JPS6345445B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for local continuous heat treatment of steel pipes. It has already become common to provide a heat treatment line consisting of an annealing or normalizing device in an ERW tube welding line to anneal or normalize the ERW tube seam portion. In this above-mentioned line, assuming that the power of the welding equipment (not shown) is approximately 600KW, it would take approximately
Approximately 2000KW of power is required. Therefore, there is a problem in that the power required for the annealing or normalizing device in the line becomes very large. What is particularly important is that the temperature difference between the inner and outer surfaces of the ERW tube must not become large. For example, it is well known that when a seam portion is heated above a certain temperature, the metallic properties of the surface deteriorate. That is, if the seam portion is heated to 1050° C. or higher, the metal crystal grains become coarse and the strength decreases. There was also the problem that the toughness could not be recovered if the temperature was lower than 950°C. The present invention has been made to solve the above-mentioned problems, and uses an annealing or normalizing device in an ERW pipe welding line to maintain the temperature difference between the inner and outer surfaces of the seam within a predetermined value. The purpose of the present invention is to provide a continuous local heat treatment method for steel pipes that does not cause deterioration of metal properties and enables power saving. Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a perspective view showing the configuration of an embodiment of the present invention. In the figure, reference numeral 1 denotes an electric resistance welded tube with a wall thickness of 12 mm or more, which is being transported in the two directions shown by the arrows. 3,
4 and 5 are inductors having a length of 400 mm to 1500 mm for annealing and normalizing, and these inductors 3, 4, and 5 are arranged in series at a predetermined interval. The arrangement is ERW tube 1
The time required for a predetermined portion of to pass between the inductors (heat penetration time) is set to be 5 seconds or more.
The heat penetration time (T) is determined with reference to a formula obtained from the following experimental results. T>9.5×10 ^-5 ×t ^4However , t is the wall thickness of the tube. Here, we will discuss the heat penetration time. As shown in Fig. 1, when an inductor is disposed on the outer periphery of the ERW pipe 1 facing the seam part to perform induction heating of the seam part, as shown in Fig. 5, the current penetration depth from the outer surface is An induced current is generated in a portion corresponding to the distance Δ, and heating is performed by this induced current. When the wall thickness t of the electric resistance welded tube 1 is greater than the current penetration depth Δ, the temperature increase on the inner diameter side depends on the diffusion (penetration) of heat from the outer circumferential side toward the radial direction of the tube.
As shown in the temperature curves of the outer surface and inner surface in the experimental examples shown in FIGS. 2 to 4, which will be described later, the ERW tube 1 being transferred is induction heated by the inductors 3, 4, and 5, and the outer surface The temperature of the inner surface increases, creating a temperature difference between it and the inner surface temperature. Therefore, in order to reduce this temperature difference and average the temperature between the inner and outer surfaces, a predetermined interval is provided between the inductors 3, 4, and 5, and the next stage inductor is arranged. While the electric resistance welded tube 1 passes through the gap between the inductor and the next stage inductor, heat is diffused and permeated from the outer circumferential side to the inner circumferential side, and the temperature on the inner surface side increases. That is, the time it takes to pass between the inductors is the heat penetration time. Therefore, the larger the distance between the inductors and the longer the heat penetration time, the smaller the temperature difference between the outer and inner surfaces.
This is preferable because the temperature is averaged. but,
On the other hand, since heat is also diffused in the circumferential direction, the power required to heat the seam area becomes large, the heat treatment line becomes long, and the equipment and space (floor area) required for it becomes large. This brings about the disadvantage of becoming. Therefore, the heat penetration time is set practically based on the results of the experimental examples shown in Figures 2 to 4, which will be described later, and the spacing between the inductors is determined accordingly. Since it depends on the rate of heat diffusion and penetration, the heat penetration time T calculated from the above-mentioned empirical formula is used as a reference for steel. Electric power of a predetermined frequency is supplied from a high frequency inverter 6 and a medium frequency inverter 7 to the inductors 3 and 4 arranged as described above. In other words, high frequency power from 700Hz to 3000Hz is supplied to the inductor 3,
Heating is performed until the average temperature of the seam portion of the tube 1 at the inlet of the next stage inductor 4 reaches 600°C to 700°C. Next, medium frequency power of 300Hz to 700Hz is supplied to the inductor 4, and the pipe 1 at the inlet of the next stage inductor 5 is
Heating is performed until the average temperature of the seam reaches 700°C to 750°C. Next, for inductor 5, 700Hz to 3000
The inverters 6 and 7 are set to supply high frequency power of Hz. Next, by comparing the calculated simulation results and the measured simulation results for the ERW tube seam, we were able to improve the accuracy of the simulation. Tables 1 and 2 show the results of various calculation simulations regarding the conductor width.

