JPH1058161A - Welded steel tube and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel tube having a superior seam quality and surface and having an excellent grooving resistance, as well as the manufacture of such tube using an induction heating method capable of high productivity. SOLUTION: A steel strip 1 is preheated, continuously formed into an open tube 7 by a group of forming rolls 3; with both edges of the open tube 7 applied by edge preheating in the temperature range of not less than the Curie point or desirably not more than 1,300 deg.C through induction heating, and by edge heating in the range of not less than 1,300 deg.C and not more than the melting point also through induction heating; and then, abutted, pressure-welded by squeeze rolls, and formed into a steel tube 8. In this case, it is desirable that, after the pressure welding, the joined part is held at 1,300 deg.C or above for 0.03sec or longer or for duration corresponding to oxygen concentration in the atmosphere. It is also desirable that the preheating of the steel strip 1 is carried out at 800 deg.C or below and that the edge preheating, heating and pressure welding are performed in the atmosphere with the oxygen concentration lower than the air. As a result, an acceleration factor αbecomes 1.4 or less in a corrosion test as the index of grooving resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a welded steel pipe and a method for manufacturing the same, 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. Also, at the joint of the ERW steel pipe, the strip steel edge is pushed out to the inner and outer surfaces of the pipe by the pressure at the time of upsetting, and is locally deformed, and the molten steel is discharged as described above to form a bead. At the joint, the non-metallic inclusions are deformed in the radial direction by such local deformation, and are likely to crack and become defects when the pipe is deformed.

[0007] The formed beads are cut,
As a result of this bead cutting, non-metallic inclusions (mainly MnS-based) inside the steel strip are often exposed on the surface of the joint at the joint as shown in FIG. Furthermore, in the joint, a S-enriched portion is likely to be formed around the nonmetallic inclusions (MnS-based inclusions) exposed on the surface due to the rapid thermal quenching thermal cycle,
In a corrosive environment, a local battery is formed and becomes a starting point of corrosion. The starting point of this corrosion increases in depth and depth and grows into a corrosion hole as shown in FIG. 9B.

Further, MnS-based inclusions accumulate in the joint, and the macroscopically, the joint becomes an anode and the base material becomes a cathode. Are acceleratedly corroded and continuously pitted. As described above, the ERW steel pipe has problems such as a high incidence of defects at the joint and low pitting corrosion resistance.

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. This method
After heating the continuously supplied steel strip to about 1300 ° C in a heating furnace, it is formed into a tube using a forming roll to form an open tube, and then high-pressure air is blown to both edges of the open tube to scale off the end face. After that, oxygen is blown to the end face by a welding horn, the end face is heated to about 1400 ° C. by the heat of oxidation, and then the both end end faces are abutted by a forging roll to solid-phase join to produce a steel pipe. (Eg, Third Edition Iron and Steel Handbook, Vol. III, (2), pp. 1056-1092).

[0010] However, in this method for producing a forged steel pipe, the scale-off of the end face is not perfect, so that the scale bites into the forged joint butting 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.

[0011] Since the steel strip is heated to a high temperature, scale is formed on the surface of the pipe, 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.

[0012]

DISCLOSURE OF THE INVENTION The present invention advantageously solves the above-mentioned problems, and provides a welded steel pipe having excellent seam quality and surface skin and having excellent pitting resistance and a welded steel pipe having such characteristics. An object of the present invention is to propose a method of manufacturing a steel pipe by an induction heating method, which can be manufactured with productivity.

[0013]

SUMMARY OF THE INVENTION The present invention relates to a steel pipe in which a strip is continuously formed and abutted and joined, wherein the surface roughness of the steel pipe is 20 μm or less in Rmax, and the cross-section or the inner or outer surface of the joined portion is formed. Excellent in seam quality and surface skin, characterized in that the ratio of the area ratio of non-metallic inclusions to the area ratio of non-metallic inclusions in the base material is 1.5 or less.
It is a welded steel pipe excellent in grooving corrosion resistance of which is 1.4 or less.

Further, the present invention provides a method for forming a steel strip by using a forming roll.
It is continuously formed into an open tube, and both ends of the open tube are formed.
Of the steel pipe to be heated
In the manufacturing method, the steel strip is preheated to form an open pipe.
Then, both sides of the open tube are cured by induction heating.
Heating to a temperature range of the Lie point or higher and preferably lower than 1300 ° C.
1300 ° C after induction preheating and induction heating
Above, subjected to edge heating to heat the temperature range below the melting point,
A seam product characterized by being pressed by the squeeze roll.
A method for producing a steel pipe having excellent quality and surface skin,
Preferably, the steel is preheated at a temperature of 800 ° C or less. Ma
The edge preheating, the edge heating and the pressure welding
Is in an atmosphere with an oxygen concentration lower than the atmosphere or has a dew point of-
It is preferably performed in an atmosphere of 10 ° C. or lower. After the crimping,
Time t at which the joint is maintained at 1300 ° C or higher _k(Sec)
0.03sec or more or the following formula (1) t_k≧ a · exp ｛−b · [O_Two]^c｝ (1) (Here, O_Two: Oxygen concentration in the atmosphere (vol%), a =
0.079, b = 1.5, c = -0.14).
is there.

