JP3684684B2 - Steel pipe manufacturing method - Google Patents

Description

【００３１】
また、帯鋼を1300℃に加熱したのち、鍛接により60.5mmφの鍛接管とし、従来例（No.11 ）とした。実施例と同様に鋼管の偏平高さ比、表面粗さRmaxを測定し、表１に併記した。
試験No.1、No.2の本発明例では、偏平高さ比 0.3以下、表面粗さRmax 10 μm 以下であり、従来例の試験No.11 の鍛接管では、偏平高さ比0.56、表面粗さRmax 37.5 μm であるのに対し向上している。本発明の範囲を外れると、試験No.3、No.4のように、偏平高さ比が大きくなり、また、試験No.10 のように、表面粗さRmaxが大きくなる。さらに、試験No.5のように、エッジ部端面が溶融すると余盛が形成され、ビード切削する必要が生じるため、造管速度が100m/minに低下する。
【００３２】
また、試験No.6は、エッジ部を1300℃を超える温度に予熱したが、シーム品質及び表面肌は優れ、造管速度の低下もなかった。
また、本発明例の生産性は、30ton/hrと高く、ビード切削する従来の電縫管の生産性が15ton/hrであるのに対し、生産性が著しく向上している。
本発明例の試験No.1、No.8、No.12 では、圧接シーム部の管内面に0.5 〜1.5mm の増肉がみられたが、圧接シーム部近傍を管内外から圧延ロールで圧延し、0.2mm 以内に減肉し、鋼管寸法の規格範囲内となった。
【００３３】
また、試験No.2、No.9、No.13 は、圧接位置において管外面にスクイズロールを、管内面に圧延ロールをそれぞれ当接させ、材料を上下方向に拘束することによって、圧接シーム部の増肉が0.1mm 以下で鋼管寸法の規格範囲内となり、圧接以降の圧延が不要であった。
本発明例の試験No.12 、No.13 では、帯鋼のエッジ処理（具体的にはミーリングによる切削加工）を実施し、エッジ部角を直角とした。エッジ処理を行った試験No.12 、No.13 では、エッジ処理を行わなかった他の試験No.1、No.2に比べ偏平高さ比が小さくなっている。
【００３４】
本発明例の試験No.14 では、エッジ加熱および固相圧接時の雰囲気中の露点を−20℃に制御した。これにより、雰囲気中の露点制御を行わなかった試験No.9に比べ偏平高さ比が小さくなっている。
【００３５】
【発明の効果】
本発明によれば、オープン管の両エッジ部を固相圧接可能温度域に安定的に保持でき、優れたシーム品質および表面肌を有する鋼管を高い生産性で製造できるという格段の効果を奏する。
【図面の簡単な説明】
【図１】 本発明の実施に好適な鋼管製造設備列の１例を示す説明図である。
【図２】固相圧接接合部のシーム品質に及ぼす圧接後1300℃以上に保持される時間ｔ_k と雰囲気中の酸素濃度との関係を示すグラフである。
【図３】固相圧接時のスクイズロール、圧接シーム部内面拘束用ロールと圧接接合部との位置関係を示す断面図である。
【図４】固相圧接後の鋼管断面形状の例を示す断面図である。
【図５】本発明の実施に好適な設備列の模式的部分断面側面図である。
【図６】固相圧接後の圧接シーム部外面形状の１例を示す断面図である。
【図７】鋼の比透磁率の温度依存性を示す特性図である。
【符号の説明】
１ 帯鋼
３ 成形ロール群
４ エッジ予熱用誘導加熱コイル
５ エッジ加熱用誘導加熱コイル
６ スクイズロール
７ オープン管
８ 鋼管
９ 圧接シーム部
１０ 圧接シーム部圧延用ロール
１０ａ 圧接シーム部外面圧延用ロール
１０ｂ 圧接シーム部内面圧延用ロール
１０ｃ 圧接シーム部圧延用ロール支持棒
１１ａ 圧接シーム部内面拘束用ロール
１１ｂ 圧接シーム部内面拘束用ロール支持棒
１２ 圧接シーム部外面ウェルドライン[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for manufacturing a steel pipe, and more particularly to a method for manufacturing a steel pipe by solid phase pressure welding.
