JPH105856A - Manufacture of steel tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a steel tube with a superior seam and surface quality by preheating a formed open tube, successively preheating and heating the edge through induction heating, and then pressure-welding by a squeeze roll. SOLUTION: A steel strip 1 is preheated by a preheating furnace 2 as necessary, and formed continuously by a group of forming rolls 3 into an open tube 7. This open tube 7 is preheated at about 400-650 deg.C using a tubular body preheating device 13 provided with an induction coil. Then, an edge preheating is performed in which both edges of the open tube 7 is heated at a temperature above Curie point by an induction heating coil for edge preheating 4. In addition, an edge heating is applied to the edges by heating above 1,300 deg.C but below the melting point by an induction heating coil for edge heating 5. Then, solid phase pressure-welding is carried out by a squeeze roll 6 installed at a position abutting on the pressure-welding seam part to form a steel tube 8. It is desirable that the edge preheating, edge heating and pressure-welding are performed in the atmosphere with an oxygen conc. less than the air or with a dew point at -10 deg.C or below.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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-state pressure welding.

[0002]

2. Description of the Related Art Welded steel pipes are formed by forming a steel plate or a steel strip into a tube and welding the seams thereof, and are made by various manufacturing methods from a small diameter to a large diameter.
Examples include electric resistance welding (electric resistance welding), forge welding, 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) utilizing 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 pipe, and then the end faces of both edges of the open pipe are heated by high-frequency induction heating to a temperature equal to or higher than the melting point of the steel. (See, for example, the third edition of the Iron and Steel Handbook, Vol. III (2), 1056-10).
92).

In the above-described method of manufacturing an electric resistance welded tube utilizing high frequency induction heating, since the end surfaces of both edges of the open tube are heated to the melting point of the steel or higher, the molten steel flows under the influence of electromagnetic force, and the generated oxidation occurs. An object is caught in the abutment weld and welding defects such as penetrators or molten steel scatter (flash)
There is a problem that is easy to occur. For this issue,
For example, Japanese Patent Application Laid-Open No. 2-299782 proposes a method for manufacturing an electric resistance welded steel pipe having two heating devices. The first heating device heats the temperature of both side edges of the open tube to the Curie point or higher, and the second heating device further heats to the melting point or higher.
A steel pipe is manufactured by butt welding both edges with a squeeze roll. Also, in Japanese Patent Application Laid-Open No. 2-299832, a current having a frequency of 45 to 250 kHz is passed by a first heating device to preheat both side edges, and 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 resistance welded pipe manufacturing apparatus which manufactures a steel pipe by abutting both edges.

[0004] However, although these electric resistance welded pipe manufacturing techniques suggest that the edges are uniformly heated, since both edges are heated to the melting point of the steel or higher, the molten steel is hardened at the time of impact welding. Is discharged to the inner and outer surfaces of the tube to form a bead. Therefore, it is necessary to remove the weld bead on the inner and outer surfaces of the pipe after the impact welding, and most of the bead is removed by cutting with a bead cutting tool.

[0005] For this reason, in this method, loss of material and time occurs due to adjustment of the cutting amount of the bead cutting tool. Since the bead cutting tool is a consumable, it needs to be changed every 3000-4000m bead cutting length, depending on the pipe forming speed. Therefore, it is necessary to change the bite for 3-5 minutes every hour. Forced to stop the line.

In particular, in high-speed pipe forming in which the pipe forming speed exceeds 100 m / min, the life of the bead cutting tool is short and the frequency of replacement is high. There was a problem that bead cutting became a bottleneck and high-speed pipe making was not possible, resulting in low productivity. On the other hand, there is a method for producing a forged steel pipe having extremely high productivity for a relatively small diameter steel pipe. In this method, a continuously supplied steel strip is heated to about 1300 ° C in a heating furnace, and then formed into a tubular shape with a forming roll to form an open pipe. After performing the scale-off, oxygen is blown to the end face by the welding horn, and the end face is heated to about 1400 ° C. by the heat of oxidation, and then the end faces of both edges are abutted by a forging roll to perform solid-state joining. This is a method for producing a steel pipe (for example, Third Edition Iron and Steel Handbook, Vol. III (2), pp. 1056-1092).

