High-strength steel pipe for piles and method for manufacturing the same.

Abstract

To provide a steel pipe for a high strength steel pipe pile being a steel pipe having API X100 grade or higher and excellent in toughness, with lower surface hardness.SOLUTION: A steel pipe for a high strength steel pipe pile has a prescribed component composition satisfying a prescribed relational expression. A microstructure includes 80% or more of a bainite structure, 20% or less of an island martensite, 20% or less of total area ratio of a balance of one or more kinds from retained austenite, pseudo pearlite and ferrite, 5 or more pieces / μm2 of composite carbide including one or more kinds from Nb, Ti and Mo with an equivalent circle diameter of 20 nm or less, yield strength of 690 MPa or more, tensile strength of 760 MPa or more, and a surface hardness 350HV10 or less at a position 1 mm from a surface.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a steel pipe for high-strength steel pipe piles having a strength of API X100 grade or higher (JIS SBHS700 grade or higher) used for port structures and flood control dikes, and is particularly characterized by excellent strength and toughness.

Background Art

[0002] In steel pipe piles used for port structures and flood control dikes, etc., the steel pipe is required to ensure high strength, particularly high strength and high toughness to prevent destruction due to external pressure at the seabed during excavation. In recent years, with the increase in the size of structures, the demand for high-strength steel pipes of API X100 grade or higher has also been increasing.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, since the steel plate described in Patent Document 1 compensates for the strength reduction due to PWHT treatment by the precipitation of Cr carbides during PWHT, a large amount of Cr addition is required. This not only results in high material costs but also causes problems such as deterioration of weldability and toughness. On the other hand, the steel pipe described in Patent Document 2 focuses on improving the properties of the seam weld metal and no special consideration is given to the base metal. Also, generally, in high-strength grades of X100 or higher, with the increase in surface hardness, pipe expansion cracks during pipe expansion after pipe welding often become a problem, and radical countermeasures are required. The object of the present invention is to solve the problems of the prior art and to provide a high-strength steel pipe for high-strength steel pipe piles that is of API X100 grade or higher, has excellent toughness, and has reduced surface hardness. [Means for solving the problem]

[0005] The features of the present invention, which solves these problems, are as follows. [1] A steel pipe for high-strength steel pipe piles having a base material portion and a welded portion, The base material is, by mass %, C: 0.04~0.10%, Si: 0.01~0.5%, Mn: 1.5~2.5%, P: 0.015% or less, S: 0.0020% or less, Al: 0.08% or less, Cr: 0.01~0.5%, Mo: 0.01~0.5%, Ti: 0.005~0.025%, Nb: 0.005~0.08%, N: 0.001~0.010%, O: 0.0050% or less, It contains, The composition has the following characteristics: the Ceq value expressed by equation (1) below satisfies Ceq value ≥ 0.48; the P value expressed by equation (2) below satisfies P value ≥ 0.15; and the Q value, the total mass % of Cr and Mo expressed by equation (3) below, satisfies Q value ≥ 0.50 and Cr ≤ Mo, with the remainder being Fe and unavoidable impurities. The microstructure consists of bainite (over 80%), island martensite (less than 20%), and the remainder consists of one or more of the following: retained austenite, pseudo-pearlite, and ferrite, with the total area ratio of the remainder being 20% ​​or less. A composite carbide containing one or more of Nb, Ti, and Mo, with an equivalent circle diameter of 20 nm or less, is present in a quantity of 5 particles / μm. 2 That's all. A high-strength steel pipe for piles, characterized by a yield strength of 690 MPa or higher, a tensile strength of 760 MPa or higher, and a surface hardness of 350 HV10 or less at a position 1 mm from the surface. Ceq value=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5...(1) However, the element symbols in equation (1) represent the mass percentage of each contained element. P value=(Mo / 95.9+Nb / 92.91+V / 50.94+Ti / 47.9) / (100 / 55.85)×100...(2) However, the element symbols in equation (2) represent the mass percentage of each contained element. Q value = Cr + Mo ···(3) However, the element symbols in equation (3) represent the mass percentage of each contained element. [2] The above component composition is further expressed in mass%, V: 0.005~0.1%, Cu: 0.5% or less, Ni: 0.5% or less, Ca: 0.0005~0.0035%, REM: 0.0005~0.0100%, A steel pipe for high-strength steel pipe piles as described in [1], characterized by containing one or more types selected from B:0.002% or less. [3] A steel material having the component composition described in [1] or [2], After heating to a temperature of 1100-1300°C and hot rolling, cooling is started above the Ar3 point, and accelerated cooling is performed at an average cooling rate of 20°C / s or higher until the cooling stop temperature is below 300°C, thereby forming a steel sheet. A method for manufacturing high-strength steel pipes for steel piles, characterized by using the aforementioned steel plate as a material, forming it into a cylindrical shape in the longitudinal direction of the steel plate, welding the butt joint where one end face of the steel plate is joined to the other end face from the inner and outer surfaces in the longitudinal direction one layer at a time in order to form a tubular shape, and then expanding the pipe, wherein the yield strength is 690 MPa or more, the tensile strength is 760 MPa or more, and the surface hardness at a position 1 mm from the surface is 350 HV10 or less. [4] A method for manufacturing steel pipes for high-strength steel pipe piles according to [3], characterized in that the pipe is reheated to 100-300°C after expansion. [Effects of the Invention]

