Steel pipe and its manufacturing method

Abstract

To provide a steel pipe and a manufacturing method of the same in which flaring processability is improved relative to conventional ones.SOLUTION: A steel pipe 9 includes a butt joint surface 10 where an end part of a steel plate is joined with the other end part thereof. At a position of 100 μm from an inner surface of the steel pipe to an external surface, a ratio of average Si concentration (mass%) within 5 μm to both sides from the butt joint surface in a vertical direction of the butt joint surface to average Si concentration (mass%) of a base material is 1 / 2 or less, and a ratio of average Mn concentration (mass%) within 5 μm to both sides from the butt joint surface in a vertical direction of the butt joint surface to average Mn concentration (mass%) of a base material is 1 / 2 or less.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a steel pipe (forge-welded steel pipe) suitable for piping and a method for manufacturing the same. [Background technology]

[0002] Forged steel pipes, which are inexpensive and have excellent workability, have been used in piping facilities such as water pipes. In recent years, in order to reduce the labor required for construction, a common method of joining such fluid-carrying pipes is to flare the pipe ends and use flanges. Flaring is a process in which the ends of steel pipes are expanded to form a flange, and since this process involves applying excessive tension in the circumferential direction of the steel pipe, excellent workability (hereinafter also referred to as "flaring workability") is required.

[0003] In butt-welded steel pipes, the ends of the steel strips are butted together after forming, but if oxides remain at the butt joint, the joint strength is weakened and cracks occur during flaring. Oxides are broadly divided into two types: oxides of the main component Fe and O, and oxides of additive elements such as Mn and Si and O. As methods for suppressing such oxides, for example, Patent Document 1 describes a method of spraying inert gas, and Patent Document 2 describes a method of applying a scale inhibitor. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 8-224614 [Patent Document 2] Japanese Patent Application Publication No. 8-243635 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in these methods, oxides of additive elements such as Mn and Si that are generated at lower oxygen partial pressures remain, and flaring workability is not fully satisfied. Therefore, the methods proposed in Patent Documents 1 and 2 are not yet sufficient as methods for preventing the generation of oxides that reduce the flaring workability of steel pipes at the abutment portion. The present invention aims to provide a steel pipe and a manufacturing method thereof in order to solve the problem of suppressing the above-mentioned decrease in flaring workability. [Means for solving the problem]

[0006] The present inventors have conducted research into a manufacturing method for reducing oxides of contained elements remaining in the abutment portion.

[0007] First, the inventors of the present invention have investigated the optimal manufacturing method for the method of heating steel plates and butting and forge-welding the edges in order to achieve the above-mentioned goal. As a result, when the end of the steel plate is heated just before butting, a scale of Fe and O is first formed. If heating is continued, the scale of Fe and O melts, and Si and Mn diffuse into this molten scale, and oxides of these elements are formed. At this time, Si and Mn combine with O, and a phase deficient in Si and Mn is formed in the lower part of the layer where the oxide is formed. When this molten oxide is discharged by upsetting or by blowing high-pressure air, a layer with lower Si and Mn concentrations remains in the butt part of the steel pipe. Upsetting is a process in which the ends of steel plates are pressed together and joined when manufacturing steel pipes from steel plates. By going through this process, elements that easily combine with oxygen during joining are deficient in the vicinity of the butt part, so that oxides of the contained elements are not formed in the butt part, and it has become possible to manufacture steel pipes (forge-welded steel pipes) with excellent flaring workability.