[0002]
[Prior art]
Welded steel pipes are formed by forming a steel plate or steel strip into a tubular shape and welding the seam. The welded steel pipes are manufactured by various manufacturing methods from small to large diameters. , Forging and electric arc welding.
For small to medium diameter steel pipes, an electric resistance welding method (electric resistance welded steel pipe, electric resistance welded pipe) using high frequency induction heating is mainly used. In this method, a steel strip is continuously supplied, formed into a tubular shape with a forming roll to form an open tube, and then both edge portions of the open tube are heated to a temperature higher than the melting point of the steel by high frequency induction heating, and then a squeeze roll. In this method, the end surfaces of both edge portions are abutted and welded to produce a steel pipe (for example, third edition Steel Handbook, Volume III (2) pages 1056-1092).
[0003]
In the above-described method for manufacturing an ERW pipe using high-frequency induction heating, the end surfaces of both edges of the open pipe are heated to the melting point or higher of the steel, so that the molten steel flows under the influence of electromagnetic force, and the generated oxide is impinged. There has been a problem that welding defects such as a penetrator or molten steel scattering (flash) are likely to occur due to being caught in the joint weld.
In order to solve this problem, for example, Japanese Patent Laid-Open No. 2-299782 proposes a method for producing an electric resistance welded steel pipe having two heating devices. The temperature of both edge parts of the open pipe is heated above the Curie point with the first heating device, further heated above the melting point with the second heating device, and both edge portions are abutted and welded with a squeeze roll to produce a steel pipe. To do. In Japanese Patent Laid-Open No. 2-99783, a current of a frequency of 45 to 250 kHz is supplied by a first heating device, both edge portions are preheated, further heated to a melting point or higher by a second heating device, and squeezed by a squeeze roll. There has been proposed an electric welded tube manufacturing apparatus for manufacturing a steel pipe by abutting welding of both edge portions.
[0004]
However, although these ERW pipe manufacturing techniques suggest that the edges are heated uniformly, both edges are heated above the melting point of the steel. A bead is formed on the inner and outer surfaces. Therefore, it is necessary to remove the weld beads on the inner and outer surfaces of the pipe after the abutting welding, and most of them are cut and removed by a cutting tool for bead cutting.
[0005]
For this reason, in this method,
(1) Loss of material and time occurs by adjusting the cutting amount of the bead cutting tool.
(2) Since the cutting tool for bead is a consumable item, it varies depending on the pipe making speed, but it is necessary to change the cutting tool every bead cutting length of 3000 to 4000 m. Therefore, it takes 3 to 5 minutes every hour. Forced to stop line for byte exchange.
[0006]
(3) Particularly in the case of a high speed pipe making with a pipe making speed exceeding 100 m / min, the life of the bead cutting tool is short and the replacement frequency is high.
For example, bead cutting has become a bottleneck, and there is a problem that productivity is low because high-speed pipe making is impossible.
On the other hand, there is a forged steel pipe manufacturing method having extremely high productivity for a relatively small diameter steel pipe. In this method, the continuously supplied steel strip is heated to about 1300 ° C in a heating furnace, then formed into a tubular shape with a forming roll to form an open pipe, and then high pressure air is blown to both edge portions of the open pipe to end face After performing the scale-off, oxygen is blown to the end face with a welding horn, the end face is heated to about 1400 ° C with the heat of oxidation, and the end faces of both edges are brought into contact with a forging roll and solid phase bonded. This is a method for producing a steel pipe (for example, 3rd Edition Steel Handbook, Volume III (2) 1056-1092).
[0007]
However, in this forged steel pipe manufacturing method,
(1) Since the scale-off of the end face is not complete, scale biting occurs in the forging contact portion, and the strength of the seam portion is considerably inferior to that of the base material portion. For this reason, in the flatness test, a flat height ratio h / D = 2t / D (t: plate thickness) can be achieved with an electric-welded steel pipe, whereas a flat height ratio h / D is inferior to about 0.5 with a forged steel pipe. It will be a thing.