[0006] As described above, the present invention provides a steel pipe with high strength of API X100 grade or higher and excellent toughness. For this reason, it is suitable for use in steel pipes for steel piles. [Modes for carrying out the invention]

[0007] The inventors conducted a detailed study on the microstructural changes of steel pipes made from steel plates in order to achieve both reduced surface hardness and high strength and toughness. Generally, welded steel pipes have strict chemical composition restrictions from the standpoint of weldability, so high-strength steel of X65 grade or higher is manufactured by hot rolling followed by accelerated cooling. As a result, the microstructure is mainly bainite, or contains island-like martensite (Martensite-Austenite constituent) within the bainite, but an increase in hardness due to the high cooling rate of the surface is unavoidable. In addition, there is a method of precipitating Cr carbides etc. during PWHT to compensate for the decrease in strength due to tempering, but the carbides easily coarseen, causing a decrease in toughness. Thus, it is clear that there are limits to ensuring strength and toughness through transformation strengthening while reducing surface hardness. Therefore, the inventors diligently researched microstructural morphologies that can achieve both reduced surface hardness and high strength and toughness, and as a result have obtained the following findings. By achieving an optimal balance of Cr and Mo, transformation strengthening and precipitation strengthening can be utilized simultaneously, suppressing the reduction in internal strength of the plate caused by reducing the cooling rate to lower surface hardness. In other words, by ensuring that the bainite structure in the microstructure of the steel plate is 80% or more, island martensite is 20% or less, and the remainder consists of one or more of retained austenite, pseudo-pearlite, and ferrite, with the total area ratio of the remainder being 20% ​​or less, surface hardness can be reduced while improving strength and toughness after pipe formation and expansion.

[0008] The steel pipe for high-strength steel pipe piles (steel pipe) of the present invention will be described in detail below. First, the component composition of the base material of the steel pipe will be described. In the following description, units indicated by % are mass % unless otherwise specified.

[0009] [Composition] C: 0.04 - 0.10% C is an element that increases the strength of steel. To obtain the desired microstructure and achieve the desired strength and toughness, a content of 0.04% or more is required. The C content is preferably 0.05% or more. On the other hand, if the content exceeds 0.10%, the weldability deteriorates, weld cracking is likely to occur, and the toughness of the base metal and the HAZ toughness decrease. Therefore, the C content is limited to 0.10% or less. Preferably, it is 0.07% or less.

[0010] Si: 0.01 - 0.5% Si acts as a deoxidizer and is an element that further increases the strength of the steel by solid solution strengthening. However, if the Si content is less than 0.01%, there is no such effect, so the Si content is 0.01% or more. Preferably, the Si content is 0.05% or more. On the other hand, a content of Si exceeding 0.5% significantly deteriorates the HAZ toughness. Therefore, the Si content is 0.5% or less. Incidentally, preferably, the Si content is 0.2% or less.

[0011] Mn: 1.5 - 2.5% Mn is an element that enhances the hardenability of steel and has the effect of improving strength and toughness. Since a content of 1.5% or more is required, the Mn content is 1.5% or more. The Mn content is preferably 1.8% or more. On the other hand, a content of Mn exceeding 2.5% may deteriorate the weldability. Therefore, the Mn content is limited to 2.5% or less. Incidentally, the Mn content is preferably 2.0% or less.

