases. Furthermore, austenite recrystallization becomes insufficient, resulting in residual coarse austenite with high hardenability, a low number density in the high KAM value region, and a high area ratio of the high KAM value region with a circle equivalent diameter of 5.0 μm or higher. Therefore, the interval is preferably 50 s or less. The interval is more preferably 45 s or less. While a shorter interval is preferable, excessive reduction increases manufacturing costs and production load, so the interval is preferably 5 s or more. Here, the reduction interval refers to the time interval between reductions on the same location of the rolled material and the next reduction (the time interval between rolling passes). Cooling may be performed between each reduction. This cooling may be air-cooled, or water-cooled if necessary.

[0074] If the number of reductions at 1000°C or higher and 1100°C or lower is small, the austenite will coarsen and the average grain size will increase. Furthermore, recrystallization of austenite will be insufficient, resulting in residual coarse austenite with high hardenability, a low number density of high KAM value regions, and a high area ratio of high KAM value regions with equivalent circle diameters of 5.0 μm or higher. Therefore, the number of reductions is preferably 5 or more. The number of reductions is more preferably 7 or more. Since an excessively large number of reductions results in a low reduction rate for each reduction, the number of reductions is preferably 15 or less.

[0075] If the total reduction in finish rolling is low, the austenite will coarsen and the average crystal grain size will increase. Therefore, the total reduction is preferably 50% or more. The total reduction is more preferably 55% or more. If the total reduction is high, the average crystal grain size will decrease. Therefore, the total reduction is preferably 80% or less. The total reduction is more preferably 75% or less.

[0076] The total reduction in the finish rolling mentioned above refers to the sum of the reductions in each rolling pass in the finish rolling.

[0077] The finished plate thickness is preferably 5 mm or more and 40 mm or less from the viewpoint of ensuring the necessary reduction rate and controlling the temperature of the steel plate.

[0078] If the finish rolling end temperature is low, the temperature will fall below the ferrite transformation start temperature during finish rolling, resulting in the formation of a large amount of worked ferrite and a decrease in ductility. On the other hand, if the finish rolling end temperature is high, fine austenite grains will not be obtained and the average crystal grain size will become large. Therefore, the finish rolling end temperature is preferably 750°C or higher and 850°C or lower. The finish rolling end temperature is more preferably 770°C or higher and 830°C or lower.

[0079] In the cooling step, for example, cooling is performed at an average cooling rate at the center of the plate thickness of 5°C / s to 40°C / s, and a cooling stop temperature of 400°C to 650°C.

[0080] If the average cooling rate is low, the structure becomes coarse and the average crystal grain size becomes large. The bainite fraction also decreases. If the average cooling rate is high, the martensite fraction in the surface layer increases, the number density of high KAM value regions increases, and the area ratio of high KAM value regions with a circle equivalent diameter of 5.0 μm or more increases. Therefore, the average cooling rate is preferably 5°C / s or more and 40°C / s or less. The average cooling rate is more preferably 10°C / s or more and 35°C / s or less. Unless otherwise specified, the average cooling rate is defined as [(cooling start temperature - cooling stop temperature) / cooling time from the cooling start temperature to the cooling stop temperature].

[0081] If the cooling stop temperature is low, the martensite fraction in the surface layer increases, the number density of high KAM value regions increases, and the area ratio of high KAM value regions with an equivalent circle diameter of 5.0 μm or more increases. If the cooling stop temperature is high, the structure becomes coarse and the average crystal grain size increases. In addition, the bainite fraction decreases. Therefore, the cooling stop temperature is preferably 400°C or higher and 650°C or lower. The cooling stop temperature is more preferably 450°C or higher and 600°C or lower.

[0082] Next, the hot-rolled steel sheet is formed into a cylindrical shape by cold rolling, and then subjected to electric resistance welding, followed by a heat treatment step.