However, in this method for producing a forged steel pipe, the scale-off of the end face is not perfect, so that the scale is caught in the forged joint portion, and the strength of the seam portion is considerably inferior to that of the base material portion. For this reason, in the flattening test, the flattened height ratio h / D = 2t / D (t: plate thickness) can be achieved with an ERW steel pipe, while the flattened height ratio h / D is inferior to about 0.5 with a forged steel pipe. It will be.

[0008] Since the steel strip is heated to a high temperature, scale is formed on the pipe surface, and the surface skin is poor. The pipe making speed is 300m /
Although the productivity is high as short as min or more, the seam quality and the surface skin are poor, and there is a problem that products requiring strength reliability and surface quality such as JIS STK cannot be manufactured.

[0009]

DISCLOSURE OF THE INVENTION The present invention advantageously solves the above-mentioned problems, and enables a steel pipe having excellent seam quality and surface skin to be manufactured with high productivity. The purpose is to propose.

[0010]

SUMMARY OF THE INVENTION According to the present invention, a steel pipe is manufactured by continuously forming a strip from a forming roll into an open pipe, heating both edges of the open pipe, and abutting and joining with a squeeze roll. In the method, prior to heating both edges, the open pipe, or the strip and the open pipe, are preheated, and then both edges of the open pipe are heated to a temperature above the Curie point by induction heating. After edge preheating, 1300 ° C by induction heating
Above, subjected to edge heating to heat the temperature range below the melting point,
A method for producing a steel pipe having excellent seam quality and surface texture characterized by being pressed against the squeeze roll, wherein the edge preheating is preferably performed by heating to a temperature equal to or higher than the Curie point and lower than 1300 ° C. Preferably, the preheating of the pipe, the preheating of the strip and the open pipe is carried out at a temperature below 800 ° 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 less. After the pressure contact, the time t _k which joint is held above 1300 ° C. (sec) is, 0.03sec more or the following formula _{(1) t k ≧ a ·} exp {-b · [O _2] ^c} ...... ( 1) (where, O ₂ : oxygen concentration in the atmosphere (vol%), a =
0.079, b = 1.5, c = -0.14).

Further, in the present invention, at the time of the pressing, the seam portion pipe may be restrained from inside and outside of the tube to suppress the increase in the thickness of the seam portion. Further, in the present invention, after the pressing, the vicinity of the pressing seam portion may be rolled. Further, in the present invention, after the pressing,
The outer surface may be smoothed by removing a minute concave portion on the outer surface of the press-seam portion.

It is preferable that the strip steel has been subjected to an edge treatment in which an end face of the edge portion is flattened and an angle formed between the end face of the edge portion and the surface of the strip steel is a predetermined angle. Further, the edge treatment of the end face of the steel strip may be performed before or after forming by a forming roll.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION A strip is continuously formed by a forming roll into an open pipe. For molding, a known method using a molding roll can be suitably applied. In the present invention, the open pipe after molding is preheated. Further, the strip may be preheated before the strip is formed, and the open pipe after the forming may be preheated.

In the preheating, the temperature difference between the edge portion and the mother pipe near the edge portion is reduced during edge heating performed later, and the temperature and temperature distribution of the edge portion are easily adjusted to a temperature range where solid phase pressure welding is possible in the solid phase pressure welding stage. Do it so you can maintain it. As the preheating, any of a method using a heating furnace, an induction heating method using an induction coil, and a resistance heating method using current can be suitably applied.

The preheating temperature is set to a temperature range of 800 ° C. or less for both the steel strip and the open pipe. When preheating the steel strip and the open pipe, two-stage preheating is preferably performed in which the preheating of the steel strip is performed at a temperature at which the heat load on the forming apparatus is low and suitable for forming, and the preheating of the open pipe is further increased.
If the preheating exceeds 800 ° C., a large amount of scale is formed on the entire surface of the pipe including the edges to be joined, and the seam quality and surface skin of the steel pipe are deteriorated. Therefore, the upper limit of the preheating temperature is set to 800 ° C. If the preheating temperature is less than 400 ° C., the edge temperature and the temperature distribution during pressure welding cannot be maintained in the temperature range where solid-state pressure welding is possible, since there is much heat diffusion from the edge to the mother pipe side during edge heating. If the preheating temperature exceeds 650 ° C., scale on the surface of the open tube is easily generated, and therefore, the preheating temperature is preferably set to a temperature range of 400 to 650 ° C.