[0012] P: 0.015% or less, S: 0.0020% or less In the present invention, P and S are inevitable impurities, and the upper limits of their contents are defined. P, if the content is high, causes significant center segregation and deteriorates the toughness of the base metal, so it is 0.015% or less. Preferably, the P content is 0.008% or less. S, if the content is high, significantly increases the amount of MnS formed and deteriorates the toughness of the base metal, so it is 0.0020% or less. Preferably, the S content is 0.0008% or less. Note that the lower the content, the more preferable, and it may be 0%.

[0013] Al: 0.08% or less Al acts as a deoxidizing agent during steelmaking, but a content exceeding 0.08% leads to a decrease in toughness, so it should be kept below 0.08%. Preferably, the Al content is 0.05% or less. As a lower limit, it is preferable that the Al content be 0.01% or more.

[0014] Cr: 0.01~0.5% Like Mo, Cr is an effective element for obtaining sufficient strength even at low carbon levels, but high levels degrade weldability, so the Cr content is specified as 0.5% or less. Preferably, the Cr content is 0.4% or less. As a lower limit, the Cr content is 0.01% or more. Preferably, the Cr content is 0.1% or more.

[0015] Mo: 0.01~0.5% Mo is an important element in this invention. By including 0.01% or more, it suppresses pearlite transformation during cooling after hot rolling, while forming fine composite carbides with Ti, Nb, and V, significantly contributing to increased strength. Therefore, the Mo content should be 0.01% or more. Preferably, the Mo content should be 0.2% or more. However, since a Mo content exceeding 0.5% leads to deterioration of the toughness of the heat-affected zone during welding, the Mo content is specified to be 0.5% or less.

[0016] Ti: 0.005~0.025% Ti, like Mo, is an important element in this invention. Including 0.005% or more Ti forms a composite precipitate with Mo, significantly contributing to increased strength. Therefore, the Ti content is set to 0.005% or more. However, a Ti content exceeding 0.025% leads to deterioration of the toughness of the heat-affected zone and the base material. For this reason, the Ti content is specified as 0.025% or less.

[0017] Nb: 0.005~0.08% Nb improves toughness by refining the microstructure and, together with Mo, forms composite precipitates, contributing to increased strength. However, since there is no effect if the Nb content is less than 0.005%, the Nb content should be 0.005% or higher. Preferably, the Nb content is 0.02% or higher. On the other hand, if the Nb content exceeds 0.08%, the toughness of the heat-affected zone of the weld deteriorates. For this reason, the Nb content is specified to be 0.08% or less.

[0018] N: 0.001~0.010% N normally exists as an unavoidable impurity in steel, but the presence of Ti forms TiN. In order to suppress the coarsening of austenite grains due to the pinning effect of TiN, it is necessary for N to be present in the steel at a concentration of 0.001% or more, and the N content should be 0.001% or more. Preferably, the N content is 0.002% or more. On the other hand, if it exceeds 0.010%, TiN decomposes in the welded area, especially in the region heated to 1450°C or higher near the weld bond, and the adverse effects of solid-solution N become significant, degrading the HAZ toughness. Therefore, the N content should be 0.010% or less. Preferably, the N content is 0.005% or less.

[0019] O: 0.0050% or less In this invention, oxygen (O) is an unavoidable impurity, and its content is limited. To suppress the formation of coarse inclusions that adversely affect toughness, the O content should be 0.0050% or less. Preferably, the O content is 0.0030% or less.

[0020] Ceq value ≥ 0.48, P value ≥ 0.15 The Ceq value is expressed by equation (1) below using the mass percentage of the alloying element, and the P value is expressed by equation (2) below. In particular, the Ceq value correlates with the strength of the base material and is often used as an indicator of strength. The P value is used as an indicator of precipitation strengthening. Since the high strength of API X100 grade cannot be obtained if the Ceq value is less than 0.48 or the P value is less than 0.15, the specifications are set so that the Ceq value ≥ 0.48 and the P value ≥ 0.15. Ceq value=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5...(1) However, the element symbols in equation (1) represent the mass percentage of each contained element. The P value can also be determined by the following formula (2), which uses the mass percentage content of Mo, Ti, Nb, and V. The element symbols in formula (2) below represent the mass percentage of each contained element. The P value represents the total atomic percentage of Mo, Ti, Nb, and V contained in the steel, and is determined by the ratio of the sum of the number of atoms of Mo, Ti, Nb, and V contained in the steel to the total number of atoms of Fe, Mo, Ti, Nb, V, and other alloying elements. P value=(Mo / 95.9+Nb / 92.91+V / 50.94+Ti / 47.9) / (100 / 55.85)×100...(2) (2) The element symbols in equation (2) represent the mass percentage of each element contained.

