JPH0235609B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a method and apparatus for bending a metal tube. In particular, the present invention relates to a metal tube bending device that easily prevents deformation of the bent portion due to bending, particularly a decrease in wall thickness on the back side of the bent portion, with a simple operation when bending metal tubes by high-frequency heating.

[Background of the invention]

In conventional high-frequency induction heating bending, the high-frequency heating coil that heats the bent part is a circular coil whose shape is the same on the back side of the bending process and on the back side of the bending process. It becomes almost uniform. Therefore, the yield point, tensile strength, and elongation of the metal tube due to external loads in this high-frequency heating region are substantially uniform. Therefore, when a bending moment is applied to the high-frequency induction heating area by fixing one end of the pipe to be processed to an arm and pushing out the pipe from the other end with a pushing device, the bending moment that occurs will be on the back side of the bending part and on the back side of the bending part. Because of this, the same amount of plastic deformation occurs on the dorsal and ventral sides of the bent part, and an amount equivalent to the increase in thickness on the ventral side of the bent part tends to decrease in thickness on the back side of the bent part. be. This tendency shows that when the length of the arm that draws a constant arc is shortened and the bending radius is set small, the increase in thickness on the return side of the bend and the decrease in thickness on the back side of the bend become particularly large. Of these, the amount of thinning on the back side of the bent portion is large, which means that when using a bent pipe, in order to maintain the specified wall thickness required for the design, an amount to compensate for this thinning amount is required. This problem has to be dealt with by adding an additional layer to the thickness of the metal tube before bending, which is a disadvantage in the use of bent tubes. In order to eliminate this disadvantage,
By applying a braking force to the central axis of the arm that draws a constant arc, the pushing force from the pushing device that pushes out the pipe to be bent and the compressive force due to the braking force applied to the arm are applied to the pipe to be bent, and the back side of the bent part is It is generally a good practice to reduce the decrease in wall thickness. This method requires the installation of a brake generator in order to apply a braking force to the arm shaft. This increases the manufacturing cost of the bending device, which is unfavorable in terms of equipment investment. Further, by applying this braking force to the central axis of the arm, a large torsional moment is applied to the shaft, so there is a drawback that the central axis of the arm needs to be made thicker than necessary. On the other hand, instead of these bending methods that apply compressive force to the pipe to be bent, in metals, the yield point and tensile strength decrease and the elongation increases in the hot working region of about 900℃. In the warm working region of about 400℃, the yield point and tensile strength are higher and the elongation is lower than in the hot working region. By lowering the processing temperature, we can increase the deformation resistance on the back side of the bent part and reduce thickness loss.
During bending, the heating part is cooled by spraying water on the high circumferential back heating part on the back side of the bending part, and the bending temperature of the back side of the bending part in the bending moment applying part is removed from the hot working area. A method is used to increase the yield point and tensile strength of the back side of the bending part, keep the elongation low, and suppress the amount of thinning on the back side of the bent part. In this method, in addition to the cooling ring provided on the arm clamp side of the high-frequency heating ring, a local cooling nozzle is provided on the pushing device side of the high-frequency heating ring to locally cool the local area on the back side of the bending part in a width of approximately 60 mm. However, increasing the yield point and tensile strength on the back side of the bending part has the effect of increasing deformation resistance and reducing the amount of thinning on the back side of the bending part, but in addition to the high-frequency heating coil and normal cooling ring setting, It requires nozzle settings for local cooling, which increases pre-processing setup time, causes unevenness in the bent pipe material due to local nozzle setting errors, and causes cooling water to flow down the lower surface of the pipe due to gravity. This may cause non-uniformity in the material of the bent member. In addition, since the cooling water is controlled mechanically by adjusting the opening of the valve, uniform cooling performance cannot always be expected due to fluctuations in water pressure.Also, the high-frequency bending method does not always process items with the same diameter. , and because the coils are not always continuously bent, the coils must be removed and set, so it is not always possible to set the high-frequency heating coil, cooling ring, and local cooling ring in the same position. , it requires fine adjustment of these three coils and the cooling ring, and has the disadvantage of requiring a skilled person.
In addition, in high-frequency heating bending processing, depending on the degree of processing of the cooling nozzle, there are differences in the direction of cooling water blowing out and the amount of cooling water blowing out from each hole, which causes an imbalance in the cooling range and an imbalance in the heating width. It is clear that uniformity is produced, but there is also the drawback that the processing accuracy and manufacturing accuracy of the nozzle may lead to non-uniformity in the finished state or material of the bent pipe. Furthermore, as a processing method for creating a temperature difference between the back side of the bent part and the ventral side of the bent part,
By shifting the central axes of the pipe to be bent, the high-frequency heating ring, and the cooling ring, and making the gap between the high-frequency heating ring and the surface of the pipe to be bent larger on the back side of the bending part and smaller on the ventral side of the bending part, the back of the bending part can be made smaller. A method has been devised in which the temperature on the dorsal side of the bent part is lowered and the temperature on the ventral side is increased by decreasing the magnetic flux density on the side and increasing the magnetic flux density on the returning side of the bent part, but due to the gap difference, heating Gap setting is important because temperatures vary widely. Therefore, it is not possible to start bending immediately by simply setting the coil, but requires fine adjustment by an expert. For this reason, there is a drawback that setup work before bending increases. Further, the usefulness of this method for providing a gap differs depending on the material, and although it is possible to easily create a temperature difference in stainless steel, it has the disadvantage that it is difficult to create a temperature difference in carbon steel. In addition, even if the material is the same, as long as a perfectly circular high-frequency heating coil is used, if the gap is set at a temperature that is convenient for suppressing thinning on the back side of the bent part, the temperature drop in the neutral part and the gap will be minimal on the recovery side. Therefore, there is a disadvantage that there is a limit on the heating temperature difference, which limits the processing conditions.