[0008] The present invention has been completed based on such findings, and employs the following means to solve the above problems. [1] A steel pipe having abutment surfaces where the end of a steel plate and the other end are joined, A steel pipe in which, at a position 100 μm from the inner surface toward the outer surface, the ratio of the average Si concentration (mass%) within 5 μm on each side from the abutment surface in a direction perpendicular to the abutment surface to the average Si concentration (mass%) in the base material is 1 / 2 or less, and the ratio of the average Mn concentration (mass%) within 5 μm on each side from the abutment surface in a direction perpendicular to the abutment surface to the average Mn concentration (mass%) in the base material is 1 / 2 or less. [2] A method for producing a steel pipe according to the above [1], The method includes a step of forming the edge portion of a steel plate, heating the steel plate in a heating furnace, and then forming the steel plate into a tubular shape while butting and forging the edges of the formed steel plate, A manufacturing method for steel pipes in which the temperature T (℃) of the edge of the steel plate before the abutment satisfies formula (1) defined by the Si (%) and Mn (%) of the steel plate and the time t (sec.) from the end of heating to the abutment. -22×{Si(%)}×{t(sec.)}-14×{Mn(%)}×{t(sec.)}+1530 <T(℃)< -17×{Si(%)}×{t(sec.)}-7×{Mn(%)}×{t(sec.)}+1542...(1) In formula (1), Si (%) and Mn (%) are the contents (mass%) of each element in the steel sheet, T (°C) is the temperature of the edge parts of the steel sheets before abutment, and t (sec.) is the time from the end of heating to the time of abutment. [3] A method for manufacturing a steel pipe as described in [2], in which, before the abutment, a gas having an oxygen concentration of 20% or more and 40% or less in volume fraction is supplied to the edge portion of the steel plate at a spray pressure of 0.50 MPa or more. Effect of the Invention

[0009] The present invention can provide a steel pipe having improved flaring workability compared to conventional steel pipes and a method for manufacturing the same. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram showing an example of a steel pipe manufacturing facility. [Diagram 2] FIG. 2 is a diagram showing a cross section (C direction cross section) of a steel pipe taken at a right angle to the rolling direction. [Diagram 3] Fig. 3(a) is a diagram showing an enlarged view of the measurement area 20 in Fig. 2. Fig. 3(b) is a diagram showing an enlarged view of the measurement area 21 in Fig. 2. [Figure 4] Fig. 4(a) is a diagram showing a steel pipe before flaring, and Fig. 4(b) is a diagram showing a steel pipe after flaring. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, an embodiment of the present invention will be described.

[0012] The steel pipe of the present invention is manufactured by using steel plates and butting and forging the edges of the steel plates. The steel plate as the raw material is preferably a steel strip, but cut plates such as thin plates and thick plates can also be used.

[0013] The process for manufacturing a forge-welded steel pipe from a steel plate (in this case, a steel strip) is shown in Figure 1. Using Figure 1, the details of the process for manufacturing a steel pipe (forge-welded steel pipe) from a steel strip will be explained based on the steel pipe manufacturing line equipment.

[0014] The steel strip 2 discharged from the coil 1 is passed through a looper 3, and the edge of the steel strip 2 is formed by an edge forming machine 4. Although not particularly limited in the present invention, the edge can be formed by forming the edge with an upset roll or by cutting the edge. Thereafter, the entire width of the steel strip is heated in a heating furnace 5, and the edge is continuously formed into a tubular shape by a forming forge welding machine 6. Just before the edge is butted and forged, gas is blown from a nozzle 7, and the edge is heated until the temperature of the steel strip edge is within the range of formula (1), and then forged and joined. The steel strip is then reduced to a desired outer diameter by a diameter reducing rolling roll 8 in a reducing rolling mill, and a steel pipe (forged welded steel pipe) 9 is manufactured. FIG. 2 is a diagram showing a cross section (C direction cross section) of the steel pipe in a direction perpendicular to the rolling direction. The forged welded steel pipe 9 has a forged welded butt joint 10. When blowing the gas, it is preferable that the gas contains an oxygen concentration of 20% to 40% by volume fraction, and the blowing pressure is 0.50 MPa or more. In this equipment line, the ends of the steel strip are forge welded together and then subjected to reducing rolling to finish the forge-welded steel pipe 9, but there are also cases where the forge-welded steel pipe is finished without undergoing reducing rolling.