【table】

[Table] From the above results, it is possible to increase total efficiency and reduce costs by narrowing the conductor width of the inductor, increasing the frequency of the current applied to the inductor, and reducing the number of inductor stands. Ta. Based on the above evaluation, we conducted an experiment with the arrangement of the inductor taken into consideration, and the results are shown below. Figures 2 to 4 show the experimental results. In addition, in FIGS. 2 to 4, experiments were conducted using examples using four and five inductors. In addition, in this experiment, in accordance with the purpose of the present invention, the temperature difference between the inner and outer surfaces of the seam was
This is to check the power consumption (heating) when the temperature is maintained within ℃. Figure 2 shows an electric resistance welded tube with an outer diameter of 406 mm and a wall thickness of 15.9 mm.
Inductors 31 to 34 with a length of 900 mm are each 1300 mm,
When the inductors 31 to 34 are arranged in series with intervals of 1250 mm and 1450 mm, and the gap between each inductor 31 to 34 and the ERW tube is set to 7.5 mm, the inductors 31 to 3
In addition to selecting the heating power PG (KW) supplied to 4.4, the frequency f (Hz) of the heating current, and the conductor width W (mm) of the inductor as shown in Table 3, the heating time 4.38 sec and the cooling time (heat penetration The relationship between the position of the inductor in the axial direction of the ERW tube and the temperature of the outer and inner surfaces of the ERW tube seam section is shown when the time is about 6 seconds.

[Table] The difference between FIG. 3 and FIG. 2 is that an inductor 35 is further provided, the distance between the inductors 34 and 35 is maintained at 1500 mm, and the power supplied to the inductors 31 to 35 is Table 4 shows the frequency and conductor width of the inductor.
The relationship between the position of the inductor in the axial direction of the electric resistance welded tube and the temperature of the outer and inner surfaces of the welded portion of the electric resistance welded tube is shown in the following figure.

[Table] Also, Fig. 4 differs from Fig. 2 in that when the power and frequency supplied to inductors 31 to 34 and the conductor width of the inductors are selected as shown in Table 5, The relationship between the axial position of the ERW tube and the temperature between the outer and inner surfaces of the welded portion of the ERW tube is shown.

[Table] In all of the three examples shown in FIGS. 2 to 4 above, the temperature difference between the inner and outer surfaces of the seam portion satisfies within 100°C, so any condition may be used in terms of this temperature. However, the consumption (heating) power is the lowest under the conditions shown in FIG. Therefore, the conditions shown in FIG. 2 are good. As for the frequency, as shown in Table 1, only high frequency requires less heating power, but in order to reduce the temperature difference between the inner and outer surfaces of the seam, a combination of heating including medium frequency is used. When the average temperature is below 600 to 700 degrees Celsius, the entire outer and inner diameter of the tube is a magnetic region, so even if the medium frequency is 300 Hz to 700 Hz, the current penetration depth Δ does not increase. Therefore, in any case, the only way to reduce the temperature inside and outside the seam is to rely on heat diffusion through heat conduction. Therefore, 700Hz has good heating efficiency.
Heating (consumption) uses high frequency from 3000Hz to
This is appropriate from the viewpoint of reducing power consumption. In the temperature range where the average temperature of the ERW tube exceeds 600 to 700℃ and is slightly higher than this, the outer diameter side of the tube is a non-magnetic region (approximately 730℃ at the inlet of the inductor 33 in Fig. 2), and the inner diameter side of the tube is a non-magnetic region. is a magnetic region (approximately 700 to 750 degrees Celsius at the inlet of the inductor 34 in Fig. 2), and these regions are 730 to 780 degrees Celsius.
It becomes a mixed state at ℃. For this reason, from 300Hz
The medium frequency of 700Hz increases the penetration depth of the current and heats as much of the inner surface as possible, including the magnetic region, to reduce the temperature difference between the inside and outside of the tube. In a temperature range where the above-mentioned average temperature is 700 to 750°C or higher, the entire region becomes a non-magnetic region as a result of heating. Therefore,
Even at high frequencies of 700Hz to 3000Hz, a large current penetration depth is achieved, so there is no temperature difference between the inside and outside.
Therefore, it is more appropriate to use a high frequency from 700Hz to 3000Hz in terms of reducing heating power. As described above, the following has become clear from the first to third experimental examples. (1) The temperature difference between the outside and inside of the seam of the ERW pipe is 100
In order to keep the heat treatment temperature to about 1000℃ while keeping it within ℃, the width of the conductor of the inductor should be changed from 35mm to 24mm.
The power applied to the inductor by
It can be reduced from about 2200KW to about 1620KW. (2) In order to reduce power consumption while keeping the temperature difference between the inner and outer surfaces at the seam part of the final heating inductor within 100℃ immediately after heating, the frequency of the current applied to each inductor should be 1000Hz, 1000Hz, and 500Hz. ,
It is better to use the order of 1000Hz. (3) The heating pattern for each inductor is to increase the temperature of the inner surface of the seam part until it becomes non-magnetic, with a heat penetration time (passage time between inductors) of about 6 seconds after power is turned on, and then to the final position. The heating of the inductor is
Preferably, this is done at 1000Hz. (4) When an electric resistance welded tube with a wall thickness of 15.9 mm and an outer diameter of 406 mm is heat-treated at a speed of 13 m/min, four inductors are installed in a space of 8000 mm, and the power supplied to these inductors is approximately 2000 KW compared to the conventional one. From, it can be reduced to 1620KW. Note that the number of inductors arranged in the seam portion is not limited to three or four. (i) Number of inductors to be heated until the average temperature of the seam area at the inlet of the next stage inductor is 600 to 700℃ (ii) The average temperature of the seam area at the inlet of the next stage inductor is 700 to 750℃ Number of inductors that perform heating up to °C (iii) The number of inductors that perform subsequent heating can each be one or more. Next, we will discuss the reason for limiting the frequency added to the inductor. Penetration depth Δ of induced current in induction heating of steel materials
depends on temperature and frequency according to the generally known formula shown in the following table. Δ=5.03√(cm) However, ρ: Specific resistivity (μΩ-cm) μ: Relative permeability f: Frequency