Next, both edges of the open tube are edge preheated. Edge preheating is performed by an induction heating method. 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 magnetic permeability of the steel shown in FIG. 7, when the steel is heated above the Curie point, the steel undergoes magnetic transformation from a ferromagnetic material to a paramagnetic material, and the relative magnetic permeability (vs. vacuum ratio) is close to 1. Becomes On the other hand, the induced current penetration depth S
Is given by the following equation (2).

S = α {ρ / (μ _rf )} ^1/2 (2) where, S: penetration depth (m), ρ: resistivity (Ω · m),
μ _r: relative permeability, f: Frequency (kHz), α: a constant. Therefore, by heating the edge portion to a temperature higher than the Curie point, the penetration depth S is increased, and the temperature distribution in the pressure contact surface is directed to be uniform. Therefore, the edge portion is preheated to a temperature range higher than the Curie point. From the viewpoint of heating energy efficiency, the heating is preferably performed at a temperature equal to or higher than the Curie point and lower than 1300 ° C., but there is no inconvenience even if the temperature is higher than 1300 ° C. However, if the temperature rises at this stage, only the corners become higher than the melting point, and beads (surplus) are generated at the time of joining, so that high-speed pipe forming may not be possible. It is preferable to carry out in a temperature range of less than.

The edge preheating is performed in the atmosphere or in an atmosphere in which the oxygen concentration is lower than that in the atmosphere (in a shield atmosphere).
Either one may be used, but from the point of seam quality, it is preferable to be in a shield atmosphere. The edge preheating is preferably performed in an atmosphere having a dew point of −10 ° C. or less. Both edge portions of the open tube 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 the edge heating is an induction heating method using an induction coil from the viewpoint of energy efficiency. For the edge heating, it is preferable to dispose an appropriate size of the impeder in the open tube from the viewpoint of the heating efficiency, but the edge heating can be performed even when the size of the impeder is reduced or the impeder is not arranged. In this case, the tube body other than the edge portion is also easily heated.

The temperature at the end surfaces of both edges of the open tube is controlled by adjusting the output of the induction heating coil. If the temperature of the edge heating is lower than 1300 ° C., the joining of the end face of the edge portion becomes insufficient and the seam quality is deteriorated. Further, if the temperature of the end face of the edge portion exceeds the melting point of the pipe material, the molten steel forms beads (extra-bulb) inside and outside the pipe at the time of abutment joining, so that bead cutting is required. For this reason, the edge heating is performed in a temperature range of 1300 ° C. or higher and lower than the melting point where solid-state pressure welding can be performed. Preferably,
It is 1350 ° C. or higher and lower than the melting point, more preferably 1400 ° C. or higher and lower than the melting point.

The solid-phase pressure welding referred to in the present invention means a pressure welding in which bead (surplus) swelling is suppressed and bead cutting is not required. In the present invention, the edge heating temperature is preferably the temperature in the solid phase region in order to suppress the amount of bead (surplus) swelling, but there is no inconvenience even in the solid-liquid two-phase region below the melting point where some liquid phase exists. .

In order to make the temperature distribution of the edge portion at the time of induction heating uniform, in the present invention, preferably, the edge of the steel strip is refined, the edge face is flattened, and the edge face and the steel strip surface are preferably flattened. Is preferably a predetermined angle. The predetermined angle is preferably 60 to 120 degrees. The edge shaping may be performed before the coil is paid off, before the coil is paid off and formed into an open pipe by a forming roll, or after the coil is formed. The edge treatment is preferably performed by cutting with an edge mirror, polishing with a grinder, rolling with an edger roll, or the like.

The open pipe whose both edge portions are heated to the above-mentioned solid-state pressure-contactable temperature range is abutted on both edge portions by a squeeze roll and solid-phase pressed. As shown in FIG. 3 (a), the pressure welding is performed by setting a squeeze roll at a position where the squeeze roll is in contact with the outer surface of the pressure-welded joint pipe, and as shown in FIG. 3 (b),
As shown in FIG. 3 (c), a method in which a squeeze roll is installed at a position where the squeeze roll does not abut against the outer surface of the press-joined portion and the outer surface side is at a position where the squeeze roll is in contact with the press-joined portion. There is a method of installing and performing, but in any case, no inconvenience occurs.

