JPH0632285B2 - Method and device for induction heating of metal tube - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an induction heating method for a metal tube, and particularly to a temperature difference in the cross-sectional direction of the tube after heating due to uneven thickness of the tube material (hereinafter referred to as “unbalanced heat”). ) And an induction heating method for realizing uniform heating and a device used for the same.

[Prior art]

Heating of metal tubes is roughly classified into continuous furnace and batch furnace,
If the continuous furnace is further divided by a pipe feeding mechanism, it is classified into a disk roller type (barrel furnace), a hearth roller type (roller hearth furnace) and a walking beam type. All of these are methods to transfer heat to the tube by radiation and convection in the furnace, so the temperature of the tube cannot exceed the furnace wall or convection temperature, and even the treated material with a slight uneven wall thickness is uniformly heated. be able to.

However, since it takes a long time to heat and it is exposed to the ambient atmosphere for a long time, there were many problems in terms of surface oxidation, adjustment of metal structure and stability of physical properties.

On the other hand, recently, a method of heating a metal tube by an induction heating coil in a short time and in a clean atmosphere has been proposed. However, unlike the previous method, this method uses the principle of generating an eddy current in the metal tube by electromagnetic induction and heating it by Joule heat, so when there is a large uneven thickness in the circumferential direction, the thin wall part Has a possibility of reaching an excessively high temperature compared to others, and it is very difficult to control the temperature, and it was not suitable for the production of metal pipes under severe temperature conditions.

A major factor that makes it difficult to control the temperature of the metal tube by the induction heating method is the deviation of the center axis between the heating coil and the metal tube (hereinafter referred to as eccentricity) and the uneven thickness of the metal tube in the circumferential direction.
The eccentricity due to the eccentricity can be eliminated by heating the metal tube while rotating it, because the heat is uniformly transferred over the entire circumference of the metal tube. Since the uneven thickness in the circumferential direction cannot be avoided to some extent at the current level of pipe manufacturing technology, the problem in the induction heating depends on how to reduce the uneven heat due to the uneven thickness.

In the induction heating method, as a method for reducing the occurrence of uneven heat generation in a metal tube having uneven thickness, the frequency of the induction heating power source is lowered to increase the penetration depth (Japanese Patent Laid-Open Nos. 52-14486 and 53-53). -10141), or the thickness distribution in the circumferential direction is measured with a wall thickness gauge, or the temperature distribution in the circumferential direction is measured with a thermometer (Japanese Patent Laid-Open No. Sho.
Proposals have been made to minimize uneven heat distribution by using methods such as 53-114712 and JP-A-55-128539).

Among the former methods, one is to provide a soaking coil outside the heating coil, which is excited at a frequency different from this, and move the soaking coil to heat the thick portion. Is to prevent uneven heat by using different frequencies for the high temperature range and the low frequency range.

One of the latter two methods is to detect the thick portion by a wall thickness gauge and always keep the thick portion on the upper surface in the coil to prevent unbalanced heat. The deviation is detected, and thereby the induction coil is eccentric to prevent the deviation of heat.

[Problems to be solved by the invention]

In the former method, it is necessary to lower the frequency and increase the penetration depth, but lowering the frequency lowers the efficiency of heat transfer from the heating coil to the tube, so there is no choice but to raise the frequency to some extent. The penetration depth could not be increased so much. Further, when two types of frequencies are required, a separate power source is required, and the equipment cost becomes expensive.

Further, in the latter case, the actual control is difficult due to the problems of the accuracy of the thermometer and the wall thickness gauge, the problem of the controllability with respect to various temperature distributions in the circumferential direction of the steel pipe, the delay of the responsiveness of the heating coil position correcting device, and the like.

Furthermore, eccentricity is roughly divided into eccentricity eccentricity such as core misalignment of the inner and outer diameters of the metal tube, the outer circumference of the metal tube has a perfect circular shape, and the inner circumference has a substantially elliptical shape concentric with the outer circumference, Although there is a facing uneven thickness in which the thickness of the facing portion has the same tendency, the latter method of the conventional methods cannot deal with this facing uneven thickness.

A first object of the present invention is to reduce uneven heat generated due to uneven thickness when heating a metal tube having uneven thickness without requiring special temperature control with inexpensive equipment.

A second object of the present invention is to effectively prevent uneven heat distribution by short-time heating by heating a metal tube to each part in the longitudinal direction at a short vertical distance and performing forced cooling a plurality of times. is there.