[0008]
(2) Since the steel strip is heated to a high temperature, scale is generated on the tube surface, and the surface skin is poor.
The pipe making speed is as fast as 300m / min or more, and the productivity is high, but the seam quality and the surface skin are poor, and there is a problem that it is impossible to manufacture those requiring strength reliability and surface quality such as JIS STK. .
[0009]
[Problems to be solved by the invention]
An object of the present invention is to propose a method of manufacturing a steel pipe by an induction heating method, which can advantageously solve the above problems and can manufacture a steel pipe having excellent seam quality and surface texture with high productivity.
[0010]
[Means for Solving the Problems]
The present invention provides a steel pipe manufacturing method in which a steel strip is continuously formed with a forming roll to form an open pipe, both edge portions of the open pipe are heated and abutted and joined with a squeeze roll, and both edges of the open pipe are used. The part is subjected to edge preheating that heats to a temperature above the Curie point by induction heating, then edge heating that heats to a temperature range of 1300 ° C or higher and below the melting point by induction heating, and press-contacts with the squeeze roll (however, , Excluding the case of preheating the strip steel and / or the open pipe and the case of drawing after the pressure welding) .
[0011]
In the present invention, the edge preheating is preferably performed at a temperature not lower than the Curie point and lower than 1300 ° C.
The edge preheating, the edge heating and the pressure welding are preferably performed in an oxygen concentration atmosphere lower than the atmosphere or in an atmosphere having a dew point of −10 ° C. or lower. In the present invention, after the pressure welding, the time t _k (sec) during which the joint is held at 1300 ° C. or more is 0.03 sec or more or the following formula (1)
t _k ≧ a · exp {−b · [O ₂ ] ^c } (1)
It is preferable that (O ₂ : oxygen concentration in the atmosphere (vol%), a = 0.079, b = 1.5, c = −0.14) is satisfied.
[0012]
Moreover, in this invention, a seam part pipe material may be restrained from the inside and outside of a pipe | tube at the time of the said pressure welding, and seam part thickness increase may be suppressed.
In the present invention, the vicinity of the pressure seam portion may be rolled after the pressure welding.
Moreover, in this invention, after the said pressure welding, you may remove the micro recessed shape part of a pressure welding seam part outer surface, and may smooth an outer surface.
[0013]
The steel strip is preferably subjected to an edge treatment in which the edge surface of the edge portion is flattened and the angle formed between the edge surface of the edge portion and the surface of the steel strip is a predetermined angle. Further, the edge treatment of the end face of the steel strip may be performed before or after forming with a forming roll.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
The steel strip is continuously formed by a forming roll to form an open tube. For forming, a generally known method using a forming roll can be suitably applied.
Next, edge preheating is performed on both edges of the open pipe.
Edge preheating is an induction heating method.
[0015]
By this edge preheating, the temperature of the edge portion is set to the Curie point or higher, preferably lower than 1300 ° C.
From the temperature dependence of the relative permeability of the steel shown in FIG. 7, when the steel is heated above the Curie point, the steel undergoes a magnetic transformation from a ferromagnetic material to a paramagnetic material, and the relative permeability (to vacuum ratio) is close to 1. It becomes. On the other hand, the penetration depth S of the induced current is given by the following equation (2).
[0016]
S = α {ρ / (μ _r f)} ^1/2 (2)
Here, S: penetration depth (m), ρ: resistivity (Ω · m), μ _r : relative permeability, f: frequency (kHz), α: constant.
Therefore, by heating the edge portion to the Curie point or higher, the penetration depth S increases, and the temperature distribution in the pressure contact surface becomes uniform. Therefore, the edge portion is preheated to a temperature range above the Curie point. From the viewpoint of energy efficiency, the edge preheating is preferably performed at a temperature higher than the Curie point and lower than 1300 ° C, but there is no problem even when the temperature is higher than 1300 ° C. However, if the temperature is increased rapidly at this stage, only the corners will exceed the melting point, and beads (excess) will be generated during joining, which may prevent high-speed pipe making. Edge preheating is more than the Curie point and less than 1300 ° C. It is more preferable to carry out in the temperature range.
