JP2009173985A - Heat-treatment method of electric resistance welded steel pipe - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To overcome the difficulty of efficiently heating an electric resistance welded steel pipe across its entire wall thickness to a target temperature in a conventional heat-treatment (seam annealing) method of a welded part of the pipe. <P>SOLUTION: A plurality of stands 15, each of which having induction coils 11a and 13a disposed at both inside and outside of the pipe 5, is disposed in a direction in which the pipe is passed, and the welded part is subjected to induction heating from both inside and outside of the pipe. At a stand between a first stand and a stand other than the last stand, the welded part is heated until the inside and outside temperatures of the welded part at the outlet side of the stand exceed an Ac<SB>3</SB>transformation temperature, and at the following stands, the welded part is heated so that the inside and outside temperatures of the welded part are kept at a prescribed temperature higher than the Curie point and that the center of thickness of the welded part reaches a target temperature. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an electric resistance welded steel pipe, and in particular, for the purpose of improving the workability and toughness of the welded portion, a heat treatment method for the welded portion of the electric resistance welded pipe with high accuracy and high efficiency. About.

  Usually, as shown in FIG. 1, the electric resistance welded steel pipe 5 is formed by forming a coiled steel strip 1 into a tubular shape by a roll forming machine 2 via an uncoiler 6 and a leveler 7, and then abutting the plate end faces to form a high frequency. It is manufactured as a steel pipe that is electro-welded with a welding machine 3, is cut with a bead part cutting machine 8, is adjusted to a predetermined diameter with a sizer 9, and is cut into a predetermined length with a pipe cutting machine 10. . In this electric welding, a high-frequency current is passed to the butt portion by a high-frequency welding machine (induction heating device) 3, Joule heat is concentrated to melt the butt portion, and the squeeze roll 4 is crimped to form a welded portion. A sewn steel pipe 5 is used. Since the formed weld is rapidly heated and cooled rapidly, it has a composition, structure, and strength different from those of the base material (steel strip), and other parts (base material part) have workability, toughness, and corrosion resistance. ) Is in a reduced state compared to). Therefore, in order to make the workability, toughness, and corrosion resistance of the welded portion equal to or higher than that of the base material portion, it is common to perform a heat treatment called seam annealing on the welded portion.

A device called a seam annealer is used for the seam annealing. The seam annealer is usually constructed by arranging a plurality of high-frequency induction heating stands with induction coils on the outer surface side of the pipe in the direction of the pipe, generating magnetic flux with the induction coil, and thereby in the welded part and its surroundings. An induced current is generated and heated by the resistance heat generation.
However, since the conventional seam annealer heats only from the outer surface side of the tube, it is difficult to uniformly heat the entire wall thickness, which inevitably causes a temperature difference between the inner and outer surfaces of the tube. In addition, this temperature difference increases as the thickness of the tube increases, and when the wall becomes thicker, one of the inner and outer surfaces of the tube does not reach the target temperature and often becomes a problem.

  For example, when manufacturing a thick and high toughness line pipe, increasing the input power to the induction coil and increasing the induced current in order to sufficiently heat the inside of the pipe, the temperature of the outer surface of the pipe becomes too high and the crystal In some cases, the grains become coarse and the toughness decreases. In addition, the method of reducing the temperature difference between the inner and outer surfaces of the pipe by increasing the pipe speed to an extremely low speed and ensuring sufficient heat transfer time after heating by the seam annealer increases the number of high frequency induction heating stands. It is necessary to use equipment, and the equipment cost increases, the amount of input power increases, and the running cost increases. Further, when the pipe making speed is extremely low, defects are likely to increase in the ERW welded part, and there is a problem that the characteristics of the welded part are deteriorated. In addition, when the production line is stopped due to troubles during pipe making, the equipment is long, so there is a problem that non-uniformity of the ERW welds increases and the yield decreases.

For such a problem, for example, Patent Document 1 discloses that the outer surface temperature of the welded portion is (Ac ₃ points + 100 ° C.) to (Ac ₃ points + 300 ° C.) with a high-frequency induction heating device in a thick-walled electric-welded steel pipe welded portion. So that the temperature range of Ac ₁ point to (Ac ₃ point + 100 ° C.) is obtained when the first normalizing is performed and the outer surface temperature of the welded portion is subsequently lowered to Ac ₁ point or less. A method for manufacturing a thick-walled electric-welded steel pipe is described in which a heat treatment consisting of second normalizing and annealing is performed. According to this technique, a thick ERW steel pipe having extremely excellent weld toughness can be manufactured without adding a complicated and long processing time.