A third object of the present invention is to combine an induction heating coil for a metal tube and a refrigerant injector on the downstream side of the metal tube in the longitudinal direction, thereby making it possible to use a simple facility at a low cost and to produce an existing tube. It can be easily applied to the line.

[Means for solving problems]

A metal pipe induction heating method according to a first aspect of the present invention detects a temperature of a metal pipe in a process of alternately performing induction heating and forced cooling of the metal pipe while vertically traversing the metal pipe, and detects the metal based on the detected temperature. It is characterized in that the induction heating output to the metal tube and the amount of the refrigerant injected into the metal tube are controlled in order to reduce the unbalanced heat of the tube.

An induction heating device for a metal tube according to a second aspect of the invention is provided with a plurality of annular induction heating coils and respective induction heating coils which are arranged with a predetermined gap in the longitudinal direction of the metal tube and surround the transportation area. At least one induction heater having a refrigerant injector for injecting a refrigerant onto the surface of the metal tube through the space, and to reduce the unbalanced heat of the metal tube based on the temperature of the metal tube measured at the outlet side of the induction heater. And a control device for controlling the output of the induction heating coil and the amount of the refrigerant injected from the refrigerant injector into the metal pipe.

The induction heating device for a metal tube according to the third aspect of the present invention is more preferable than the annular induction heating coil and the induction heating coil that surround a plurality of sets of metal tube transport areas that are alternately arranged along the longitudinal direction of the metal tube. Located on the downstream side in the longitudinal direction of the metal tube, a refrigerant injector for injecting a refrigerant into the metal tube, and detecting the temperature of the metal tube, based on the detected temperature induction heating coil output, to the metal tube from the refrigerant injector. And a control device that controls the amount of the injected refrigerant.

[Operation] When a metal tube having an uneven thickness in the circumferential direction is induction-heated and then forced to be uniformly cooled in the circumferential direction, the thin portion is easily cooled and the thick portion is difficult to be cooled. Therefore, by forcibly cooling, the unbalanced heat caused by induction heating is canceled out,
The temperature of the tube can be homogenized in the circumferential direction.

[Principle of Invention]

 Hereinafter, the principle of the present invention will be specifically described with reference to the drawings.

FIG. 1 is a schematic sectional view of a metal or the like having an uneven thickness in the circumferential direction, which is drawn for explaining the principle of the present invention. The cross section of a metal pipe of unit length is divided into n equal parts in the circumferential direction, and the divided pieces are numbered 1, 2, ... i-1, i, i + 1 ,.
t _max is the overall maximum wall thickness, and t _min is the same minimum wall thickness. ti is the thickness of the divided piece i, and ri is the average radius of the divided piece i. Further, Q _1i is the amount of heat transferred by convection from the outside of the divided piece i, and Q _2i and Q _3i are the divided piece i and the adjacent divided pieces i.
-1, heat conduction heat quantity in the tube with i + 1, Q _4i is adiabatic heat quantity.

The reason why the cooling effect of the thin portion is larger than that of the thick portion by the forced cooling is as follows.

Now, as an initial condition, it is assumed that the metal pipe having uneven thickness is in a uniform temperature state throughout. Cooling is applied thereto by the amount of heat transferred by convection, Q _1i, so that the temperature θ _i of the divided piece _i becomes unit time Δt
If θ _i ′ is reached after a second, the difference approximation internal is obtained from the heat transfer theory as follows.

However C: Specific heat of metal tube mi: Mass of divided piece i λ: Thermal conductivity of metal tube λ _A : Thermal conductivity of refrigerant θ _A : Temperature of refrigerant Nu: Nusselt number From the above formulas (2) to (5), when cooling is evenly applied from the outside in the circumferential direction, Q _1i is constant irrespective of the segment number,
Since the initial state of Q _2i and Q _3i is uniform, Q _2i = Q _3i = 0
Is. Therefore, from equation (1) Becomes From equation (6), the temperature drop amount θ _i ′ −θ _i per unit time Δt depends on the divided piece mass mi. That is, when the convection heat transfer heat quantity Q _1i is the same, the thin wall portion with a small divided piece mass mi has a larger temperature drop amount θ _i ′ −θ _i than the thick wall portion. That is, the temperature drop in the thin-walled portion is larger than that in the thick-walled portion. Therefore, if the metal pipe having the unbalanced heat whose temperature in the thin-walled portion is high is forcibly cooled, the unbalanced heat is reduced.