[0015] In the present invention, the edge portions of the steel plates are heated before they are butted, and at the same time, a gas containing oxygen is sprayed at high pressure, thereby making it possible to remove Fe oxides from the butt surface (forge welded butt portion or butt portion) 10 during butt welding, and to manufacture a butt-welded steel pipe 9 without forming oxides of elements such as Si, Mn, etc. in the butt portion 10.

[0016] First, the specific element regulations for steel pipe (fork-welded steel pipe) 9 will be described.

[0017] The ratio of the average Si concentration (mass%) in the range of 5 μm on each side from the abutment surface in the direction perpendicular to the abutment surface to the average Si concentration (mass%) of the base material is 1 / 2 or less, and the ratio of the average Mn concentration (mass%) in the range of 5 μm on each side from the abutment surface in the direction perpendicular to the abutment surface to the average Mn concentration (mass%) of the base material is 1 / 2 or less. The lack of Si and Mn from the abutment portion 10 can suppress the generation of Si and Mn oxides, which are generated even at low oxygen partial pressure during abutment. In particular, the highest stress is applied to the inner surface of the steel pipe during flaring, and cracks that occur during flaring propagate from the inner surface. Therefore, if the ratio of the average Si concentration (mass%) in the range of 5 μm on each side from the abutment surface in the direction perpendicular to the abutment surface 10 to the average Si concentration (mass%) in the base material is 1 / 2 or less at a position 100 μm from the inner surface of the steel pipe toward the outer surface, and the ratio of the average Mn concentration (mass%) in the range of 5 μm on each side from the abutment surface in the direction perpendicular to the abutment surface 10 to the average Mn concentration (mass%) in the base material is 1 / 2 or less, the generation of Si and Mn oxides in the abutment portion can be suppressed, and the flaring workability satisfies the desired value. Therefore, at a position 100 μm from the inner surface of the steel pipe toward the outer surface, the ratio of the average Si concentration (mass%) in the range of 5 μm on each side from the abutment surface in the direction perpendicular to the abutment surface 10 to the average Si concentration (mass%) of the base material is 1 / 2 or less, and the ratio of the average Mn concentration (mass%) in the range of 5 μm on each side from the abutment surface in the direction perpendicular to the abutment surface 10 to the average Mn concentration (mass%) of the base material is 1 / 2 or less. Preferably, the above ratio is 3 / 8 or less. Note that, since the desired flaring workability cannot be obtained unless both Si oxides and Mn oxides are suppressed, it is necessary to set both the ratio of the average Si concentration (mass%) in the range of 5 μm on each side from the abutment surface in the direction perpendicular to the abutment surface 10 to the average Si concentration (mass%) of the base material and the ratio of the average Mn concentration (mass%) in the range of 5 μm on each side from the abutment surface in the direction perpendicular to the abutment surface 10 to the average Mn concentration (mass%) of the base material to 1 / 2 or less.

[0018] Furthermore, since Si and Mn are particularly likely to segregate in the central part of the thickness of the steel plate, these elements are also likely to segregate in the central part of the thickness of the abutment part 10, and a large amount of oxide is generated, making it impossible to satisfy the desired flare workability. Therefore, it is preferable that the ratio of the average Si and Mn concentration (mass%) in the range of 5 μm on each side from the abutment surface in the direction perpendicular to the abutment surface 10 to the average concentration (mass%) in the base material in the central part of the thickness of the abutment part 10 is 1 / 2 or less. The range of 5 μm on each side from the abutment surface in the direction perpendicular to the abutment surface 10 refers to the range up to a position 5 μm away from the abutment surface on each side, and the total length of the above range is 10 μm.

[0019] Next, a method for manufacturing a steel pipe will be described.

[0020] Forming the edges of the steel plate Since the edges of the as-manufactured steel sheet are uneven, which is a factor that reduces the joining strength, it is necessary to shape the edges by forming them in order to ensure a certain joining strength. The forming method is not particularly limited, but examples thereof include rolling with a roll and cutting.