[0017]
The edge preheating may be performed in the atmosphere or in an atmosphere in which the oxygen concentration is reduced from the atmosphere (in the shield atmosphere), but in the shield atmosphere, the shield atmosphere is preferable. The edge preheating is preferably performed in an atmosphere having a dew point of −10 ° C. or lower.
In addition, since edge preheating is heated from an open tube at room temperature, the temperature gradient from the part other than the preheating part is large. Therefore, in order to set the temperature of the edge part to a predetermined temperature, the width of the preheating part should be widened to some extent. Is preferred.
[0018]
Both edge portions of the open pipe subjected to the edge preheating are further subjected to edge heating for heating to a temperature range of 1300 ° C. or higher and lower than the melting point.
The heating method of edge heating is an induction heating method using an induction coil from the viewpoint of energy efficiency.
In the edge heating, it is preferable to dispose an impeder of an appropriate size in the open tube from the viewpoint of heating efficiency, but the edge heating is possible even when the impeder is reduced in size or not provided. In this case, the tubular body other than the edge portion is also easily heated.
[0019]
The temperatures of the end faces of both edges of the open tube are controlled by adjusting the output of the induction heating coil.
If the temperature of the edge heating is less than 1300 ° C., the joining of the end faces of the edge part becomes insufficient and the seam quality is deteriorated. Further, when the temperature of the end face of the edge part exceeds the melting point of the pipe material, the melted steel forms a bead (excess) on the inside and outside of the pipe at the time of abutting joining, and thus bead cutting is required. For this reason, the edge heating is set to a temperature range in which solid-state pressure welding is possible at 1300 ° C. or higher and lower than the melting point. The melting point is preferably 1350 ° C. or higher and lower than the melting point, more preferably 1400 ° C. or higher and lower than the melting point.
[0020]
The solid-phase pressure welding referred to in the present invention means pressure welding that suppresses the swell of the bead (excess) and does not require bead cutting.
In the present invention, the edge heating temperature is preferably in the solid phase region in order to suppress the amount of bead (excess) buildup, but there is no problem even in the solid-liquid two phase region below the melting point where some liquid phase exists. .
[0021]
In order to make the temperature distribution of the edge part at the time of induction heating uniform, in the present invention, preferably, the edge of the steel strip is refined, the edge part end face is flattened, and the angle formed between the edge part end face and the steel strip surface Is preferably a predetermined angle. The predetermined angle is preferably 60 to 120 degrees. The edge trimming may be performed either before paying off the coil, before paying off the coil and forming it into an open tube with a forming roll, or after forming. The edge treatment is preferably performed by cutting with an edge mirror, polishing with a grinder, or rolling with an edger roll.
[0022]
The open pipes whose both edge portions are heated to the above-mentioned temperature range capable of solid-phase pressure welding are brought into contact with each other by a squeeze roll and solid-phase pressure-welded. As shown in FIG. 3 (a), the pressure welding is performed by installing a squeeze roll at a position where it abuts on the outer surface of the pressure welding joint tube, and as shown in FIG. 3 (b), the squeeze roll is placed on the pressure welding joint tube. As shown in FIG. 3 (c), there is a method in which the squeeze roll is installed on the outer surface side and a roll or the like is installed in a position where the inner surface side comes into contact with the press-contact joint as shown in FIG. However, there is no inconvenience in any case.
[0023]
Edge heating and solid-phase pressure welding may be performed either in the air or in an atmosphere in which the oxygen concentration is reduced from the air (in a shield atmosphere), but from the viewpoint of seam quality, a shield atmosphere is preferable. In addition, the edge heating and the solid-phase pressure welding are preferably performed in an atmosphere having a dew point of −10 ° C. or less from the viewpoint of seam quality.