[Example 1] Fig. 2 is a schematic sectional view of an induction heater used in this example. The induction heater 1 includes an induction heating coil 2 and a refrigerant injector 3, and the induction heating coil 2 is a conductor. Is spirally wound with a diameter larger than that of a metal tube. The refrigerant injector 3 includes a case 3a, a nozzle 3b, and a refrigerant inlet 3c. The case 3a has a cylindrical outer cylinder, and both end surfaces thereof have the same outer diameter as the outer cylinder but a larger inner diameter than the metal tube 2 It is composed of two hollow discs and an inner cylinder having an outer diameter slightly larger than the inner diameter of the disc, and the induction heating coil 2 is provided on the inner surface of the inner cylinder. A refrigerant distribution chamber 3d is provided in a space surrounded by an inner cylinder, an outer cylinder, and two discs, and a nozzle having a refrigerant injection hole is provided from the inner cylinder toward the gap between the turns of the induction heating coil 2. A plurality of chambers 3b are arranged so that the metal tubes can be forcibly and uniformly cooled in the circumferential direction. Further, a plurality of refrigerant introduction ports 3c are formed on the outer peripheral surface of the case 3a, and the refrigerant 4 is fed from a refrigerant supply device (not shown) to the refrigerant introduction port 3c.
c, Passes through the nozzle 3b through the refrigerant distribution chamber 3d, and is uniformly jetted to the outer peripheral surface of the metal tube 6 in the circumferential direction. In this way, the metal tube 6 is forcibly and uniformly cooled in the circumferential direction by the refrigerant 4.

As a result, the metal tube 6 is subjected to the heating by the induction heating coil 2 and the forced cooling by the refrigerant 4 at each portion in the longitudinal direction alternately in a large number of times within a short longitudinal distance.

The metal tube used for the test was made of SUS304, the outer diameter was 15.9 mm,
The thick side inner thickness was 1.2 mm, the thin side wall thickness was 1.0 mm, and the length was 100 mm. The induction heating high-frequency dispatcher was capable of applying a high-frequency output of 15 kW 75 kHz. Further, air was used as the refrigerant.

In the experimental method, the metal tube 6 was loaded into the induction heating device 1, a predetermined high frequency output was applied, and after heating for a certain period of time, it was cooled.

FIG. 3 and FIG. 4 are graphs showing the results of experiments conducted to verify the above effects, in which the vertical axis represents temperature and the horizontal axis represents time.

In FIG. 3, heating and forced cooling are performed simultaneously, and forced cooling is performed after a predetermined time has elapsed. In FIG. 4, only heating is performed for the same time as in FIG. 3, and then natural cooling is performed. It is a thing. In the figure, the solid line shows the temperature change in the thick portion, and the broken line shows the temperature change in the thin portion. As is apparent from the comparison between FIG. 3 and FIG. 4, the temperature drop in the thin portion is promoted by the forced cooling following the heating.

Further, when heating and forced cooling are simultaneously performed as shown in FIG. 3, temperature reversal between the thin portion and the thick portion occurs, and the temperature difference after heating is the same as in FIG. The temperature of the meat part is 80 ℃ higher than the temperature of the thin part. Therefore, this temperature difference is 0
It was found that induction heating that reduces unbalanced heat can be achieved by appropriately setting the forced cooling air volume, wind speed, etc. so that the temperature becomes ℃.

[Embodiment 2] FIG. 5 is a layout view showing a second embodiment. The metal tube 6 is provided with a heating device 2 and a refrigerant injector 3 arranged on the downstream side thereof in a direction indicated by an outline arrow by a transfer device (not shown).
Traverse in the induction heaters 1, 1, 1 arranged in a stand tandem. The induction heating coil 2 has the same structure as that of the above-described embodiment, the refrigerant injector 3 has a cylindrical shape, and the refrigerant 4 is introduced from the refrigerant inlet port formed in the case as in the above-mentioned embodiment, It is jetted from the nozzle toward the metal tube 6 through the distribution chamber. Therefore, the metal tube 6 is alternately heated and forcibly cooled at a cycle time determined by the transport speed and the arrangement of the induction heater 1.

FIG. 6 is a graph showing the results of alternately performing heating and forced cooling three times, with the vertical axis representing temperature and the horizontal axis representing time. However, in this example, forced cooling was not performed during heating.

As a comparison target, heating and natural cooling were also performed at the same cycle time (not shown).