[0083] In the heat treatment process, induction heating is applied from the outer surface of the electric resistance welded portion so that, for example, the total heating time at 900°C or higher on the outer surface of the electric resistance welded portion is 100 seconds or less, and the temperature rise rate on the inner surface of the electric resistance welded portion is 3°C / s or more. The heating method is not limited to the above method, and the entire pipe may be heated in a heating furnace or by induction heating. In this case, the temperature history is controlled to be uniform throughout the pipe. The temperature rise rate is an average temperature rise rate in a temperature range of 400 to 800°C. Next, water cooling is applied from the outer surface of the electric resistance welded portion so that, for example, the average cooling rate on the inner surface of the electric resistance welded portion is 10°C / s or more and 50°C / s or less. The cooling method is not limited to the above method, and may include cooling from the inner surface, cooling the entire pipe in a water bath, or cooling the entire pipe by spraying from both the inside and outside surfaces. In this case, the temperature history is controlled to be uniform throughout the pipe. The average cooling rate is an average cooling rate in a temperature range of 400 to 700°C. Then, if necessary, tempering may be carried out to adjust the hardness.

[0084] If the total heating time at 900°C or higher on the outer surface of the electric resistance weld is long, the structure becomes coarse and the average crystal grain size becomes large. Furthermore, the proportion of coarse martensite on the outer surface increases, the martensite connects with each other, the number density of high KAM value regions decreases, and the area proportion of high KAM value regions with a circle equivalent diameter of 5.0 μm or more increases. Therefore, the total heating time is preferably 100 seconds or less. The total heating time is more preferably 90 seconds or less. If the total heating time is short, there is a risk that the structure will not be completely austenitized during heating, so the total heating time is preferably 10 seconds or more.

[0085] If the heating rate on the inner surface of the electric resistance weld is low, the structure will become coarse and the average crystal grain size will become large. Furthermore, the number density of high KAM value regions on the outer surface will be low, and the area ratio of high KAM value regions with a circle equivalent diameter of 5.0 μm or more will be high. Therefore, the heating rate is preferably 3°C / s or more. The heating rate is more preferably 5°C / s or more. If the heating rate is too high, there is a risk that the structure will not be completely austenitized during heating, so the heating rate is preferably 50°C / s or less.

[0086] If the average cooling rate on the inner surface of the electric resistance weld is low, the structure becomes coarse and the average crystal grain size becomes large. Also, the bainite fraction becomes low. Therefore, the average cooling rate is preferably 10°C / s or more. The average cooling rate is more preferably 15°C / s or more. If the average cooling rate is high, the martensite fraction on the inner surface becomes high, the number density of high KAM value regions becomes high, and the area proportion of high KAM value regions with a circle equivalent diameter of 5.0 μm or more becomes high. Therefore, the average cooling rate is preferably 50°C / s or less. The average cooling rate is more preferably 40°C / s or less.

[0087] Whether a steel pipe is an electric resistance welded steel pipe or not can be determined by cutting the electric resistance welded steel pipe perpendicular to the pipe axis, polishing and corroding the cut surface including the weld (electric resistance weld), and observing it under an optical microscope. If the width of the molten and solidified part of the weld (electric resistance weld) in the pipe circumferential direction is 1.0 μm or more and 1000 μm or less across the entire pipe thickness, it is an electric resistance welded steel pipe.

[0088] Here, the etching solution should be selected appropriately depending on the steel composition and type of steel pipe. Furthermore, as shown in the schematic diagram of FIG. 5, the molten solidified portion can be visually recognized as a region 7 in FIG. 5 that has a different structural morphology and contrast from the base material portion 5 and the weld heat-affected zone 6. For example, the molten solidified portion of an electric resistance welded steel pipe made of carbon steel or low alloy steel can be identified as a white region observed under an optical microscope in the cross section corroded with nital. Furthermore, the molten solidified portion of a UOE steel pipe made of carbon steel or low alloy steel can be identified as a region containing a cellular or dendritic solidification structure under an optical microscope in the cross section corroded with nital.

[0089] The steel pipe pile joint of the present invention is produced by cutting the electric resistance welded steel pipe to a predetermined length, and then providing an appropriate mechanical joint means (mechanical means for connecting steel pipes to each other) by cutting to form a joint of the desired shape, such as a tubular shape.