[Table] When heating and heat-treating the seam area, if the wall thickness of the pipe is thin (previously 12 mm or less), the temperature difference between the inner and outer surfaces is small and has not caused any particular problems in the past. In recent years, as ERW pipe welding technology has improved, even thicker pipes are increasingly manufactured by ERW pipe welding, and a problem has arisen in that the temperature difference between the inner and outer surfaces becomes larger. In addition, in order to reduce this temperature difference between the inside and outside surfaces, for example, the frequency of the heating power can be changed.
If measures such as lowering the frequency to 700 Hz or lower, increasing the spacing between inductors or increasing the number of inductors, or increasing the conductor width of the inductors, the amount of power required for heating will decrease. There was a problem that the number of units increased or the equipment became large-scale. It should be noted that this problem becomes most noticeable in tubes with a wall thickness of 12 mm or more. Furthermore, the range of wall thickness for electric resistance welded pipes is generally up to about 20 mm. As mentioned above, the heating method of the present invention has the following features: (i) Below 600 to 700°C, the penetration depth of the current does not increase significantly even if the frequency is lowered, so the heating efficiency is high and the required electric power is low. It is best to heat it above. If it exceeds 3000Hz, the penetration depth will become extremely small, making it unsuitable. (ii) When heated until the average temperature of the seam area at the inlet of the next stage inductor reaches 700 to 750℃, the outer circumferential side becomes a non-magnetic region and the inner circumferential side becomes a magnetic region.
The penetration depth extends not only to the non-magnetic area but also to the magnetic area, and the entire surface is heated as much as possible (from 12 mm to 20 mm).
It is best to set the frequency to 700Hz or less so that the However, considering heating efficiency, a range of 300 to 700 Hz is sufficient. (iii) When the average temperature exceeds 750°C, almost the entire area becomes a non-magnetic region, so a sufficiently large penetration depth can be obtained even at high frequencies. As a result, it is appropriate to heat at 700 to 3000 Hz, which provides good heating efficiency. Note that the frequency at this time may be selected depending on the wall thickness within the range of each of the above-mentioned frequencies. In addition, as shown in Figure 1, the above
The power source for heating in (i) and (iii) can be shared. This allows equipment costs to be reduced. In summary, the present invention provides an electric resistance welded pipe heat treatment line for thick-walled pipes, in which at least three inductors are formed to have a predetermined conductor width and are arranged in series at intervals, and the frequency of the electric power applied to the inductors is The average temperature of the seam at the inlet of the next stage inductor is 600~
The frequency is 700 to 3000Hz until the temperature reaches 700℃, and then the temperature of the seam at the inlet of the next stage inductor is 700 to 3000Hz.
The temperature was set to 300 to 700 Hz until the temperature reached 750°C, and then again to 700 to 3000 Hz. Therefore, the present invention maintains the temperature difference between the inner and outer surfaces of the ERW pipe seam portion to a small value and performs annealing. It is possible to standardize and also has the effect of reducing power consumption.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing the configuration of an embodiment of the present invention, and FIGS. 2, 3, and 4 are ERW tube axes of inductors in the first, second, and third experimental examples, respectively. FIG. 5 is a diagram showing the relationship between the directional arrangement position and the temperature between the outer surface and the inner surface of the welded portion of the electric resistance welded tube, and FIG. 5 is a diagram for explaining the current penetration depth. 1... ERW pipe, 2... Movement direction of the ERW pipe, 3,
4, 5... Inductor, 6... High frequency inverter, 7
...Medium frequency inverter.

Claims (1)

[Claims] 1. In an ERW tube heat treatment line for locally annealing and normalizing the seam portion of the ERW tube to be transferred, at least three inductors are placed at the seam portion along the direction of transfer of the ERW tube. Arranged facing each other, the average temperature of the seam at the inlet of the next stage inductor is 600℃
Power at a frequency of 700Hz to 3000Hz is supplied to the inductor to induction heat the seam area until the temperature reaches 700℃ to 700℃, and then the average temperature of the seam area at the entrance of the next stage inductor reaches 700℃ to 750℃. until
Power with a frequency of 300Hz to 700Hz is supplied to the inductor to inductively heat the seam area, and subsequent heating in the temperature range of 700℃ to 750℃ or higher starts from 700Hz again.
A continuous local heat treatment method for steel pipes, which is characterized by supplying a frequency of 3000 Hz to an inductor to perform induction heating at seam areas.