The inventors have found that the seam quality of the steel pipe changes with the time t _k during which the joint is held at 1300 ° C. or higher after the pressure welding. And t _k on the seam quality (flat height ratio h / D), the relationship between the oxygen concentration shown in Fig. From Figure 2, t _k in accordance becomes longer, it can be seen that the seam quality is improved. Further, in accordance with the oxygen concentration in the atmosphere is reduced, in order to obtain the same seam quality t _k it is understood that it may be shortened.
[0024]
This time t _k (sec) is preferably 0.03 sec or more when edge preheating, edge heating, and solid phase pressure welding are performed in the atmosphere. On the other hand, when edge preheating, edge heating, and solid-phase pressure welding are performed in an atmosphere having a lower oxygen concentration than in the atmosphere (in a shield atmosphere), t _k is preferably set to a time that satisfies the following formula (1). .
t _k ≧ a · exp {−b · [O ₂ ] ^c } (1)
Here, O ₂ : oxygen concentration (vol%) in the atmosphere, a, b, c: constants, and in the case of low carbon steel, a = 0.079, b = 1.5, c = −0.14. More preferably, a = 0.23, b = 1.4, c = −0.17.
[0025]
This time t _k controls the heating temperature of both edges of the open tube during edge preheating and the heating width above the Curie point, and further the heating temperature of the edge surfaces of both edges during edge heating. By adjusting the temperature distribution in the circumferential direction of the pipe from the end face to the center of the pipe, the cooling rate of the seam after solid-phase pressure welding is adjusted and controlled.
In the pressure welding seam portion formed by solid phase pressure welding, depending on whether or not the squeeze roll is in contact with the outer surface of the pressure welding joint, the ultimate temperature of the edge portion, or the degree of pipe circumferential direction drawing by the squeeze roll, FIG. As shown in b), an increase in thickness of 5% or more of the tube thickness may occur inside or outside the pipe of the seam portion. In such a case, it is preferable to reduce the thickness of the vicinity of the thickened seam portion by rolling at an appropriate place after the pressure welding. Rolling in the vicinity of the thickened seam portion is performed, for example, from the inside and outside of the tube by a pressure seam portion rolling roll 10 shown in FIG. The pressure seam rolling roll 10 includes a pressure seam outer roll 10a and a pressure seam inner roll 10b, and 10b is supported by a pressure seam roll support rod 10c.
[0026]
Further, among the above-mentioned pressure welding methods, by adopting a method of bringing a roll or the like into contact with the inner and outer surfaces of the pressure welded portion, the material is restrained in the vertical direction, the thickness increase due to pressure welding is suppressed to less than 5%, and rolling after pressure welding is performed. Can be eliminated. For example, the material is restrained from inside and outside the pipe by the squeeze roll 6 and the pressure seam portion inner surface restraining roll 11a shown in FIG. The pressure seam portion inner surface restraining roll 11a is supported by a pressure seam portion inner surface restraining roll support bar 11b.
[0027]
In the welding seam part formed by solid phase welding, regardless of whether the welding part is rolled or not, depending on the degree of edge of the steel strip, the accuracy of edge adjustment of the steel strip, the pressure welding method or the degree of thickness increase by pressure welding As shown in FIG. 6, a minute concave portion called a weld line having a depth of about 0.2 mm may be formed on the outer surface, which adversely affects the appearance and seam quality. In such a case, it is preferable to smooth the outer surface by removing the weld line at an appropriate place after the pressure welding. The removal of the weld line is performed by performing processing such as cutting and polishing. Further, the welding line may be removed either before or after the rolling when the press-thickened portion is rolled.
[0028]
As described above, according to the present invention, both edge portions of the open tube can be stably held in a temperature range in which solid-state pressure welding is possible, and then have excellent seam quality and surface skin by solid-phase pressure welding with a squeeze roll. Steel pipes can be manufactured with high productivity.
[0029]
【Example】
An equipment row suitable for carrying out the present invention shown in FIG. 1 was used.