In addition, the bending method that creates these temperature differences requires a skilled person to adjust the nozzle and gap, and is a high-frequency heating bending method that is difficult to employ in today's world where the number of skilled people is decreasing.

[Purpose of the invention]

The present invention has been made in order to solve the problems of the prior art, and aims to lower the processing temperature on the back side of the bent part during high-frequency bending and increase the processing temperature on the ventral side of the bent part. It is an object of the present invention to provide a metal pipe bending device that can easily minimize side wall thinning through simple work.

[Summary of the invention]

The present invention provides a metal tube bending apparatus comprising an annular high-frequency heating coil and a means for applying a bending moment to a metal tube to be bent passed through the inside of the annular high-frequency heating coil. The annular high-frequency heating coil is a metal tube bending device characterized in that the width of the bending back side of the metal tube to be bent in the longitudinal direction is wider than the width of the bending antinode in the longitudinal direction. With such configuration requirements, the annular high-frequency heating coil has a wide coil cross-sectional area on the dorsal side and a narrow coil cross-sectional area on the ventral side, so the current density is low on the dorsal side and low on the backward side. becomes higher. For this purpose, by passing the metal tube to be bent through the center of the annular high-frequency heating coil, high-frequency induction heating can be easily applied to the metal tube to be bent, with low temperature on the back side and high temperature on the ventral side. This makes it possible to easily suppress the amount of thinning of the back side of the bent portion.

[Embodiments of the invention]

In the present invention, as shown in FIG. 1, for a constant input current value I, the coil cross-sectional area is changed from the ventral side S ₁ to the dorsal side S ₂ , and when the ratio is α, the dorsal cross-sectional area is changed. Dorsal coil cross-sectional area (S ₂ )/ventral coil cross-sectional area (S ₁
)=α...(1), and the ratio is 1/α on the ventral side.

Here, if the current density is ψ, it is expressed by the following relational expression.

ψ=High frequency current value/Coil cross-sectional area (S)...
(2) Therefore, on the ventral side of the bend, equation (3) is obtained, and on the dorsal side, equation (4) is obtained.
The formula becomes

ψ ₁ = I/S ₁ ...(3) ψ ₂ = I/S ₂ ...(4) Here, ψ ₁ : ventral current density ψ ₂ : dorsal current density. From equations (1), (3), and (4), ψ ₂ = ψ ₁ /α (5), and the dorsal current density ψ ₂ changes depending on the ratio of the dorsal and ventral cross-sectional areas.