[0021] Q value ≥ 0.50, Cr ≤ Mo The Q value is given by equation (3) below and serves as an indicator of the amount and morphology of Cr and Mo-based alloy carbides precipitated. If the combined Q value of Cr and Mo is less than 0.50, the high strength of API X100 grade cannot be obtained, so the Q value is specified as ≥ 0.50. Also, if Cr > Mo, the amount of Cr-based alloy carbides precipitated becomes excessive, and the required amount of Mo-based alloy carbides precipitated cannot be secured, so the high strength of API X100 grade cannot be obtained, so the Cr ≤ Mo is specified. Q value = Cr + Mo ···(3) However, the element symbols in equation (3) represent the mass percentage of each contained element.

[0022] In this invention, for the purpose of further improving the strength and toughness of the steel sheet, one or more of the following V, Cu, Ni, Ca, REM, and B may be included.

[0023] V: 0.005% or more and 0.1% or less Like Nb, V forms composite precipitates with Mo, contributing to increased strength. However, if the V content exceeds 0.1%, the toughness of the heat-affected zone deteriorates. Therefore, when adding V, the V content should be specified as 0.1% or less. Preferably, the V content is 0.06% or less. On the other hand, the lower limit for the V content is 0.005% or more. Preferably, the V content is 0.01% or more.

[0024] Cu: 0.5% or less While copper (Cu) is an effective element for improving toughness and increasing strength, high concentrations can degrade weldability; therefore, when added, the Cu content should be kept below 0.5%.

[0025] Ni: 0.5% or less Ni is an effective element for improving toughness and increasing strength, but if present in large quantities, it reduces PWHT resistance. Therefore, if Ni is added, the Ni content should be 0.5% or less.

[0026] Ca: 0.0005~0.0035% Ca is an effective element for improving toughness by controlling the morphology of sulfide inclusions, but its effect is insufficient if the Ca content is less than 0.0005%; therefore, the Ca content should be 0.0005% or higher. On the other hand, if the Ca content exceeds 0.0035%, the effect saturates, and it actually degrades toughness by reducing the cleanliness of the steel; therefore, if Ca is added, the Ca content should be specified as 0.0035% or less.

[0027] REM: 0.0005~0.0100% Rare earth metals (REMs) are also effective elements for improving toughness by controlling the morphology of sulfide inclusions in steel. However, their effect is insufficient if the REM content is less than 0.0005%, so the REM content should be 0.0005% or higher. On the other hand, if the REM content exceeds 0.0100%, the effect saturates, and it actually degrades toughness by reducing the cleanliness of the steel. Therefore, if REMs are added, the REM content should be specified as 0.0100% or less.

[0028] B: 0.002% or less B segregates at austenite grain boundaries and suppresses ferrite transformation, thereby contributing to preventing a decrease in HAZ strength in particular. However, the effect saturates if the B content exceeds 0.002%, so if added, the B content should be 0.002% or less.

[0029] The remainder, excluding the above, is substantially composed of Fe. The statement that the remainder is substantially composed of Fe means that, as long as the effects of the present invention are not lost, materials containing unavoidable impurities and other trace elements may be included within the scope of the present invention.

[0030] [Microstructure, precipitation morphology of complex carbides] The microstructure of the base material of the steel pipe of the present invention must consist of 80% or more bainite structure, 20% or less island martensite, and the remainder consisting of one or more of retained austenite, pseudo-pearlite, and ferrite, with the total area ratio of the remainder being 20% ​​or less. To achieve strength, the bainite structure must be 80% or more. Preferably, the bainite structure is 90% or more. The island martensite must be 20% or less. Preferably, the island martensite is 10% or less. The remainder must consist of one or more of retained austenite, pseudo-pearlite, and ferrite, with the total area ratio of the remainder being 20% ​​or less. Preferably, the remainder is 10% or less. The remainder may be 0%, or if it exceeds 0%, it must consist of one or more of retained austenite, pseudo-pearlite, and ferrite. Note that pseudo-pearlite refers to a finely fragmented pearlite structure and is not lamellar pearlite with a eutectoid composition. Furthermore, as a composite carbide that serves as an indicator for equation (2), to improve strength, there are 5 composite carbides / μm that contain one or more of Nb, Ti, and Mo, and have an equivalent circle diameter of 20 nm or less. 2 The above must be sufficient. Preferably, the composite carbide is 10 particles / μm 2 That's all.