Further, in the present invention, at the time of the pressing, the seam portion tube 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.

Preferably, the strip is 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.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The welded steel pipe of the present invention is a steel pipe which is formed by continuously forming a strip, butted and solid-phase pressed. The surface roughness of the steel pipe should be 20 μm or less in Rmax. In the present invention, to form after heating the steel strip in the warm zone, to solid-phase pressure welding,
The surface roughness of the steel pipe is approximately the same as the surface roughness of the steel strip.

The ratio of the area ratio of the non-metallic inclusions to the cross-section or the inner and outer surfaces of the joint portion of the steel pipe and the area ratio of the non-metallic inclusions in the base metal part is 1.5 or less. If the ratio of the area ratio of inclusions to the base material in the cross section of the joint exceeds 1.5, the incidence of defects at the joint increases. If the ratio between the area ratio of inclusions on the inner and outer surfaces of the pipe and the area ratio of inclusions in the base material exceeds 1.5, the starting point of corrosion increases, the joints are corroded in a groove shape, and the pitting corrosion resistance is deteriorated.

The pitting corrosion resistance is evaluated by an acceleration factor obtained by a corrosion test. The corrosion test was performed for one year using a rotating immersion type corrosion tester with a peripheral speed of 25 m / min in artificial seawater (ASTM D-1141-52) at a temperature of 40 ° C and air atmosphere (dissolved oxygen concentration: about 5 ppm). Do. Then, the acceleration coefficient α is determined from the change in the cross-sectional shape before and after the corrosion. As shown in FIG. 8, α is the following equation based on the thickness h ₂ before the corrosion of the grooved portion, the thickness h _2a after the corrosion, the thickness h ₁ before the corrosion other than the grooved portion, and the thickness h _1a after the corrosion. α = (h ₂ −h _2a ) / (h ₁ −h _1a ).

If the acceleration coefficient α in the corrosion test is 1.4 or less, the pitting corrosion resistance becomes good. When the ratio of the area ratio of the nonmetallic inclusions on the inner and outer surfaces of the pipe to the area ratio of the nonmetallic inclusions in the base metal part is 1.5 or less, α becomes 1.4 or less. Next, a method for manufacturing the above-described welded steel pipe will be described.
In the present invention, the strip is preheated prior to forming the strip.

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 can be easily adjusted to the 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 of the steel strip is performed in a temperature range of 800 ° C. or less. Preheating at a temperature higher than 800 ° C causes a large amount of scale to be generated on the surface of the steel strip, and deteriorates both the seam quality and the surface skin of the steel pipe. 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. However, if the preheating temperature exceeds 650 ° C, scale on the surface of the steel strip is likely to be formed, so that the preheating temperature is 400 to 6 ° C.
Preferably, the temperature range is 50 ° C.

The preheated steel 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. Then, preheat both edges of the open tube. Edge preheating is performed by an induction heating method. This edge preheating reduces the temperature of the edge,
The temperature should be higher than the Curie point and 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 penetration depth S of the induced current 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. However, at this stage, if the temperature is raised to a temperature range of 1300 ° C or more at a stretch, only the corners become the melting point or more, and beads (surplus) may occur at the time of joining. It is preferred to preheat the edge to the area.

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 of the end face of each edge 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 term "solid-phase pressure welding" as used in the present invention means a pressure welding in which bead (surplus) is suppressed and no bead cutting is 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) rising, but may be the temperature in the solid-liquid two-phase region slightly lower than the melting point where a liquid phase exists.

In order to make the temperature distribution of the edge portion uniform during induction heating, in the present invention, preferably, the edge of the strip is refined, the end face of the edge is flattened, and the edge face of the strip and the surface of the strip are preferably removed. 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 edges are heated to the above-mentioned solid-state pressure-contactable temperature range is abutted on both edges 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 atmosphere or in an atmosphere having a lower oxygen concentration than the atmosphere (in a shield atmosphere), but preferably in a shield atmosphere from the viewpoint of seam quality. Further, the edge heating and the solid-state 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 after pressed, by the time t _k which joint is held above 1300 ° C., the seam quality of a steel pipe was found to vary. And t _k on the seam quality (flat height ratio h / D), the relationship between the oxygen concentration 2
Shown in 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-state 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.