FIG. 2 shows the change in circumferential current density of the heating coil. As shown in this figure, it can be seen that the current density is high on the back side of the bend, and the current density is low on the back side of the bend.

Further, the amount of heat generated (Q) in the pipe to be bent is proportional to the applied current density ψ.

Calorific value (Q) ∝ Current density (ψ) ...(6) Therefore, the distribution of the circumferential calorific value Q of the heated part is as shown in Figure 3, with a high calorific value on the ventral side of the bend and a high calorific value on the back side of the bend. It can be seen that the calorific value is low. The processing temperature (T) of the pipe to be bent and the amount of heat generated (Q) are in a proportional relationship.

Processing temperature (T) ∝ Calorific value (Q) ...(7) Figure 4 shows the processing temperature distribution of the pipe to be bent. The broken line indicates the case where there is no heat conduction within the wall of the tube to be bent, and the solid line indicates the case where there is heat conduction within the wall of the tube to be bent. By continuously changing the cross-sectional area of the coil in this way, it is possible to keep the processing temperature of the pipe to be processed high on the ventral side of the bent portion and low on the back side of the bent portion.

FIG. 5 shows the cross-sectional shape of the heating coil in the present invention, and FIG. 6 shows the dorsal side cross-sectional shape and the ventral side cross-sectional shape of the heating coil. When using coil materials of the same thickness, the temperature distribution shown in FIG. 4 can be obtained by making the coil width _l2 on the back side of the bending part wider than the coil width l1 on the ventral side of the bending part.

FIG. 7 shows an example of the relationship between the stress difference Δσ between the back side and the ventral side of the bent portion and the thickness reduction suppression amount Δe of the deformation resistance at the processing temperature. This results in
It is clear that the greater the stress difference Δσ, the greater the amount of suppression of wall thinning. Furthermore, this stress difference Δσ
is dependent on temperature, and the temperature dependence of the deformation resistance value is shown in FIG.

Examples of the method of applying a temperature difference according to the present invention are as follows for stainless steel pipes and carbon steel pipes.

For stainless steel, if the bending radius is twice the diameter, the heating temperature on the back side of the bent part should be 1000
By processing at a heating temperature of 800°C or higher on the ventral side of the bent part, the deformation resistance difference Δσ = 7Kg/
mm ² , and it is possible to obtain a 9% reduction in thickness.
In normal bending, the amount of thickness loss is around 20%, so the total amount of thickness loss can be improved to around 11%.

For carbon steel, when the bending radius is twice the diameter, the heating temperature on the back side of the bent part should be 700℃ or higher,
By performing processing at a heating temperature of 500℃ or less on the ventral side of the bending part, the deformation resistance difference Δσ = 7Kg/mm ² , and a 9% reduction in thickness can be obtained. Since the amount of thinning is around 20%, the total amount of thinning can be improved to around 11%. In this way, carbon steel,
For both stainless steel, it is possible to improve the amount of thinning by creating a temperature difference of approximately 200℃, and the tapered high-frequency heating coil shape that satisfies this condition is
It will look like this:

High temperature side (ventral side) bending temperature (function of current value)
Let T ₁ (I) be the ratio α of the dorsal coil cross-sectional area S ₂ and the ventral coil cross-sectional area S ₁ of the tapered high-frequency heating coil.
is shown in equation (8).

α=T ₁ (I)/T ₁ (I)−ΔT ……(9) Here, ΔT: Temperature difference between the dorsal side and the ventral side Therefore, in the previous example, for stainless steel, α=1.25 …… (10) For carbon steel, α=1.4...(11)

However, the shape of the tapered high-frequency heating coil in this embodiment for stainless steel is shown in FIG. 9, and the shape of the tapered high-frequency heating coil in this embodiment for carbon steel is shown in FIG. 10.