[0031] By controlling the microstructure morphology in this way, it is possible to achieve API X100 grade high strength.

[0032] Furthermore, the microstructural morphology of the steel pipes used for high-strength steel pipe piles (steel pipes) described above must be satisfied regardless of their position in the thickness direction of the steel pipe.

[0033] [Characteristics of the base material of steel pipes for high-strength steel pipe piles] The characteristics of the steel pipe for high-strength steel pipe piles of the present invention are a yield strength of 690 MPa or more, a tensile strength of 760 MPa or more, and a surface hardness of 350 HV10 or less at a position 1 mm from the surface. Next, an example of a method for manufacturing a high-strength steel pipe pile of the present invention using steel of the above composition will be described.

[0034] [Manufacturing conditions] This invention is a technology for manufacturing high-strength steel pipes that utilizes transformation strengthening due to bainite transformation during accelerated cooling, and further enhances strength by combining it with precipitation strengthening using fine carbides of Nb, Ti, and Mo that precipitate during hot rolling and before and after accelerated cooling. This allows for high strength without containing large amounts of alloying elements, reduces surface hardness, and exhibits excellent strength characteristics after tempering. The steel pipe for high-strength steel pipe piles of the present invention is manufactured by using a steel material having the above-mentioned component composition, heating it at a heating temperature of 1100 to 1300°C, then hot rolling it, starting cooling at an average cooling rate of 20°C / s or more from the Ar3 point or higher, and accelerating cooling to a cooling stop temperature of less than 300°C, and using the resulting steel plate as the material. After forming it into a cylindrical shape in the longitudinal direction of the steel plate, the butt joints where one end face of the steel plate is joined to the other are welded one layer at a time in the longitudinal direction from the inner and outer surfaces to form a tubular shape, and then expanding the pipe. As a result, the bainite structure in the microstructure of the steel pipe can be 80% or more, island martensite 20% or less, and the remainder can be one or more of retained austenite, pseudo-pearlite, and ferrite, with the total area ratio of the remainder being 20% ​​or less, thereby obtaining the desired mechanical properties.

[0035] The heating temperature must be 1100°C or higher to solidify coarse precipitates such as Nb carbides. From the viewpoint of suppressing excessive coarsening of austenite grain size, the heating temperature should be 1300°C or lower.

[0036] Furthermore, by initiating accelerated cooling of the steel from the austenite single-phase region and stopping the cooling at the lowest possible temperature below the bainite transformation completion temperature, it becomes possible to most effectively combine and utilize transformation strengthening and precipitation strengthening. If cooling is started from the two-phase region below the Ar3 point, polygonal ferrite will be present, resulting in a significant decrease in strength; therefore, the cooling start temperature should be above the Ar3 point. The Ar3 point is defined by equation (4) below. Ar3=910-310C-80Mn-20Cu-15Cr-55Ni-80Mo...(4) (4) The element symbols in equation (4) represent the mass percentage of each element contained.

[0037] Furthermore, the average cooling rate in accelerated cooling must be 20°C / s or higher. Here, the average cooling rate refers to the average cooling rate from the cooling start temperature (Ar3 point or higher) to a temperature below 300°C (cooling stop temperature). In particular, if the cooling stop temperature exceeds 300°C, the desired strength characteristics cannot be obtained, so the cooling stop temperature should be 300°C or lower. From the viewpoint of manufacturing efficiency, the lower limit of the cooling stop temperature is preferably 100°C or higher.

[0038] The steel plate described above (steel plate for high-strength steel pipe piles) is used as the material. After forming the steel plate into a cylindrical shape along its longitudinal direction, the butt joint where the two ends of the steel plate meet is welded longitudinally, one layer at a time from the inner and outer surfaces, and then expanded to form a tube. Note that "one layer at a time from the inner and outer surfaces" means that one layer of weld metal is applied from the inner side, followed by one layer of weld metal from the outer side. At this time, the inner weld metal and the outer weld metal must overlap in the cross-section of the welded joint.