[0021] The temperature T (℃) of the edge of the steel plate before the collision satisfies the formula (1) consisting of the Si (%), Mn (%) of the steel plate and the time t (sec.) from the end of heating to the collision. -22×{Si(%)}×{t(sec.)}-14×{Mn(%)}×{t(sec.)}+1530 <T(℃)< -17×{Si(%)}×{t(sec.)}-7×{Mn(%)}×{t(sec.)}+1542...(1) In formula (1), Si (%) and Mn (%) are the contents (mass%) of each element in the steel sheet, T (°C) is the temperature of the edge of the steel sheet before abutting, and t (sec.) is the time from the end of the heating process to the completion of abutting. During abutment, the higher the temperature of the edges, the more the diffusion of Si and Mn from the base material to the oxide scale progresses, but a high concentration of Si and Mn in the base material also increases the difference in concentration between the oxide scale and the base material, making diffusion easier. In addition, the longer the time from the end of the heating process to the completion of abutment, the more the diffusion progresses, making it easier for the concentration of Si and Mn at the abutment to decrease, and a specified Si and Mn concentration ratio can be obtained within a range of 5 μm on each side of the abutment surface in the direction perpendicular to the abutment surface. On the other hand, if the temperature of the edge exceeds the specified value and approaches the melting point of the base material, the strength of the steel pipe abutment part decreases and some melting occurs, the steel pipe cannot withstand the stress of abutment and deforms excessively, the cross section cannot maintain its circular shape, and the steel pipe cannot be manufactured. Or, a large amount of oxide scale is generated and cannot be completely removed during abutment, which leads to a decrease in workability. If the Si and Mn concentrations of the base material increase, the melting point decreases and the temperature at which the abutment part cannot be formed decreases because the abutment part cannot withstand the stress during abutment. In addition, the longer the time until the abutment is completed, the more oxide scale is generated, which has the same effect as when the temperature increases, and there are cases where the oxide scale cannot be completely removed. Therefore, the higher the Si and Mn concentrations of the base material and the longer the time until abutment, the lower the temperature range of the edge part required during abutment, and the specified steel pipe can be obtained by satisfying formula (1).

[0022] In order to ensure diffusion time into the oxide scale, the time t (sec.) from the end of the heating step to the completion of the collision is preferably 1.0 s or more, and in order to prevent excessive oxide scale formation, it is preferably 5.0 s or less.

[0023] Before the abutment, gas with an oxygen concentration of 20% or more and 40% or less in volume fraction is blown onto the edges of the steel sheets, in other words, a specified gas is supplied to the edges of the steel sheets, so that the edges are heated by an exothermic oxidation reaction before the abutment. However, heating can also be achieved by other methods such as high-frequency heating using an electromagnetic coil, flame radiation, or laser irradiation.

[0024] Oxygen concentration: 20% to 40% (optimal conditions) By dissolving oxygen in the oxide scale during heating, Si and Mn become more depleted, and the ratio of the average Si concentration (mass%) in the range of 5 μm on each side from the abutment surface in the direction perpendicular to the abutment surface 10 to the average Si concentration (mass%) in the base material and the ratio of the average Mn concentration (mass%) in the range of 5 μm on each side from the abutment surface in the direction perpendicular to the abutment surface 10 to the average Mn concentration (mass%) in the base material become smaller. If the oxygen concentration is less than 20%, the effect of promoting deficiency is small, and if it exceeds 40%, the excessive oxide generated reduces the strength, so it is preferable that the gas blown onto the steel sheet contains an oxygen concentration of 20% to 40% by volume. Preferably, the oxygen concentration is 25% to 40%.

[0025] The pressure of the gas to be sprayed is 0.50MPa or more (optimal conditions) In addition to exhausting the oxides and liquid phase from the steel sheets immediately before they are butted, there is another method of removing the oxides and liquid phase from the steel sheets by upsetting, namely, spraying a gas containing oxygen. By increasing the pressure of the gas spray, it is possible to remove more oxides and liquid phase before upsetting and to reduce the Si and Mn concentrations in the butt joint. For this reason, it is preferable to set the pressure of the gas sprayed onto the steel sheets to 0.50 MPa or more, and more preferably 0.55 MPa or more.