The steel strip 1 having a thickness of 3.5 mm is continuously formed by the forming roll group 3 to form the open pipe 7, and then the edge preheating is performed by the edge preheating induction heating coil 4 under the conditions shown in Table 1 at both edges of the open pipe. Furthermore, edge heating is performed by the induction heating coil 5 for edge heating, and solid-phase pressure welding is performed with a squeeze roll 6 installed at a position where it abuts against the pressure seam portion, and a pipe size: 60.5 mmφ × 3.5 mmt, standard: STKM11A steel pipe 8 did. The seam quality and surface skin of the manufactured steel pipe 8 are investigated, and the results are also shown in Table 1. The seam quality was evaluated based on the flat height ratio of the steel pipe (h / D, h: flat height mm, D: outer diameter mm of the steel pipe). Moreover, the surface roughness of the steel pipe was evaluated by the surface roughness Rmax (μm). For some of the steel pipes, edge preheating, edge heating and solid phase pressure welding were performed in a shield atmosphere.
[0030]
[Table 1]

[0031]
Also, after heating the strip steel to 1300 ° C, it was made into a forged welded tube of 60.5mmφ by forge welding, which was a conventional example (No. 11). The flat height ratio and surface roughness Rmax of the steel pipe were measured in the same manner as in the examples, and are also shown in Table 1.
In the present invention examples of test No. 1 and No. 2, the flat height ratio is 0.3 or less and the surface roughness Rmax is 10 μm or less. In the conventional welded test tube of test No. 11, the flat height ratio is 0.56, the surface Roughness is improved compared to Rmax 37.5 μm. Outside the scope of the present invention, the flat height ratio increases as in Test No. 3 and No. 4, and the surface roughness Rmax increases as in Test No. 10. Furthermore, as in test No. 5, when the edge of the edge portion melts, a surplus is formed, and it becomes necessary to perform bead cutting, so the pipe making speed is reduced to 100 m / min.
[0032]
In Test No. 6, the edge portion was preheated to a temperature exceeding 1300 ° C., but the seam quality and surface skin were excellent, and the pipe making speed was not reduced.
Further, the productivity of the example of the present invention is as high as 30 ton / hr, and the productivity of the conventional ERW pipe for bead cutting is 15 ton / hr, while the productivity is remarkably improved.
In tests No. 1, No. 8, and No. 12 of the present invention example, a thickness increase of 0.5 to 1.5 mm was observed on the inner surface of the pipe of the pressure seam, but the vicinity of the pressure seam was rolled with a rolling roll from inside and outside the pipe. However, the thickness was reduced to within 0.2mm, and it was within the standard range of steel pipe dimensions.
[0033]
Test No.2, No.9, and No.13 are the press seam parts by holding the squeeze roll on the outer surface of the pipe and the rolling roll on the inner surface of the pipe at the press contact position, and restraining the material in the vertical direction. The thickness increase of 0.1 mm or less was within the standard range of steel pipe dimensions, and rolling after pressure welding was unnecessary.
In tests No. 12 and No. 13 of the examples of the present invention, the edge treatment of the steel strip (specifically, cutting by milling) was performed, and the edge portion angle was set to a right angle. In tests No. 12 and No. 13 where edge processing was performed, the flat height ratio was smaller than in other tests No. 1 and No. 2 where edge processing was not performed.
[0034]
In Test No. 14 of the present invention example, the dew point in the atmosphere during edge heating and solid-phase pressure welding was controlled at -20 ° C. As a result, the flat height ratio is smaller than test No. 9 in which the dew point control in the atmosphere was not performed.
[0035]
【The invention's effect】
According to the present invention, both edge portions of an open pipe can be stably held in a temperature range in which solid phase pressure welding is possible, and a remarkable effect is achieved that a steel pipe having excellent seam quality and surface skin can be manufactured with high productivity.
[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is an explanatory diagram showing an example of a steel pipe manufacturing equipment line suitable for implementing the present invention.