Patent Document 2 discloses the first heating in which the temperature of the inner surface of the welded portion is (Ac ₃ points + 50 ° C.) or higher with a high-frequency induction heating device on the thick-walled ERW steel pipe welded portion, After the first heating, the outer surface temperature is cooled to below the bainite transformation end temperature of the material to be heated by water cooling or air cooling, and then the Ac ₃ transformation region is a temperature that can cover the region where the bainite structure is generated by the first heating / cooling. And the heat processing method of a thick-walled electric-welded steel pipe which performs the heat processing which consists of the 2nd heating heated to the temperature below which a bainite structure generate | occur | produces is described. According to this technique, a thick ERW steel pipe having excellent weld toughness can be manufactured without adding a complicated and long processing time.
JP 59-129729 A Japanese Patent Laid-Open No. 06-220547

In the technique described in Patent Document 1, cooling is performed to a predetermined temperature or lower after the first heating. However, this temperature is high and untransformed austenite remains, and the temperature is low during cooling after the second heating. There is a problem that the desired toughness improvement cannot be obtained due to transformation into a transformation product.
Further, in the techniques described in Patent Documents 1 and 2, since the cooling is performed after the first heating, it is necessary to lengthen the heat treatment equipment, resulting in an increase in equipment costs, inefficient production, and production costs. There is a problem of increasing the (running cost). In addition, since the cooling rate slightly changes depending on the water temperature and air temperature, it is difficult to accurately achieve the desired temperature at the time of cooling after the first heating, and it is possible to stably improve sufficient toughness. There was a problem that it was difficult to obtain.

In order to solve the above-mentioned problems (problems), the present inventors have intensively studied a heating pattern capable of efficiently performing a heat treatment capable of improving the toughness of the welded portion of the ERW steel pipe.
First, as a result of detailed examination of the heating method until the inner surface temperature and outer surface temperature of the welded part of the ERW steel pipe reach the Ac ₃ transformation point, sufficient rapid heating is possible even if the equipment length is short. In order to prevent overheating of the inner surface and prevent insufficient heating on the inner surface side, it is effective to arrange induction coils on both the inner surface side and the outer surface side and to perform induction heating not only from the outer surface side but also from the inner surface side. I figured out.

The induction heating only from conventional outer surface, in particular as the wall thickness increases, the inner surface side to reach the outer surface is the target temperature (Ac ₃ transformation point + alpha) is in many cases not be the target temperature. Therefore, conventionally, after reaching the temperature above the Ac ₃ transformation point only on the outer surface side, it is gradually cooled in the middle of the seam heat treatment, the temperature is made uniform using heat conduction, and then heated again. A method of finely adjusting the temperature on the inner surface side toward the target and a method of heating gradually while utilizing heat conduction by making the equipment length long have been inefficient production.

On the other hand, by induction heating not only from the outer surface side but also from the inner surface side, even if the wall thickness is thick, it is only necessary to heat about 1/2 of the thickness, and the heating efficiency can be remarkably improved.
Subsequently, the present inventors paid attention to the temperature at the central portion of the welded portion. If the temperature on both the inside and outside surfaces is rapidly increased, it is common practice that the temperature at the central part of the thickness is difficult to follow.

Therefore, the present inventors examined a method for quickly reaching the target temperature of the thickness center portion after the temperatures of both the inner and outer surfaces exceed the Ac ₃ transformation point. The cross section of the welded portion inner and outer surfaces temperatures in this study has reached a temperature above the Ac ₃ transformation point, electromagnetic field analysis, observing the magnetic flux distribution and the temperature distribution by heat transfer analysis and temperature measured, mow flux given to the induction heating Found that it bypasses the region of temperature above the Curie point. In other words, in order to heat the tube from both the inner and outer sides, the magnetic flux does not concentrate on both the inner and outer sides of the weld where the Curie point is exceeded. The part is heated. As a result, the thickness center part of the welded portion is efficiently heated and the heating on both the inner and outer sides is suppressed, so the temperature difference between the surface of the tube and the inside is reduced, and uniform heating becomes possible. That is why. In order to obtain this effect, induction heating may be performed so that the temperature on both the inner and outer surfaces exceeds the Curie point and is maintained at a certain appropriate temperature.