[0033] 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 the inside and outside of the pipe by a press-seam rolling roll 10 shown in FIG. The pressure-welded seam portion rolling roll 10 includes an outer surface rolling roll 10a and an inner surface rolling roll 10b, and 10b is supported by a pressure-welded seam portion rolling roll support rod 10c.

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 pipe in the press-welding method, the material is constrained in the vertical direction, and the increase in thickness due to the press-welding is 5%.
It is also possible to suppress the rolling after the pressure welding. For example, the material is restricted from inside and outside the pipe by the squeeze roll 6 and the roll 11a for restricting the inner surface of the press-contact portion shown in FIG. The roll 11a for restricting the inner surface of the press contact portion is supported by a roll support rod 11b for restricting the inner surface of the press contact portion.

In the welded seam formed by solid-phase welding, the presence or absence of rolling of the welded part depends on the degree of edge droop of the steel strip, the precision of edge adjustment 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 may be performed by performing processing such as cutting or polishing, or may be performed by irradiating an energy beam to melt the weld line. As the energy beam, a laser beam, an electron beam, a plasma beam, or the like is preferable. FIG. 10 schematically shows one embodiment of a weld line removing apparatus for irradiating a weld line with a laser beam after pressure welding. 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 in which solid pressure welding can be performed, 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 and excellent in pitting resistance can be manufactured with high productivity.

[0038]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An equipment row suitable for carrying out the present invention shown in FIG. 1 was used. After a strip steel 1 having a thickness of 3.5 mm was continuously preheated at a temperature of 400 to 650 ° C. in a preheating furnace 2, it was continuously formed by a forming roll group 3 to obtain an open pipe 7. A squeeze roll installed at a position where the both ends of the open pipe are preheated by the edge preheating induction heating coil 4 under the conditions shown in Table 1 and further subjected to edge heating by the edge heating induction heating coil 5, and are brought into contact with the pressure welding seam portion. Solid-state pressure welding was performed at 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, surface skin, non-metallic inclusion area ratio and pitting corrosion resistance of the joint and base metal of the manufactured steel pipe 8 were investigated.
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 of steel pipe mm).

Further, the evaluation of the surface skin of the steel pipe was performed by using the surface roughness R
max (μm). Nonmetallic inclusion area ratio is
The cross section of the joint and the inner and outer surfaces of the tube are determined by optical microscope observation in accordance with JIS G 0555, and are shown by the ratio of the area ratio of nonmetallic inclusions at the joint to the area ratio of nonmetallic inclusions at the base material. For the pitting corrosion resistance, an acceleration coefficient α was obtained by a corrosion test under the above test conditions. For some steel pipes, edge preheating, edge heating, and solid-state pressure welding were performed in a shield atmosphere.

[0040]

[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. 11). The characteristics of the steel pipe were investigated in the same manner as in the examples, and are shown in Table 1. In the present invention examples of Test No. 1, No. 2, No. 8, and No. 9, the flat height ratio is 0.3 or less, and the surface roughness Rmax is 10 μm or less. The flattened height ratio is 0.56 and the surface roughness Rmax is 37.5 μm, which is improved. In addition, in the example of the present invention, the area ratio of inclusions at the joining portion is low, the acceleration coefficient α of grooving is small, and the grooving resistance is excellent. On the other hand, if the ratio is out of the range of the present invention, the flat height ratio becomes large as in Tests No. 3, No. 4 and No. 7, and the surface roughness Rmax as shown in Test No. 10. Becomes larger. In addition, test N
As shown in o.5, when the end face of the edge part melts, a margin is formed and it is necessary to bead cutting, so the pipe forming speed is 100 m
/ min.

In Test No. 6, although the seam quality and surface skin were excellent, the edge portion was preheated to a temperature exceeding 1300 ° C., so that only the corner portion of the edge end surface became higher than the melting point and a margin was formed. Because of the need for bead cutting,
It dropped to 100m / min. Along with this, the area ratio of inclusions at the joint is large, and the acceleration coefficient α of pitting corrosion is increased, thereby deteriorating pitting corrosion resistance.

The productivity of the example of the present invention is as high as 30 tons / hr, and the productivity of a conventional ERW pipe for bead cutting is 15 tons / hr, whereas the productivity is remarkably improved. In Tests No. 1, No. 8, and No. 12 of the present invention example, the inner wall of the welded seam was increased by 0.5 to 1.5 mm on the inner surface of the tube, but the area near the welded seam was rolled from inside and outside the tube with rolling rolls. However, the wall thickness was reduced to within 0.2mm, and the steel pipe dimensions were within the specified range.

In Tests No. 2, No. 9 and No. 13, 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. 12 to 13 of the present invention, edge treatment of the strip (specifically, cutting by milling) was performed, and the edge angle was set to a right angle. Test No.12 with edge treatment
In No. 13 to 13, other tests No. 1 to 2 where edge processing was not performed
The flat height ratio is smaller than that of.