[0039] Furthermore, reheating the steel pipe to 100-300°C provides even more stable and high strength. Therefore, it is preferable to reheat the pipe after expansion to 100-300°C. Here, the temperature is the average temperature of the steel pipe, i.e., the average temperature calculated from the surface temperature and plate thickness. [Examples]

[0040] Steel with the chemical composition shown in Table 1 (steel grades A to M) was cast into slabs using a continuous casting method, and these were used to manufacture thick steel plates (No. 1 to 17) with a thickness of 32 mm.

[0041] [Table 1]

[0042] After the heated slabs were rolled by hot rolling, they were immediately cooled using a water-cooled accelerated cooling system. The manufacturing conditions for each steel plate (No. 1 to 17) are shown in Table 2.

[0043] [Table 2]

[0044] These steel plates were used as raw materials, formed into a cylindrical shape in the longitudinal direction, then the butt joints were welded one layer at a time from the inner and outer surfaces in the longitudinal direction, and the pipe was expanded to produce a steel pipe with an outer diameter of 40 inches (1016 mm). Some of the steel pipes were reheated to 100-300°C on a high-frequency heating line. As described above, No. 5 tensile test specimens were taken from the steel pipe manufactured in the axial direction and circumferential direction in accordance with the provisions of JIS Z 2201, and tensile strength was measured in accordance with the provisions of JIS Z 2241, determining the yield strength and tensile strength. A yield strength (0.5%YS) of 690 MPa or higher and a tensile strength (TS) of 760 MPa or higher were considered the strength required for the present invention (acceptable).

[0045] In addition, the microstructural morphology and precipitate morphology of the steel pipes were investigated, and Charpy impact test specimens were taken from the center of the plate thickness to evaluate their Charpy impact properties. A specimen for microstructure observation was taken from the half-thickness position in the thickness direction of the obtained steel plate. The observation surface was etched with nital, and microstructure observation was performed on 5 fields using a scanning electron microscope (SEM) with an acceleration voltage of 1-15kV and an observation magnification of 2000x. The tissue was identified and its fraction was determined. The fraction was the average value of the 5 fields. For microstructure identification, in the SEM image, polygonal, lath-like or granular regions were identified as bainite, white regions as island martensite, gray regions as retained austenite, regions with finely fragmented cementite as pseudo-pearlite, and polygonal granular structures as ferrite. Furthermore, specimens for precipitate observation were taken from the steel plate at a position half the thickness in the plate thickness direction, and the precipitate morphology was determined by transmission electron microscopy. The precipitate size and number density of composite carbides containing one or more Nb, Ti, and Mo were determined by the average value of 5 fields of view at a magnification of 10,000x. The components of the precipitates were analyzed by energy-dispersive X-ray spectroscopy (EDX).

[0046] In this process, Charpy impact test specimens with standard V-notch dimensions were taken from the 1 / 2 position in the pipe thickness direction of the steel pipe, perpendicular to the rolling direction, in accordance with the provisions of JIS Z 2202, and impact tests were conducted in accordance with the provisions of JIS Z 2242. Specimens with a Charpy impact energy absorption of 200 J or more at -10°C were considered good (pass). In addition, the Vickers hardness was measured at 5 points 1 mm from the outer surface of the steel pipe with a load of 10 kgf, and the average value was evaluated. The measurement results are shown in Table 3.

[0047] [Table 3]

[0048] For the toughness of the heat-affected zone (HAZ) of the weld, Charpy impact tests were performed using test specimens of actual joints. A Charpy impact test specimen with standard V-notch dimensions was taken in accordance with JIS Z 2202, perpendicular to the rolling direction, from a position 1 mm or less below the outer surface in the thickness direction of the pipe, so as to encompass the entire HAZ. The impact test was performed in accordance with JIS Z 2242. Specimens with a Charpy impact energy absorption of 100 J or more at -10°C were considered good (〇).