The present invention has been made based on the above findings and has been further studied, and is as follows.
(Claim 1)
Heat treatment of an ERW steel pipe welded portion in which the outer surface side of the welded portion is induction-heated by using an induction heating device in which a plurality of induction coils are arranged on the outer surface side of the welded portion of the ERW steel pipe along the direction of passage of the ERW steel pipe In the method, a plurality of induction coils for induction heating the inner surface side of the welded portion are further provided in the induction heating device, and the inner surface side of the welded portion is also induction heated using them. In the stand from the stand to any one stand other than the final stand, induction heating is performed so that the outer surface temperature and the inner surface temperature of the welded portion on the exit side of the one stand exceed the Ac ₃ transformation point as soon as possible, and the subsequent In this stand, the outer surface temperature and the inner surface temperature of the welded portion are maintained at a predetermined temperature exceeding the Curie point, and induction heating is performed so that the thickness center portion of the welded portion reaches the target temperature. Heat treatment method for welded part of ERW steel pipe.
(Claim 2)
High efficiency heat treatment method of the electric resistance welded steel pipe welds according to claim 1, characterized in that the target temperature and the Ac ₃ transformation point or more of the temperature.

  According to the present invention, improvement in toughness of an ERW steel pipe welded portion can be achieved inexpensively, efficiently and stably, and there is a remarkable industrial effect. In addition, according to the present invention, it is possible to shorten the equipment length of the induction heating device (seam annealer), perform heat treatment efficiently, reduce input power, and reduce manufacturing cost (running cost).

  An example of an induction heating apparatus that can be preferably used in the present invention is shown in FIG. As shown in FIG. 2A, the induction heating device 11 is preferably disposed on the exit side of the squeeze roll 4 from the viewpoint of improving productivity. As shown in FIG. 2 (b), the induction heating device 11 passes through a stand 15 supporting an induction coil 11a for induction heating the outer surface side of the welded portion of an electric resistance steel pipe (abbreviated pipe) 5 along the pipe direction. Assume that multiple stands are arranged. In the induction heating device 11, a plurality of induction coils 13a for induction heating the inner surface side of the welded portion are arranged along the tube direction.

As an installation method of the induction coil 13a on the inner surface side, for example, an impeder (not shown) installed on the inner surface side of the pipe 5 at the position of the high frequency welding machine 3 in FIG. 1 is extended to the position of the induction heating device 11 in FIG. And a method of attaching the induction coil 13a to the extended portion.
The induction coils 11a and 13a are configured such that an induction current is generated from the outer surface side and the inner surface side of the welded portion to the welded portion and the periphery thereof to locally heat the welded portion.

It is preferable to arrange thermometers 12 and 14 that can measure the outer surface temperature and the inner surface temperature of the welded portion on the exit side of each stand 15, respectively. Each stand 15 is preferably provided with a frequency variable device (not shown) so that it can adapt to a desired frequency as needed.
In FIG. 2B, as the best mode, the same number of induction coils 11a for inductively heating the outer surface side of the welded portion and induction coils 13a for heating the inner surface side of the welded portion are placed at the same pipe-direction position. Although the arrangement example is shown, the number of both the induction coil 11a and the induction coil 13a does not necessarily have to be the same, and the positions of both the induction coil 11a and the induction coil 13a are not necessarily the same. May be.

In the present invention, when heat-treating the welded portion of the pipe 5 using the induction heating device 11 described above, a stand from the first stand to any one stand other than the final stand (this is referred to as a pre-stage stand; a pre-stage stand. May be only the first stand) so that the outer surface temperature and the inner surface temperature of the welded portion on the exit side of the one stand (the front side final stand) exceed the Ac ₃ transformation point as quickly as possible. Induction heating. The frequency and input electric energy in the induction coils 11a and 13a upstream (front side) from the front side final stand exit side position are not particularly limited, and the outer surface temperature and the inner surface temperature are efficiently the target temperature (for example, directly above the Ac ₃ transformation point). The temperature may be adjusted so that the temperature can be reached. Here, the Ac ₃ transformation point was predicted from the steel composition by a prediction formula (for example, the Japan Institute of Metals: revised 3rd edition metal data book, p.152, published on Mar. 25, 1993). Use the value or the value measured experimentally. Strictly speaking, the composition of the pipe weld should be used for this prediction or measurement, but the base material composition may be used as long as the difference between the pipe base material composition and the weld composition is negligible.