The edge heating and the solid-state pressure welding may be performed in the air or in an atmosphere in which the oxygen concentration is lower than that in the air (in a shield atmosphere), but preferably in a shield atmosphere from the viewpoint of seam quality. Further, it is preferable that the edge heating and the solid-phase pressure welding are performed in an atmosphere having a dew point of −10 ° C. or less from the viewpoint of seam quality. The present inventors have found that after pressing, the time t _k at which the joint is maintained at 1300 ° C. or higher is determined by
It was found that the seam quality of steel pipes changed. 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.

This time t _k (sec) is 0.03 when the edge preheating, the edge heating, and the solid phase pressure welding are performed in the atmosphere.
It is preferable to set it to sec or more. On the other hand, if the edge preheating, edge heating, solid phase pressure is carried out in a low oxygen concentration atmosphere (in a sealed atmosphere) than atmospheric, t _k is preferably the time that satisfies the following equation (1) . t _k ≧ a · exp {−b · [O ₂ ] ^c } (1) where, O ₂ : oxygen concentration in the atmosphere (vol%), a,
b, c: constants, a = 0.079, b = 1.5 for low carbon steel
, C = −0.14. More preferably, a = 0.23, b
= 1.4 and c = -0.17.

[0026] The time t _k is an open pipe both edges of the heating temperature and the Curie point or more heating width during edge preheating,
In addition, by controlling the heating temperature of the end faces of both edges during edge heating and adjusting the temperature distribution in the pipe circumferential direction from the end faces of both edges to the center of the pipe during solid phase pressure welding, Adjust and control the seam cooling rate. 4 (a) and 4 (b), depending on whether or not the squeeze roll is in contact with the outer surface of the press-bonded portion, the temperature reached at the edge portion, or the degree of reduction in the pipe circumferential direction by the squeeze roll. As shown in b), the thickness of the pipe may be increased by 5% or more inside or outside the pipe or inside the pipe at the seam portion. In such a case, at an appropriate place after the welding,
It is preferable to reduce the thickness in the vicinity of the increased seam portion by rolling. Rolling in the vicinity of the increased seam portion is performed, for example, as shown in FIG.
Rolling is performed from inside and outside of a pipe by a press-seam rolling roll 10 shown in FIG. The pressure-welded seam portion rolling roll 10 includes an outer rolling roll 10a and an inner rolling roll 10b,
Reference numeral 10b is supported by a roll support rod 10c for rolling the press-seam portion.

Further, by adopting a method in which a roll or the like is brought into contact with the inner and outer surfaces of the press-joined portion of the tube, the material is constrained in the vertical direction, and the increase in wall thickness by press-contact is 5%.
It is also possible to suppress the rolling after the pressure welding. For example, the squeeze roll 6 and the roll 11a for restraining the inner surface of the press contact seam shown in FIG. The roll 11a for restricting the inner surface of the press contact seam is supported by a roll support rod 11b for restricting the inner surface of the press contact seam.

In the welded seam formed by the solid-phase welding, the presence or absence of rolling of the welded portion depends on the degree of edge droop of the steel strip, the precision of the edge refining of the steel strip, the method of pressure welding, or the degree of wall thickening by pressure welding. Irrespective of this, as shown in FIG. 6, a minute concave portion having a depth of about 0.2 mm called a weld line may be formed on the outer surface, which adversely affects the appearance and seam quality. In such a case, it is preferable to remove the weld line at an appropriate place after the welding to smooth the outer surface. The removal of the weld line is performed by performing processing such as cutting and polishing. Further, the removal of the weld line may be performed before or after the rolling in the case of rolling the press-welded thickened portion.

As described above, according to the present invention, both edges of the open pipe can be stably held in the temperature range where solid-state pressure welding is possible, and then the solid-state pressure welding is performed by the squeeze roll to obtain excellent seam quality and surface quality. A steel pipe having skin can be manufactured with high productivity.

[0030]