[0090] The steel pipe pile of the present invention is produced by attaching the steel pipe pile joint to a steel pipe that serves as the pile body. Specifically, the steel pipe pile of the present invention is produced by welding the steel pipe pile joint to the end of the steel pipe that serves as the pile body. Generally, a pin joint is joined to one end of the steel pipe pile, and a box joint is joined to the other end.

[0091] The shape of the steel pipe pile joint may be any as long as it has a structure that allows the pair of joints to be mechanically joined together. For example, a screw-type joint structure is conceivable. The steel pipe pile shown in FIG. 4 is installed in the ground so that the upper end of steel pipe pile 2 is a box joint 4 and the lower end of steel pipe pile 1 is a pin joint 3, as shown in FIG. 4 . When connecting the upper steel pipe pile 1 to the lower steel pipe pile 2 that has been installed in the ground, the upper steel pipe pile 1 is suspended using a crane or the like so that the lower end is the pin joint 3. The upper steel pipe pile 1 is then lowered, and the pin joint 3 at the lower end of the upper steel pipe pile 1 is inserted into the box joint 4 at the upper end of the lower steel pipe pile 2. In this state, the upper steel pipe pile 1 is rotated to screw the pin joint 3 into the box joint 4, thereby connecting the two steel pipe piles. [Example]

[0092] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0093] Molten steel having the chemical composition shown in Table 1 was melted to prepare a slab (steel material). The obtained slab was subjected to the heating process, hot rolling process, and cooling process under the conditions shown in Table 2 to obtain a hot-rolled steel sheet with the finished thickness (mm) shown in Table 2. The hot-rolled steel sheet was then formed into a cylindrical round steel pipe by roll forming, and the butt joint was electric resistance welded. The electric resistance welded portion was then subjected to a heat treatment process under the conditions shown in Table 3, and the diameter was reduced using rolls arranged above, below, and to the left and right of the round steel pipe to obtain an electric resistance welded steel pipe for steel pipe pile joints with the outer diameter (mm) and wall thickness (mm) shown in Table 3.

[0094] [Table 1]

[0095] [Table 2]

[0096] [Table 3]

[0097] Test pieces were taken from the obtained electric resistance welded steel pipe and the following measurements were carried out: average grain size measurement, number density measurement of high KAM value regions, measurement of the area ratio of high KAM value regions with an equivalent circle diameter of 5.0 μm or more, microstructure observation, tensile test, Vickers test, and cutting test. Note that the "base material" below refers to the base material located 90° away from the electric resistance weld in the circumferential direction of the pipe.

[0098] [Average grain size measurement] The average grain size was measured using the SEM / EBSD method. The measurement area was 400 μm × 400 μm, with a measurement step size of 0.1 μm. Measurement values from five fields were averaged at the center of the base material wall thickness. Based on the obtained EBSD data, the crystal orientation analysis software OIM Analysis™ was used to determine the distribution of grain boundaries and grain size, with boundaries with a misorientation of 15° or more considered as grain boundaries (high-angle grain boundaries). The average grain size was calculated as the diameter (equivalent circle diameter) of a circle with an area equal to the total measured area divided by the number of grains. Note that when calculating the average grain size, grains with a grain size of 1.0 μm or less were excluded as measurement noise.