In addition, in the embodiment according to the present invention, the coil is designed to be a perfect circle and to be longer in the coil axial direction from the ventral side of the bending part to the dorsal side. It has the effect of cooling the back side of the bending part, and by using the conventional method of locally cooling the back side of the bending part, or by changing the gap between the coil and the pipe to be bent on the back side and the ventral side of the bending part. It is possible to obtain the same effect as the method in which the back side is made low temperature and the ventral side of the bent part is made high temperature, and it is also easy to make by simply aligning the axis of the high-frequency heating coil with the pipe to be bent, which is normally done by bending. It is possible to create a temperature difference between the dorsal side and the ventral side of the bent part, and by controlling the current supplied to the high-frequency heating coil, it is possible to easily and accurately control the temperature and adjust the heating temperature. I have it too.

Further, FIG. 11 shows an example of bending operation when this embodiment is used.

One end is fixed to an arm 2 that can rotate around a rotating shaft 1 and draws a constant arc, the other end is clamped by a pushing device 3, and the metal tube 4 to be bent is guided by a guide roller 6 using a high frequency power source. The high-frequency induction heating coil 7 is inserted without being connected to the tapered high-frequency heating coil 7 connected to the high-frequency heating coil 7 connected to the high-frequency heating coil 7, and the high-frequency induction heating coil 7 is heated directly under the high-frequency heating coil by high-frequency induction heating to keep the temperature on the dorsal side of the bent part low and the temperature on the ventral side of the bent part high. The heating width is controlled by blowing out cooling water from the cooling ring 8 provided on the arm side of the tube, and the pushing device 3 pushes the tube 4 forward to apply a bending moment to the high-frequency heating area, thereby performing the bending process. This is what we do.

〔Effect of the invention〕

The present invention provides a metal tube bending device that can reduce the amount of thinning on the back side of the bending part by lowering the processing temperature on the back side of the bending part than the processing temperature on the ventral side of the bending part. This has the advantage that the coil width can be increased on the back side of the bent portion, and the coil width can be easily imparted on the ventral side of the bent portion using a high-frequency heating coil.

[Brief explanation of drawings]

Figure 1 is a diagram showing the cross-sectional area distribution of the high-frequency heating coil, Figure 2 is a diagram showing changes in the circumferential current density of the heating coil, and Figure 3 is a diagram showing the circumferential heat generation amount (Q ) A diagram showing the distribution, FIG. 4 is a diagram showing the processing temperature distribution of the tube to be bent, FIG. 5 is a cross-sectional view of the heating coil in the present invention, and FIG. 6 is a diagram showing the back side of the heating coil. FIG. 7 is a longitudinal cross-sectional view showing the cross-sectional shape and the ventral side cross-sectional shape; FIG. The figure is a diagram showing the temperature dependence of deformation resistance value, Figure 9 is a longitudinal cross-sectional view showing an embodiment of the present invention in stainless steel, and Figure 10 is a longitudinal cross-sectional view showing an embodiment of the present invention in carbon steel. 11 are front views illustrating the bending operation when this embodiment is used. S... Coil cross-sectional area, α...
Dorsal coil cross-sectional area (S ₂ )/ventral coil cross-sectional area (S ₁ ), ψ
... Current density, T... Back side of bent part, N... Neutral part of bent part, C... Ventral side of bent part, Q... Power generation amount, T... Heating temperature, l ₂ ...
Bending coil width, Δσ...Stress difference, Δe...Thickness reduction suppression amount, σγ...Deformation resistance, 1...Rotating shaft, 2...Arm, 3...Pushing device, 4...Pipe to be bent, 5
...High frequency power supply, 6...Guide roller, 7...
Tapered high-frequency heating coil, 8...cooling ring.

Claims (1)

[Claims] 1. In a metal tube bending device comprising an annular high-frequency heating coil and a means for applying a bending moment to a metal tube to be bent passed inside the annular high-frequency heating coil, the annular high-frequency heating coil comprises: A metal tube bending apparatus characterized in that the width in the longitudinal direction of the bent back side of the metal tube to be bent is wider than the width in the longitudinal direction of the bent side.