DESCRIPTION OF THE PREFERRED EMBODIMENTS 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 was continuously formed by the forming roll group 3 to form an open pipe 7, and the open pipe 7 was preheated to a temperature of 400 to 650 ° C. by a pipe preheating device 13 having an induction coil. Some steel pipes are continuously preheated at a temperature of 150 to 350 ° C. in a preheating furnace 2 at a temperature of 150 to 350 ° C., and then continuously formed by a forming roll group 3 to form an open pipe 7. Tube preheating device 13 with induction coil
To a temperature of 400-650 ° C. Then, both sides of the open pipe were subjected to edge preheating by the edge preheating induction heating coil 4 under the conditions shown in Table 1 and further to edge heating by the edge heating induction heating coil 5, and were installed at positions where they came into contact with the pressure welding seam. A solid pipe was welded with a squeeze roll 6 to obtain a steel pipe 8 having a pipe size of 60.5 mmφ × 3.5 mmt and a standard of STKM11A. The seam quality and surface skin of the manufactured steel pipe 8 were investigated, and the results are shown in Table 1. The seam quality was evaluated based on the flat height ratio (h / D, h: flat height mm,
D: Outer diameter of steel pipe mm). The evaluation of the surface skin of the steel pipe was performed based on the surface roughness Rmax (μm). For some steel pipes, edge preheating, edge heating, and solid-state pressure welding were performed in a shield atmosphere.

[0031]

[Table 1]

After the strip was heated to 1300 ° C., a forged pipe of 60.5 mmφ was formed by forging to obtain a conventional example (No. 13). With respect to this forged pipe, the flat height ratio and the surface roughness Rmax of the steel pipe were measured in the same manner as in the example, and are also shown in Table 1. Test N
o.1 to No.4, No.8, No.10, No.11, No.14 to No.16, the flat height ratio is 0.3 or less, and the surface roughness Rmax 10
μm or less, and in the conventional welded pipe of test No. 13, the flatness height ratio is 0.56 and the surface roughness Rmax is 37.5 μm, which is improved. When out of the scope of the present invention, test No. 5, N
o.6, No.9, the flat height ratio increases,
As in Test No. 12, the surface roughness Rmax increases. Further, as shown in Test No. 7, when the end face of the edge portion is melted, a margin is formed and it is necessary to perform bead cutting, so that the pipe forming speed is reduced to 100 m / min.

In Test No. 8, the edge portion was preheated to a temperature exceeding 1300 ° C., but the seam quality and surface skin were excellent.
There was no decrease in the tube forming speed. 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, but the productivity is remarkably improved. In Tests No. 1, No. 3 and No. 14 of the present invention example, a wall thickness of 0.5 to 1.5 mm was found on the inner surface of the pipe of the press-seam seam.
The area near the welded seam is rolled from inside and outside of the pipe with rolling rolls,
The thickness was reduced to within 2mm, and the steel pipe dimensions were within the specified range.

In Tests No. 2, No. 4, and No. 15, the squeeze roll was brought into contact with the outer surface of the tube and the rolling roll was brought into contact with the inner surface of the tube at the pressed position to restrain the material in the vertical direction. When the thickness of the welded seam was 0.1 mm or less, it was within the standard range of steel pipe dimensions, and rolling after the welding was unnecessary. In Test Nos. 14 and 15 of the present invention, edge treatment of the steel strip (specifically, cutting by milling) was performed, and the edge portion angle was made a right angle. Test with edge treatment
No. 14 and No. 15 have smaller flat height ratios than other tests No. 1 and No. 2 in which no edge treatment was performed.

In Test No. 16 of the present invention, the dew point in the atmosphere during edge heating and solid-phase pressure welding was controlled to -20.degree.
As a result, the test N in which the dew point control in the atmosphere was not performed
The flat height ratio is smaller than o.11.

[0036]

According to the present invention, both edges of an open pipe can be stably held in a temperature range in which solid pressure welding can be performed, and a steel pipe having excellent seam quality and surface skin can be manufactured with high productivity. Has the effect of

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing one example of a steel pipe manufacturing facility line suitable for carrying out the present invention.

Fig. 2 Effect of post-welding on seam quality of solid-state welded joints
00 is a graph showing the relationship between the oxygen concentration in the time t _k and the atmosphere is maintained above ° C..

FIG. 3 is a cross-sectional view showing a positional relationship between a squeeze roll, a roll for restraining the inner surface of a press-seam portion, and a press-joint portion during solid-phase press welding.

FIG. 4 is a cross-sectional view illustrating an example of a cross-sectional shape of a steel pipe after solid-phase pressure welding.

FIG. 5 is a schematic partial sectional side view of an equipment row suitable for carrying out the present invention.

FIG. 6 is a cross-sectional view showing an example of the outer shape of a press-seam portion after solid-phase press-bonding.