[0026] The composition of the base metal of the steel pipe 9 of the present invention is not particularly limited, but preferably contains, by mass%, C: 0.01-0.12%, Si: 0.1-0.5%, Mn: 0.2-1.0%, P: 0.02% or less, S: 0.01% or less, with the balance being Fe and unavoidable impurities. The reasons for limiting each component are described below.

[0027] C: 0.01 to 0.12% If the C content is 0.01% or more, the desired strength or more (yield strength YS in the tube axial direction: 200 MPa or more, the same applies below) will be obtained. Therefore, the C content is preferably 0.01% or more. More preferably, it is 0.02% or more. On the other hand, if the C content exceeds 0.12%, the ductility decreases and the desired flare workability may not be obtained. Therefore, the C content is preferably 0.12% or less.

[0028] Silicon: 0.1 to 0.5% Since Si is a ferrite former element, the inclusion of Si produces a large amount of ferrite structure with a high diffusion coefficient during joining. If the Si content is less than 0.1%, the ferrite phase fraction during joining is small and the diffusion coefficient cannot be increased, resulting in a decrease in the joining strength of the abutment portion 10 and failure to obtain the desired flare workability. Therefore, the Si content is preferably 0.1% or more. On the other hand, if the Si content exceeds 0.5%, a large amount of Si remains even if a Si- and Mn-deficient layer is formed as described above, and the impact force decreases due to the formation of Si oxide, which deteriorates the flare workability. Therefore, the Si content is preferably 0.5% or less.

[0029] Mn: 0.2-1.0% When Mn is contained at 0.2% or more, the desired strength can be obtained. Therefore, the Mn content is preferably 0.2% or more. On the other hand, when Mn exceeds 1.0%, as described above, even if a depleted layer of Si and Mn is formed, a large amount of Mn remains, and oxides are formed, which reduces the impact force and reduces the flaring workability. Therefore, the Mn content is preferably 1.0% or less.

[0030] P:0.02% or less If the P content exceeds 0.02%, the joining strength of the abutting portion 10 decreases, and the desired flare workability may not be obtained. Therefore, the P content is preferably 0.02% or less. There is no particular lower limit, but the P content may be 0%.

[0031] S: 0.01% or less If the S content exceeds 0.01%, the joining strength of the abutting portion 10 decreases, and the desired flare workability may not be obtained. Therefore, the S content is preferably 0.01% or less. There is no particular lower limit, but the S content may be 0%. In addition to the above, Al, Nb, Cr, Mo, and Ti may also be contained.

[0032] Next, the measurement method for each value will be explained with reference to FIG.

[0033] Steel pipe wall thickness Fig. 2 is a diagram showing a cross section (C-direction cross section) of a steel pipe in a direction perpendicular to the rolling direction. As shown in Fig. 2, the wall thickness of the steel pipe is measured in the C-direction cross section of the steel pipe 9 at three wall thickness measurement positions 11 at 3 o'clock, 6 o'clock, and 9 o'clock in the circumferential direction when the position of the abutment part 10 is set as 0 o'clock, using a single ball micrometer or the like, and the average value of the three points is defined as the wall thickness t of the steel pipe.

[0034] Steel pipe outer diameter As shown in FIG. 2, the outer diameter 12 of the butt-welded steel pipe before flaring at the abutment portion 10 is measured with a vernier caliper or the like, and this value is regarded as the outer diameter of the butt-welded steel pipe before flaring.

[0035] Oxygen concentration The oxygen concentration of the air or oxygen-mixed air blown from the nozzle 7 is measured by an oxygen concentration meter placed just before the outlet of the nozzle 7, and the measured value is regarded as the oxygen concentration.