[0099] [Measurement of number density in the high KAM value region] The number density of high KAM value regions was measured using the SEM / EBSD method. The measurement area was 400 μm × 400 μm, with a measurement step size of 0.1 μm. Based on the obtained EBSD data, a distribution image of KAM values (KAM map) was obtained using the crystal orientation analysis software OIM Analysis™. Here, the KAM (Kernel Average Misorientation) value was calculated using the following method. For each measurement point (a regular hexagonal pixel), the misorientation between each pixel was calculated using the center and the three neighboring pixels (37 pixels in total), and the average of the calculated misorientation values was used as the KAM value of the central pixel. This operation was performed for all pixels in the field of view (measurement area), resulting in a KAM map. The number density of high KAM value regions was determined by analyzing the obtained KAM map using the image analysis software ImageJ 1.52p. First, the KAM map was binarized into regions with KAM values of 3.0 to 5.0 (high KAM value regions) and other regions. The number of high KAM value regions in the KAM map was then determined using the "Analyze Particles" function in the image analysis software ImageJ 1.52p. The number density of high KAM value regions was calculated by dividing the number of high KAM value regions by the area of the KAM map. However, high KAM value regions with a circular equivalent diameter of less than 2.0 μm were excluded as measurement noise. Measurements were performed in five fields of view for each of the above measurement regions at a depth of 1 mm from the inner surface of the ERW weld, a depth of 1 mm from the outer surface of the ERW weld, a depth of 1 mm from the inner surface of the base metal, and a depth of 1 mm from the outer surface of the base metal, and the average value was calculated. This average value was used as the number density of high KAM value regions in the structure at each depth. The maximum and minimum number densities of high KAM value regions at a depth of 1 mm from the inner surface of the ERW weld, a depth of 1 mm from the outer surface of the ERW weld, a depth of 1 mm from the inner surface of the base metal, and a depth of 1 mm from the outer surface of the base metal were then determined.

[0100] [Measurement of the area ratio of high KAM value regions with a circle equivalent diameter of 5.0 μm or more] The area percentage of high KAM value regions with a circular equivalent diameter of 5.0 μm or greater was determined by analyzing the KAM map obtained during the number density measurement of the high KAM value region using the image analysis software ImageJ 1.52p. First, the KAM map was binarized into regions with KAM values between 3.0 and 5.0 (high KAM value regions) and other regions. The area of the high KAM value regions in the KAM map was then calculated using the "Analyze Particles" function of the image analysis software ImageJ 1.52p. The circular equivalent diameter of each high KAM value region was also calculated from the area of each high KAM value region. The area percentage of high KAM value regions with a circular equivalent diameter of 5.0 μm or greater was calculated by dividing the total area of high KAM value regions with a circular equivalent diameter of 5.0 μm or greater by the area of the KAM map. Measurements were taken in five fields of view for each of the above measurement regions at a depth of 1 mm from the inner surface of the electric resistance weld, a depth of 1 mm from the outer surface of the electric resistance weld, a depth of 1 mm from the inner surface of the base material, and a depth of 1 mm from the outer surface of the base material, and the average values were calculated. This average value was used as the area percentage of high KAM value regions with a circle equivalent diameter of 5.0 μm or more in the structure at each depth position. The maximum and minimum area percentages of high KAM value regions with a circle equivalent diameter of 5.0 μm or more were then determined at a depth of 1 mm from the inner surface of the electric resistance weld, a depth of 1 mm from the outer surface of the electric resistance weld, a depth of 1 mm from the inner surface of the base material, and a depth of 1 mm from the outer surface of the base material.

[0101] [Structural observation] Test specimens for microstructural observation were prepared by taking specimens from the base material so that the observation surface was a cross section parallel to both the axial direction and the thickness direction of the electric resistance welded steel pipe and at the center of the wall thickness. The specimens were then polished and etched with nital. Microstructural observation was performed by observing and photographing the microstructure at the center of the wall thickness using an optical microscope (magnification: 1000x) or a scanning electron microscope (SEM, magnification: 1000x). The area fractions of bainite and the remainders (ferrite, pearlite, martensite, and austenite) were then determined from the obtained optical microscope and SEM images. The area fractions of each microstructure were calculated as the average of the values obtained from five visual fields. In the present invention, the area fractions obtained by microstructural observation were used as the volume fractions of each microstructure.

[0102] [Tensile test] JIS No. 5 full-thickness tensile test specimens were taken from the base metal of the electric resistance welded steel pipe so that the tensile direction was parallel to the pipe axis. The tensile test was conducted in accordance with the provisions of JIS Z 2241. The yield strength (MPa) was defined as the flow stress at a nominal strain of 0.5%.