FIG. 7 is a characteristic diagram showing temperature dependence of relative magnetic permeability of steel.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Strip steel 2 Preheating furnace 3 Forming roll group 4 Induction heating coil for edge preheating 5 Induction heating coil for edge heating 6 Squeeze roll 7 Open pipe 8 Steel pipe 9 Pressure welding seam part 10 Rolling pressure welding seam part rolling roll 10a Pressure welding seam part outer surface rolling Roll 10b Roll for inner surface rolling of press-welded seam part 10c Roll support rod for rolling of press-welded seam part 11a Roll for restraining inner surface of press-welded seam part 11b Roll support rod for restraining inner surface of press-welded seam part 12 Weld line for outer surface of press-welded seam part 13 Tube preheating device

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Akira Ito 1-1-1, Kawasaki-cho, Handa-shi, Aichi Prefecture Inside the Chita Works of Kawasaki Steel Corporation (72) Inventor Toshio Onishi 1-1-1, Kawasaki-cho, Handa-shi, Aichi Prefecture Kawasaki (72) Inventor Yuji Hashimoto 1-1-1, Kawasakicho, Handa-shi, Aichi Prefecture Kawasaki Steel Corporation Chita Works (72) Inventor, Nobuki Tanaka 1-1-1, Kawasakicho, Handa-shi, Aichi Prefecture Kawasaki Inside the Chita Works of Iron and Steel Corporation

Claims (13)

[Claims] 1. A method for producing a steel pipe in which a steel strip is continuously formed by a forming roll to form an open pipe, and both edges of the open pipe are heated and abutted and joined by a squeeze roll. Prior to the heating, the open pipe is preheated, and subsequently, both edges of the open pipe are subjected to an edge preheating of heating to a temperature equal to or higher than the Curie point by induction heating. A method for producing a steel pipe having excellent seam quality and surface skin, characterized in that edge heating for heating to a temperature range of 1 to 3 mm is performed and pressure welding is performed with the squeeze roll. 2. A method for manufacturing a steel pipe in which a steel strip is continuously formed by a forming roll into an open pipe, and both edges of the open pipe are heated and abutted and joined by a squeeze roll. Prior to this, the steel strip and the open pipe are preheated, and subsequently, both edges of the open pipe are subjected to an edge preheating of heating to a temperature equal to or higher than the Curie point by induction heating, and then 1300 ° C. by induction heating.
Above, subjected to edge heating to heat the temperature range below the melting point,
A method for producing a steel pipe having excellent seam quality and surface skin, wherein the steel pipe is pressure-welded with the squeeze roll. 3. The method according to claim 1, wherein the edge preheating is performed at a temperature above the Curie point of 1300.
The method for producing a steel pipe according to claim 1, wherein the steel pipe is heated to a temperature lower than ℃. 4. The preheating of the steel strip or the open pipe is performed at a temperature of 800 ° C. or less.
4. The method for producing a steel pipe according to 2 or 3. 5. The method according to claim 1, wherein the edge preheating is performed in an oxygen concentration atmosphere lower than the atmosphere. 6. The method according to claim 1, wherein the edge heating and the pressure welding are performed in an oxygen concentration atmosphere lower than the atmosphere. 7. The 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. Production method. 8. After the pressure, claim joint time t _k which is held above 1300 ° C. (sec), which is a t _k satisfying the above 0.03sec or below (1) The method for producing a steel pipe excellent in seam quality and surface skin according to 1, 2, 3, 4, 5, 6, or 7. Note that t _k ≧ a · exp ｛−b · [O ₂ ] ^c ｝ (1) where, O ₂ : oxygen concentration in the atmosphere (vol%), a = 0.
079, b = 1.5, c = -0.14. 9. The seam portion tube material is restrained from the inner and outer surfaces of the tube during the press-contact to suppress the increase in the thickness of the seam portion, and the thickness of the seam portion is suppressed. Manufacturing method of steel pipe. 10. The method according to claim 1, wherein, after the pressing, the vicinity of the pressing seam portion is rolled.
10. The method for producing a steel pipe according to 7, 8 or 9. 11. The method according to claim 1, wherein after the pressing, a minute concave portion on the outer surface of the pressing seam portion is removed to smooth the outer surface. 11. The method for producing a steel pipe according to 9 or 10. 12. The steel strip flattens an edge end face,
An edge treatment for making an angle between the edge portion end face and the surface of the strip steel a predetermined angle is performed. 9, 10 or
12. The method for producing a steel pipe according to 11. 13. The edge treatment of the end face of the steel strip before or after forming by a forming roll.
12. The method for producing a steel pipe according to 12.