[0036] Spray Pressure The pressure of the air or oxygen-mixed air sprayed from the nozzle 7 is measured with a pressure gauge or the like immediately before the outlet of the nozzle 7, and this value is taken as the spray pressure.

[0037] Average Si and Mn concentrations in the joint 3(a) is a diagram showing an enlarged view of the measurement area 20 in FIG. 2. As shown in FIG. 3(a), in the cross section of the steel pipe 9 in the C direction, a line (average Si, Mn concentration measurement line of the abutment part) 13 is drawn in a direction perpendicular to the abutment part at a position 100 μm from the inner surface side of the steel pipe along the abutment part 10 toward the outer surface side, and 25 points are measured at a pitch of 0.2 μm on the left and right sides of the abutment part 10 along this line 13, and the average Si and Mn concentrations of the measured 51 points are the average Si and Mn concentrations of the abutment part 10, respectively. The range measured above is a range of 5 μm on each side from the abutment surface in a direction perpendicular to the abutment surface 10 described above. The concentration measurement of Si and Mn can be performed using a method using characteristic X-rays, but is not particularly limited.

[0038] Average Si and Mn concentrations in the base material Fig. 3(b) is a diagram showing an enlarged view of the measurement area 21 in Fig. 2. As shown in Fig. 3(b), in the C-direction cross section of the steel pipe 9, at the 3 o'clock, 6 o'clock, and 9 o'clock positions when the abutment portion 10 is set to 0 o'clock, a normal line 14 is drawn from the outer surface of the steel pipe to the inner surface of the steel pipe, and along the normal line 14, at a position 100 μm from the inner surface side of the steel pipe toward the outer surface side, a measurement line 15 for measuring the average Si and Mn concentrations of the base material is drawn perpendicular to the normal line 14, and 25 points are measured at the intersection of the normal line 14 and the line 15, and 25 points are measured at 0.2 μm pitches on the left and right sides of the intersection point along the line 15, and the average Si and Mn concentrations of the 51 measured points are calculated. The measurements are performed at the 3 o'clock, 6 o'clock, and 9 o'clock positions, and the average of the values ​​obtained at each position is taken as the average Si and Mn concentration of the base material. The concentration of Si and Mn can be measured by a method using characteristic X-rays, but is not limited to this method.

[0039] The flare processability is evaluated by the method described in the examples. EXAMPLES

[0040] According to the manufacturing process shown in Figure 1, a forge-welded steel pipe 9 was manufactured using a material with the composition shown in Table 1. The steel strip was unwound from a coil 1, passed through a looper 3, and the edges were formed, after which the entire width of the steel strip was heated in a heating furnace 5, and continuously formed into a tubular shape in a forming forge welding machine 6, while just before the edges were butted and forged, oxygen-mixed air was blown from a nozzle to raise the temperature, forge-welded and joined, and further reduced to the desired outer diameter by a diameter reducing roll 8, to manufacture a forge-welded steel pipe 9. The temperature of the steel strip edges just before butting was set to be within the range of the above-mentioned formula (1).

[0041] The outer diameter of the butt-welded steel pipe 9 manufactured as described above was measured, and then flaring was performed using a flare processing machine. Figure 4(a) shows the butt-welded steel pipe before flaring, and Figure 4(b) shows the butt-welded steel pipe after flaring. The butt-welded steel pipe after flaring was determined to have good flaring workability when no cracks were generated when the outer diameter 16 of the butt-welded steel pipe before flaring was processed until it was 1.7 times the outer diameter 12 of the butt-welded steel pipe before flaring. For those that did not crack even when formed to 1.7 times, further processing was performed until no cracks were generated, and the outer diameter 16 of the butt-welded steel pipe after flaring at that time / the outer diameter 12 of the butt-welded steel pipe before flaring was determined as the expansion ratio. The outer diameter 16 of the butt-welded steel pipe after flaring was measured with a vernier caliper as shown in Figure 4(b) including the butt-welded abutment part 10 after flaring. The test pieces for measuring the average Si and Mn concentrations were taken from the unprocessed part by flaring in the longitudinal direction to a thickness of 5 mm. The collected test pieces were polished and measured using an electron probe microanalyzer (EPMA), which performs elemental analysis using characteristic X-rays. The acceleration voltage was 15 kv and the beam diameter was 0.2 μm, and measurements were taken on a 10 μm line centered on the abutment part 10. Table 2 shows the evaluation results.