[0103] [Vickers test] Vickers tests were performed on both the electric resistance welded portion and the base material, using a cross section perpendicular to the pipe axis as the measurement surface, according to the method described in JIS Z 2244 (2020), under a load of 10 gf. For the electric resistance welded portion, tests were performed at 201 points at 0.2 mm intervals over a 20 mm range on each side of the circumferential direction around the electric resistance welded portion, at a depth of 1 mm from the inner surface and a depth of 1 mm from the outer surface, as shown in Figure 2. For the base material, tests were performed at 201 points at 0.2 mm intervals over a 40 mm range, at a depth of 1 mm from the inner surface and a depth of 1 mm from the outer surface, at a 90° circumferential angle from the electric resistance welded portion, as shown in Figure 3. The Vickers hardness was then calculated for each indentation, and the lengths of the lines OA, OB, OC, and OD shown in Figure 1 were measured to obtain the L. max , L min , L ave The ratio of indentations with an average hardness of +40 HV or more and the ratio of indentations with an eccentricity E of 0.20 or more were then calculated at a depth of 1 mm from the inner surface of the electric resistance weld, a depth of 1 mm from the outer surface of the electric resistance weld, a depth of 1 mm from the inner surface of the base material, and a depth of 1 mm from the outer surface of the base material, out of the total number of indentations (201) at each depth. The average hardness is the average value (arithmetic mean value) of the Vickers hardness calculated for each indentation at each depth. The maximum and minimum values of the ratio of indentations with an average hardness of +40 HV or more were then calculated at a depth of 1 mm from the inner surface of the electric resistance weld, a depth of 1 mm from the outer surface of the electric resistance weld, a depth of 1 mm from the inner surface of the base material, and a depth of 1 mm from the outer surface of the base material.

[0104] [Cutting test] The cutting test was carried out by placing the electric resistance welded steel pipe on a lathe and using a rectangular parallelepiped tip, a P10 class carbide tool (JIS B 4053), to cut the outer or inner circumference of the electric resistance welded steel pipe at a cutting speed of 100 m / min, a feed rate of 0.1 mm / rev, and a cutting depth of 0.5 mm. Cutting was stopped when 1000 m had been cut, and the wear width of the flank face of the tool tip was measured.

[0105] The results obtained are shown in Tables 4 and 5.

[0106] [Table 4]

[0107] [Table 5]

[0108] In Tables 4 and 5, electric resistance welded steel pipes Nos. 1, 2, 5, 7, 9 and 10 are examples of the present invention, and electric resistance welded steel pipes Nos. 3, 4, 6 and 8 are comparative examples.

[0109] In all of the electric-welded steel pipes of the present invention, the proportion of indentations with an average hardness of +40 HV or more was 0.30 or less, and the proportion of indentations with an eccentricity E of 0.20 or more was 0.40 or less at a position 1 mm deep from the inner surface of the electric-welded weld, a position 1 mm deep from the outer surface of the electric-welded weld, a position 1 mm deep from the inner surface of the base material, and a position 1 mm deep from the outer surface of the base material (in Table 5, the ``maximum proportion of indentations with an average hardness of +40 HV or more'' was 0.30 or less, and the ``maximum proportion of indentations with an eccentricity E of 0.20 or more'' was 0.40 or less).