The outer surface temperature and the inner surface temperature of the welded portion on the stand 15 exit side may be measured by thermometers 12 and 14 arranged on the stand exit side, respectively. High-precision rapid heating is possible by adjusting the frequency and input power in the induction coil on the front stage so that the difference between the measured temperature of the thermometers 12 and 14 and the target temperature is eliminated.
Next, in the induction heating using the outer surface side and inner surface side induction coils (generally called the rear stage side induction coils) in the subsequent stand (the downstream side stand from the front side final stand exit side position), the outer surface temperature of the welded portion Heating is performed so that the central portion of the thickness of the weld reaches the target temperature while both inner surface temperatures are maintained at a predetermined temperature exceeding the Curie point.

  In order to perform such heating, an electromagnetic field analysis and a process for induction heating of a welded portion in which the outer surface side and the inner surface side are heated to the Curie point or more from the outer surface side and the inner surface side with different frequency and input power are performed. Calculate the temperature distribution and magnetic flux density distribution in the weld cross-section by heat transfer analysis (for example, literature: iron and steel, vol93, No.5 (2007), P373-378). Under a small input power and as quickly as possible, the outer surface temperature and the inner surface temperature at which the magnetic flux density is most effectively concentrated on the central thickness portion are obtained and set as the predetermined temperature, and the frequency corresponding to such a state. And the combination of input power may be set in the induction coil on the rear stage side.

The target temperature of the center thickness portion of the weld, be the external surface temperature and internal surface temperature as well Ac ₃ transformation point or above the temperature, preferably made further improve the weld toughness.

A 0.05% C-0.2% Si-1.4% Mn hot-rolled steel strip (Ac ₃ transformation point: 860 ° C.) in mass%, as shown in FIG. 1, uncoiler 6 and leveler 7 Then, it was processed by a pipe making machine having a roll forming machine 2, a high-frequency welding machine 3, a squeeze roll 4, a sizer 9, and the like to obtain an electric resistance steel pipe (outer diameter 600φmm × thickness 19.1 mm). Next, heat treatment was performed on the welded portion using an induction heating apparatus 11 as shown in FIG. The configuration of the induction heating device 11 is a configuration in which seven induction coils 11a on the outer surface side are arranged, and an induction coil 13a for induction heating the same side is arranged on the inner surface side at the same tube passage position as each induction coil 11a. . Thermometers 12 and 14 were placed on the outer surface side and inner surface side of the welded portion on the outlet side of each stand, and the outer surface temperature and inner surface temperature of the welded portion on the outlet side of each stand were measured using these.
(Conventional example)
In the conventional example, only the induction coil 11a on the outer surface side is used, and the welded portion is induction-heated only from the outer surface side. The power applied to each stand (induction coil 11a on the outer surface side) is set to the same as before (the efficiency of each induction coil 11a on the outer surface side remains unchanged from the operation experience so that the outer surface temperature of the weld becomes the target temperature (880 ° C). The heat treatment was carried out by adjusting so as to be substantially uniform. As a result, the outer surface temperature of the welded portion (measured value of the thermometer 12 on the stand outlet side) reaches the target temperature on the fifth stand outlet side, and the inner temperature of the welded portion (the stand outlet temperature on the seventh stand outlet side). Measured value on the side thermometer 14) has reached the target temperature.
(Example of the present invention)
In the example of the present invention, the induction coils 11a and 13a on the outer surface side and the inner surface side are used, and the welded portion is induction-heated not only from the outer surface side but also from the inner surface side. The input power to the first and second stands (these correspond to the previous-stage stand) is set to be larger than that to the other stands, and the outer surface temperature and inner surface temperature of the welded portion on the outlet side of the second stand Both were allowed to reach the target temperature (880 ° C.) as quickly as possible. In the subsequent stand after the third stand, the outer surface and inner surface obtained by heat transfer analysis and electromagnetic field analysis are such that the magnetic flux is most concentrated in the central portion of the wall thickness as quickly as possible with as little input power as possible. The frequency and input power to the induction coils 11a and 13a were set so as to realize a holding state at a predetermined temperature (both inner and outer surfaces are temperatures exceeding the Curie point). Moreover, since it was predicted from the analysis result that the thickness center part reaches the target temperature on the exit side of the fourth stand, power is not supplied to the induction coils on the outer surface side and the inner surface side in the fifth to seventh stands. did. As a result of the heat treatment of the welded portion set as described above, the outer surface temperature and the inner surface temperature (measured values of the thermometers 12 and 14 on the exit side of these stands) are substantially the predetermined temperature. Kept.