[0042] As can be seen from Table 2, the ratios of the average Si and Mn concentrations in the butt joint 10 to the average Si and Mn concentrations in the base material of steel pipes No. 1 to 6, 11, and 12, which are examples of the present invention, are within the appropriate range. As a result, no cracks were generated during flaring (peeling of the forge-welded parts of the flanges of the flare pipes), and the flaring workability was good (◯ or ◎). In particular, steel pipes No. 2, 4, and 6 have a gas blowing pressure of 0.56 MPa or more, a high pipe expansion ratio, and extremely high flaring workability (◎). In contrast, in steel pipes No. 7 to 10, and 13, which are comparative examples, cracks were visually confirmed in the joints before flaring to the desired outer diameter, and the flaring workability was poor (×). As for No. 13, the temperature at the time of butt exceeded the upper limit, so the edge part was extremely softened or partially melted, and it was deformed without being able to withstand the stress of butt, and the cross section could not maintain its circular shape, and the steel pipe could not be manufactured.

[0043] [Table 1]

[0044] [Table 2] [Explanation of symbols]

[0045] 1 coil 2. Steel strip 3. Looper 4 Edge forming machine 5 Furnace 6 Forming forge welding machine 7 Nozzles 8. Reducing rolls in reducing mills 9 Steel pipe (forge welded steel pipe) 10 Abutment surface (forge welded abutment part or abutment part) 11 Thickness measurement position 12 Outside diameter of forged steel pipe before flaring 13 Line (average Si, Mn concentration measurement line of the joint) 14 Normal line drawn toward the inside of the steel pipe (normal line) 15 Line (average Si, Mn concentration measurement line for base material) 16 Outside diameter of forged steel pipe after flaring 20 measurement area 21 Measurement area

Claims

1. A steel pipe having abutment surfaces where an end of a steel plate and another end of the steel plate are joined, A steel pipe, in which, at a position 100 μm from the inner surface toward the outer surface, the ratio of the average Si concentration (mass %) within 5 μm on each side from the abutment surface in a direction perpendicular to the abutment surface to the average Si concentration (mass %) in the base material is 0.03 or more and 1 / 2 or less, and the ratio of the average Mn concentration (mass %) within 5 μm on each side from the abutment surface in a direction perpendicular to the abutment surface to the average Mn concentration (mass %) in the base material is 0.02 or more and 1 / 2 or less.

2. A method for producing a steel pipe according to claim 1, The method includes a step of forming the edge portion of a steel plate, heating the steel plate in a heating furnace, and then forming the steel plate into a tubular shape while butting and forging the edges of the formed steel plate, A manufacturing method of steel pipe, in which the temperature T (°C) of the edge portions of the steel plates before abutting satisfies formula (1) defined by the Si (%) and Mn (%) of the steel plates and the time t (sec.) from the end of heating to abutting. -22×{Si(%)}×{t(sec.)}−14×{Mn(%)}×{t(sec.)}+1530 <T (° C.)< -17 x {Si (%)} x {t (sec.)} -7 x {Mn (%)} x {t (sec.)} + 1542... (1) In formula (1), Si (%) and Mn (%) are the contents (mass %) of each element in the steel sheets, T (°C) is the temperature of the edge portions of the steel sheets before abutting, and t (sec.) is the time from the end of heating to the abutting.

3. 3. The method for manufacturing a steel pipe according to claim 2, wherein, before the abutment, a gas having an oxygen concentration in volume fraction of 20% to 40% is supplied to the edge portions of the steel plates at a blowing pressure of 0.50 MPa or more.

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