[0049] In Table 3, examples No. 1 to 7 of the present invention all have component composition, microstructure morphology, and precipitate morphology that fall within the scope of the present invention, exhibit high strength with a yield strength (YS) of 690 MPa or higher and a tensile strength (TS) of 760 MPa or higher, and further exhibit good surface hardness, base material toughness, and weld heat-affected zone toughness. In No. 8, the component composition was within the scope of the present invention, but the heating temperature was outside the scope of the present invention, resulting in no dispersion and precipitation of fine carbides, and deterioration of the base material strength or toughness. In No. 9, the component composition was within the scope of the present invention, but the cooling start temperature was outside the scope of the present invention, resulting in a bainite structure area ratio of less than 80% in the steel pipe microstructure, and deterioration of the base material strength. In Nos. 10 and 11, the component composition was within the scope of the present invention, but the average cooling rate and cooling stop temperature were outside the scope of the present invention, resulting in a bainite structure fraction of less than 80% in the steel pipe microstructure, and deterioration of the base material strength and toughness. Since the component composition of Nos. 12-14 was outside the scope of the present invention, sufficient base material strength or toughness could not be obtained. No. 15 had a Q value outside the range of the present invention, and its base material strength, base material toughness, and HAZ toughness were deteriorated. In sample No. 16, the Ceq value was outside the range of the present invention, indicating a deterioration in the base material strength. In sample No. 17, the P-value and Q-value were outside the range of the present invention, indicating a deterioration in the base material strength.

Claims

1. A steel pipe for high-strength steel pipe piles having a base material portion and a welded portion, The base material is, by mass%, C: 0.04-0.10%, Si: 0.01 to 0.5%, Mn: 1.5-2.5%, P: 0.015% or less, S: 0.0020% or less, Al: 0.08% or less, Cr: 0.01-0.5%, Mo: 0.01-0.5%, Ti: 0.005 to 0.025%, Nb: 0.005-0.08%, N: 0.001 to 0.010%, O: 0.0050% or less, It contains, The composition has the following properties: the Ceq value expressed by equation (1) below satisfies Ceq value ≥ 0.48; the P value expressed by equation (2) below satisfies P value ≥ 0.15; and the Q value, the total mass % of Cr and Mo expressed by equation (3) below, satisfies Q value ≥ 0.50 and Cr ≤ Mo, with the remainder being Fe and unavoidable impurities. The microstructure consists of bainite or more (80%), island martensite or less (20%), and the remainder consists of one or more of the following: retained austenite, pseudo-pearlite, and ferrite, with the total area ratio of the remainder being 20% ​​or less. Five composite carbides containing one or more of Nb, Ti, and Mo, with an equivalent circular diameter of 20 nm or less, per μm. ２ That's all. A high-strength steel pipe for piles, characterized by a yield strength of 690 MPa or more, a tensile strength of 760 MPa or more, and a surface hardness of 350 HV10 or less at a position 1 mm from the surface. Ceq value=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5...(1) However, the element symbols in equation (1) represent the mass percentage of each contained element. P value=(Mo / 95.9+Nb / 92.91+V / 50.94+Ti / 47.9) / (100 / 55.85)×100...(2) However, the element symbols in equation (2) represent the mass percentage of each contained element. Q value = Cr + Mo ... (3) However, the element symbols in equation (3) represent the mass percentage of each contained element.

2. The aforementioned component composition is further expressed in mass%, V: 0.005-0.1%, Cu: 0.5% or less, Ni: 0.5% or less, Ca: 0.0005-0.0035%, REM: 0.0005-0.0100%, B: A steel pipe for high-strength steel pipe piles according to claim 1, characterized in that it contains one or more types selected from 0.002% or less.

3. A method for manufacturing a steel pipe for high-strength steel pipe piles according to claim 1 or 2, A steel material having the above-mentioned component composition, After heating to a temperature of 1100-1300°C and hot rolling, cooling is started from the Ar3 point or higher, and accelerated cooling is performed at an average cooling rate of 20°C / s or higher down to a cooling stop temperature of less than 300°C to form a steel sheet. A method for manufacturing high-strength steel pipes for steel piles, characterized by using the aforementioned steel plate as a material, forming it into a cylindrical shape in the longitudinal direction of the steel plate, welding the butt joint where one end face of the steel plate is joined to the other end face from the inner and outer surfaces in the longitudinal direction one layer at a time in order to form a tubular shape, and then expanding the pipe, wherein the yield strength is 690 MPa or more, the tensile strength is 760 MPa or more, and the surface hardness at a position 1 mm from the surface is 350 HV10 or less.

4. A method for manufacturing high-strength steel pipes for piles according to claim 3, characterized in that the pipe is reheated to 100 to 300°C after expansion.

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