FIG. 2 is a graph showing the relationship between the time t _k maintained at 1300 ° C. or higher after pressure welding and the oxygen concentration in the atmosphere on the seam quality of the solid-phase pressure welding joint.
FIG. 3 is a cross-sectional view showing the positional relationship between a squeeze roll, a pressure seam seam inner surface restraining roll, and a pressure welding joint during solid phase pressure welding.
FIG. 4 is a cross-sectional view showing an example of a steel pipe cross-sectional shape after solid-phase pressure welding.
FIG. 5 is a schematic partial cross-sectional side view of an equipment row suitable for implementing the present invention.
FIG. 6 is a cross-sectional view showing an example of the outer shape of a pressure seam portion after solid-phase pressure welding.
FIG. 7 is a characteristic diagram showing the temperature dependence of the relative permeability of steel.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Steel strip 3 Forming roll group 4 Edge preheating induction heating coil 5 Edge heating induction heating coil 6 Squeeze roll 7 Open pipe 8 Steel pipe 9 Pressure welding seam part 10 Pressure seam part rolling roll 10a Pressure seam part outer surface rolling roll 10b Pressure welding Roll 10c for inner surface rolling Roll support rod 11a for pressure welding seam rolling Inner surface constraining roll 11b Pressure welding seam inner surface constraining roll support rod 12 Pressure welding seam outer surface weld line

Claims (11)

In a method of manufacturing a steel pipe, in which a steel strip is continuously formed by a forming roll to form an open pipe, both edge portions of the open pipe are heated and abutted and joined by a squeeze roll, induction is applied to both edge portions of the open pipe. Apply edge preheating to heat to a temperature above the Curie point by heating, then perform edge heating to a temperature range of 1300 ° C or higher and below the melting point by induction heating, and press-contact with the squeeze roll (however, the steel strip and And / or a method for producing a steel pipe with excellent seam quality and surface texture, characterized by the fact that the preheat is applied to the open pipe and the case where drawing rolling is performed after pressure welding) .   The method of manufacturing a steel pipe according to claim 1, wherein the edge preheating is performed to a temperature not lower than the Curie point and lower than 1300 ° C.   3. The method of manufacturing a steel pipe according to claim 1, wherein the edge preheating is performed in an oxygen concentration atmosphere lower than the atmosphere.   4. The method of manufacturing a steel pipe according to claim 1, wherein the edge heating and the pressure welding are performed in an oxygen concentration atmosphere lower than the atmosphere.   5. The method of manufacturing a steel pipe according to claim 1, wherein the edge preheating, the edge heating, and the pressure welding are performed in an atmosphere having a dew point of −10 ° C. or less. The time t _k (sec) during which the joint is held at 1300 ° C. or higher after the pressure welding is 0.03 sec or more or t _k satisfying the following expression (1): The manufacturing method of the steel pipe of 4 or 5.
Record
t _k ≧ a · exp {−b · [O ₂ ] ^c } (1)
Here, O ₂ : oxygen concentration (vol%) in the atmosphere, a = 0.079, b = 1.5, c = −0.14.   The method for manufacturing a steel pipe according to claim 1, wherein the seam portion pipe material is restrained from the inner and outer surfaces of the pipe at the time of the press contact, and the seam portion is prevented from being thickened.   The method for manufacturing a steel pipe according to claim 1, 2, 3, 4, 5, 6 or 7, wherein the vicinity of the pressure seam portion is rolled after the pressure welding.   9. The method of manufacturing a steel pipe according to claim 1, wherein after the press contact, a minute concave shape portion of the outer surface of the press contact seam portion is removed to smooth the outer surface. .   The strip steel is obtained by flattening an end face of an edge portion and subjected to an edge treatment in which an angle formed between the end face of the edge portion and the surface of the strip steel is a predetermined angle. The manufacturing method of the steel pipe of 3, 4, 5, 6, 7, 8, or 9.   11. The method of manufacturing a steel pipe according to claim 10, wherein the edge treatment of the end surface of the steel strip is performed before or after forming with a forming roll.

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