A sample was cut out from the welded portion and its vicinity after the heat treatment in each of the above examples, and the cross-sectional structure was observed. As a result, in the conventional example, the inner surface side of the welded portion had a relatively fine ferrite-pearlite structure, but a bainite structure that deteriorates toughness was partially developed on the outer surface side. That is, in the conventional example with only the outer surface heating, seven stands are required to reach the target temperature exceeding the Ac ₃ transformation point for the entire thickness of the welded portion, and the outer surface side that reaches the target temperature faster than the inner surface side The inner surface was overheated before reaching the target temperature, and the structure deteriorated toughness.

On the other hand, in the example of the present invention, a relatively fine ferrite-pearlite structure was obtained over the entire thickness of the weld. That is, in the example of the present invention in which heating is performed from both the outer and inner surfaces, four stands are sufficient to reach the target temperature exceeding the Ac ₃ transformation point, and both the outer and inner surfaces are excessive. The structure was not heated to deteriorate the toughness.

Moreover, the Charpy impact test was done about the weld part after the heat processing of said each example, and toughness was evaluated. The test method was as follows.
(Charpy impact test)
An impact test piece (V-notch test piece) was collected from the welded portion of the obtained ERW steel pipe in accordance with the provisions of JIS Z 2242. The test specimens were collected at 10 locations in the longitudinal direction of the pipe, with the specimen length direction parallel to the pipe tangential direction and the specimen notch position as the center of the weld width (extended range in the pipe circumferential direction). The sample was taken from the thickness center of the weld. Using these test pieces, a Charpy impact test was performed at a test temperature of −46 ° C. in accordance with the provisions of JIS Z 2242, and the absorbed energy and the brittle fracture surface ratio were determined. The results are shown in Table 1.

  In the example of the present invention, the welding coil has a high absorbed energy value, a low brittle fracture surface ratio and excellent toughness despite the fact that the number of required stands of the induction coil is relatively small (the required equipment length of the induction heating device is relatively short). This is a reliable product tube. On the other hand, in the conventional example, although the number of required stands of the induction coil is relatively large (the required length of the induction heating apparatus is relatively long), the absorbed energy value is low, the brittle fracture surface ratio is high, and the weld zone toughness is low. It has become a product pipe with low reliability.

It is explanatory drawing which shows typically an example of the ERW steel pipe manufacturing line suitable for this invention. It is explanatory drawing which shows typically an example of the (a) installation position of an induction heating apparatus suitable for this invention, and (b) structure.

Explanation of symbols

1 Steel strip 2 Roll forming machine 3 High frequency welding machine 4 Squeeze roll 5 Pipe (ERW steel pipe)
6 Uncoiler 7 Leveler 8 Bead cutting machine 9 Sizer
10 pipe cutting machine
11 Induction heating device
11a, 13a induction coil
12,14 Thermometer
15 Stand

Claims (2)

Heat treatment of an ERW steel pipe welded portion in which the outer surface side of the welded portion is induction-heated by using an induction heating device in which a plurality of induction coils are arranged on the outer surface side of the welded portion of the ERW steel pipe along the direction of passing the ERW steel pipe In the method, a plurality of induction coils for induction heating the inner surface side of the welded portion are further provided in the induction heating device, and the inner surface side of the welded portion is also induction heated using them. In the stand from the stand to any one stand other than the final stand, induction heating is performed so that the outer surface temperature and the inner surface temperature of the welded portion on the exit side of the one stand exceed the Ac ₃ transformation point as soon as possible, and the subsequent In this stand, the outer surface temperature and the inner surface temperature of the welded portion are maintained at a predetermined temperature exceeding the Curie point, and induction heating is performed so that the thickness center portion of the welded portion reaches the target temperature. Heat treatment method for welded part of ERW steel pipe. High efficiency heat treatment method of the electric resistance welded steel pipe welds according to claim 1, characterized in that the target temperature and the Ac ₃ transformation point or more of the temperature.

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