[0110] On the other hand, the electric resistance welded steel pipe of Comparative Example No. 3 had a small minimum reduction ratio at 1000°C or higher and 1100°C or lower during the hot rolling process. As a result, the area proportion of high KAM value regions with equivalent circle diameters of 5.0 μm or higher was high, and the proportion of indentations with an average hardness of +40 HV or higher exceeded the range of the present invention. As a result, the desired machinability was not obtained. The electric resistance welded steel pipe of Comparative Example No. 4 had a large maximum value for the reduction interval (time interval between rolling passes) at 1000°C or higher and 1100°C or lower in the hot rolling process. As a result, the area proportion of high KAM value regions with a circle equivalent diameter of 5.0 μm or higher was high, and the proportion of indentations with an eccentricity E of 0.20 or higher exceeded the range of the present invention. As a result, the desired machinability was not obtained. The electric resistance welded steel pipe No. 6 of the comparative example had a small number of reductions at 1000°C or higher and 1100°C or lower in the hot rolling process. As a result, the number density in the high KAM value region was low and the proportion of indentations with an eccentricity E of 0.20 or higher exceeded the range of the present invention. As a result, the desired machinability was not obtained. The comparative electric resistance welded steel pipe No. 8 had a high average cooling rate on the inner surface during the heat treatment process. As a result, the number density of the high KAM value region was high, the total volume fraction of ferrite and bainite was low, the proportion of indentations with an average hardness of +40 HV or more exceeded the range of the present invention, and the proportion of indentations with an eccentricity E of 0.20 or more exceeded the range of the present invention. As a result, the desired machinability was not obtained. [Explanation of symbols]

[0111] 1 Steel pipe pile (upper steel pipe pile) 2 Steel pipe pile (lower steel pipe pile) 3-pin fitting 4 Box Joint 5 Base metal part 6 Weld heat affected zone 7 Melting and solidification area

Claims

1. An electric resistance welded steel pipe for a steel pipe pile joint having a base material portion and an electric resistance welded portion, When a Vickers test was carried out with a load of 10 gf at a position 1 mm deep from the inner surface of the electric resistance weld, a position 1 mm deep from the outer surface of the electric resistance weld, a position 1 mm deep from the inner surface of the base material, and a position 1 mm deep from the outer surface of the base material, At each of the positions, The ratio of the number of indentations having an average hardness of +40 HV or more is 0.30 or less, The ratio of the number of indentations having an eccentricity E of 0.20 or more is 0.40 or less, The composition of the base material is, in mass%, C: 0.020% or more and 0.200% or less, Si: 0.50% or less, Mn: 0.30% or more and 2.00% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% or more and 0.100% or less, and N: 0.0100% or less; Further optionally, Nb: 0.080% or less, V: 0.080% or less, Ti: 0.080% or less, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less, Ca: 0.0050% or less, B: 0.0050% or less, Mg: 0.020% or less, Zr: 0.020% or less, REM: 0.020% or less, the balance being Fe and unavoidable impurities; The steel structure at the center of the base material thickness is The average grain size is 2.0 μm or more, the total volume fraction of ferrite and bainite is 80% or more, and the remainder contains one or more types selected from pearlite, martensite, and austenite, The steel structures at a depth of 1 mm from the inner surface of the electric resistance weld, a depth of 1 mm from the outer surface of the electric resistance weld, a depth of 1 mm from the inner surface of the base material, and a depth of 1 mm from the outer surface of the base material are respectively: An electric resistance welded steel pipe for steel pipe pile joints, in which the number density of high KAM value regions having a KAM value of 3.0 or more and 5.0 or less is 1000 / mm 2 or more, and the area ratio of high KAM value regions having a circle equivalent diameter of 5.0 μm or more is 0.050 or less. In this case, the eccentricity E is expressed as follows: O is the intersection of the diagonal lines of the indentation, and A, B, C, and D are the vertices in counterclockwise order. The maximum, minimum, and average lengths of the line segments OA, OB, OC, and OD are expressed as L, respectively. ｍａｘ , L ｍｉｎ , L ａｖｅ When this is the case, the value is calculated by equation (1). E=(L ｍａｘ -L ｍｉｎ ) / L ａｖｅ ・・・(1)

2. The steel structure at the center of the base material thickness is The electric resistance welded steel pipe for steel pipe pile joints according to claim 1, wherein the average crystal grain size is 15.0 μm or less.

3. A tubular coupling attached to the end of a steel pipe, the coupling comprising a mechanical means for interconnecting the steel pipes, A steel pipe pile joint using the electric resistance welded steel pipe for steel pipe pile joints according to claim 1 or 2.

4. A steel pipe pile having the steel pipe pile joint according to claim 3 at an end of the steel pipe.

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