In Test No. 14 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.9. In Test No. 15 of the example of the present invention, the formed weld line was removed by operating the weld line removing device using the laser beam in FIG. The laser beam used was a 10 kW CO ₂ laser. By removing the weld line, the flat height was comparable to that without the weld line.

[0046]

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.

FIG. 8 is a conceptual diagram illustrating a change in the cross-sectional shape of a test piece due to a corrosion test.

FIG. 9 is a conceptual diagram showing a state of occurrence of a corrosion hole near a joint of an electric resistance welded tube.

FIG. 10 is a conceptual diagram illustrating an example of a weld line removal device suitable for weld line removal.

[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 Roll for pressure welding seam part rolling 10a For external rolling of pressure welding seam part Roll 10b Roll for inner surface rolling of press-welded seam portion 10c Roll support rod for rolling of seam for press-welded portion 11a Roll for restraining inner surface of press-welded seam portion 11b Roll support bar for inner surface of press-welded seam portion 12 Weld line for outer surface of press-welded seam portion 13 Laser oscillator 13a Mirror 13b Lens

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification number Agency reference number FI Technical display location // B23K 13/00 B23K 13/00 A (72) Inventor Akira Yoto 1 Kawasaki-cho, Handa-shi, Aichi 1-chome, Kawasaki Steel Corporation Chita Works (72) Inventor Toshio Onishi 1-1-1, Kawasaki-cho, Handa City, Aichi Prefecture Kawasaki Steel Corporation Chita Works (72) Inventor Yuji Hashimoto 1, Kawasaki-cho, Handa City, Aichi Prefecture No. 1 at Kawasaki Steel Corporation Chita Works (72) Inventor Nobuki Tanaka 1-1-1 Kawasakicho, Handa City, Aichi Prefecture Kawasaki Steel Corporation Chita Works (72) Inventor Yoshinori Sugie 1 Kawasakicho, Handa City, Aichi Prefecture 1-chome, Kawasaki Steel Corporation Chita Works (72) Koji Kanno 1-1-1, Kawasaki-cho, Handa-shi, Aichi Prefecture Chita Corporation Kawasaki Steel Corporation Concrete plant

Claims (14)

[Claims] Claims: 1. A steel pipe formed by continuously forming a strip of steel and abutting and joining, wherein the surface roughness of the steel pipe is 20 μm or less in Rmax.
A welded steel pipe having excellent seam quality and surface texture, wherein the ratio of the area ratio of non-metallic inclusions to the cross-section or the inner and outer surfaces of the joint portion and the area ratio of non-metallic inclusions in the base metal part are 1.5 or less. 2. A steel pipe formed by continuously forming and joining a steel strip, wherein the steel pipe has a surface roughness Rmax of 20 μm or less.
The ratio of the area ratio of non-metallic inclusions on the inner and outer surfaces of the joint to the area ratio of non-metallic inclusions on the base metal is 1.5 or less.Acceleration coefficient α by corrosion test is 1.4 or less. Welded steel pipe. 3. A method of 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. After heating the edges of the open tube to the temperature range above the Curie point by induction heating, heat both ends to 1300 ° C or higher and lower than the melting point by induction heating. A method for producing a steel pipe having excellent seam quality and surface skin, wherein edge heating is performed and pressure welding is performed with the squeeze roll. 4. The method according to claim 3, wherein the temperature range of the edge preheating is equal to or higher than the Curie point and lower than 1300 ° C. 5. The method for producing a steel pipe according to claim 3, wherein the preheating of the steel strip is performed at a temperature of 800 ° C. or less. 6. The method according to claim 3, wherein the edge preheating is performed in an oxygen concentration atmosphere lower than the atmosphere. 7. The method of manufacturing a steel pipe according to claim 3, wherein the edge heating and the pressure welding are performed in an oxygen concentration atmosphere lower than the atmosphere. 8. The method according to claim 3, 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. 9. 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) 9. The method for producing a steel pipe according to any one of 3 to 8. 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. 10. The method of manufacturing a steel pipe according to claim 3, wherein at the time of pressing, the seam portion pipe is restrained from the inner and outer surfaces of the tube to suppress the increase in the thickness of the seam portion. 11. The method for producing a steel pipe according to claim 3, wherein, after the pressing, the vicinity of the pressed seam portion is rolled. 12. The method of manufacturing a steel pipe according to claim 3, wherein after the pressure welding, a minute concave portion on the outer surface of the pressure welding seam is removed to smooth the outer surface. 13. The steel strip flattens an edge end face,
13. The method for producing a steel pipe according to claim 3, wherein an edge treatment is performed so that an angle between the end face of the edge portion and the surface of the steel strip is a predetermined angle. 14. The edge treatment of the end face of the steel strip is performed before or after forming by a forming roll.
13. The method for producing a steel pipe according to item 13.