As a result, when the natural cooling was performed, the uneven heat was 30 ° C. between the thick part and the thin part, but in this example in which heating and forced cooling were alternately performed, the uneven heat was only 5 ° C. It was

In the above two embodiments, the forced cooling is performed from the outer surface of the metal 6 or the like, but the inner surface may be cooled. As a cooling method from the inner surface, for example, a well-known quenching cooling head from the inner surface may be used. The inner and outer surfaces may be cooled at the same time.

FIG. 7 is a block diagram showing an example of the control method of the present invention. The metal tube 6 is vertically traversed in the induction heating device A having a refrigerant injector by the metal pipe carrier F in the direction indicated by the white arrow.
Heated and cooled. Then, the temperature-measuring device G on the downstream side measures each predetermined time, and the data is input to the information processing device E.

When the metal tube 6 is rotated to prevent eccentricity and bending between the heating coil and the metal tube 6 during vertical running, the average of the temperature measurement values of the same thickness tendency portion input to the information processing device E is processed. It is calculated by the device E and used as a temperature measurement value.

If the temperature measurement value input in this way has a difference from the set temperature, the power control device C or the speed control device D is fed back to adjust the output of the induction heating device A or the speed of the metal pipe transport device F.

Fig. 8 shows the high-frequency output kW required for heating from the required heating temperature.
Is a graph for calculating the temperature, and the vertical axis represents the temperature, and the horizontal axis represents the high frequency output divided by the longitudinal speed. Also, a, b, and c represent the cross-sectional area of the metal pipe, and a <b <
c.

As shown in FIG. 8, when the required heating temperature and the pipe cross-sectional area b (b in this embodiment) are determined, the value kW / V of the ratio between the high frequency output kW and the longitudinal running speed V is determined. Since the vertical running speed V is usually determined in advance by the size of pipe, material, etc.,
From this, the required high-frequency output kW for the required heating temperature is determined (see the arrow in Fig. 8).

In FIG. 9, the vertical axis represents the difference between the thin wall portion temperature θ ₁ and the thick wall portion temperature θ ₂ , that is, the uneven heat Δθ (Δθ = θ ₁ −θ ₂ ), and the horizontal axis represents the cooling amount Ar divided by the vertical running speed V. 7 is a graph for obtaining an optimum cooling amount with a numerical value Ar / V. In addition, the cross-sectional area a of the metal pipe a,
b and c are the same as described above, and a <b <c. As shown in FIG. 9, the temperature difference Δθ on the outlet side of the induction heating device A is the cooling amount Ar.
Is a function of the value Ar / V of the ratio between the vertical running speed V and the longitudinal speed V, and the cooling amount is measured by the cooling control device B via the information processing device E so that Δθ is measured by the temperature measuring device G and becomes zero (0). If Ar is controlled, the unbalanced heat Δθ can be further reduced.

Further, the thickness is measured with the thickness gauge H at the same time as the temperature measurement, and when the thin portion temperature θ ₁ is higher than the thick portion temperature θ ₂ (θ ₁ > θ
₂ ) When the cooling amount Ar is increased and vice versa (θ ₁ > θ
₂ ) If the cooling amount Aγ is reduced, the unbalanced heat Δθ can be further reduced.

Although air is used as the refrigerant in the present embodiment, the present invention is not limited to this, and an inert gas, mist or the like may be used.

Further, the temperature measuring device G and the thickness meter H do not necessarily have to be installed on the downstream side of the induction heating device A as in the illustrated example, but they are installed between the coils to speed up the control response. It is also possible to

〔effect〕

In the first aspect of the present invention, the induction heating output and the amount of the refrigerant are controlled based on the detected temperature of the metal tube, and appropriate heating and forced cooling are alternately performed on each part of the metal tube in the longitudinal direction. Therefore, it is possible to achieve uniform heating by reducing the uneven heat distribution of the metal pipe having uneven thickness.

In addition, in the second aspect of the present invention, by combining a plurality of induction heating coils and a refrigerant injector that injects a refrigerant into the metal pipe through each induction heating coil, the effects of the first aspect of the invention can be obtained. In addition, the installation can be downsized, and uniform heating can be effectively performed in a short time, which has the effects of reducing scale loss and cooling the induction heating coil itself.

Furthermore, in the third aspect of the present invention, in addition to the first effect, an induction heating coil whose output is controlled based on the temperature of the metal pipe in the longitudinal direction of the metal pipe, and a metal pipe located downstream thereof Since a plurality of refrigerant injectors whose refrigerant amount is controlled based on the temperature are provided alternately, the existing equipment can be applied to the pipe manufacturing line as it is, and the equipment cost can be reduced.

[Brief description of drawings]

FIG. 1 is a drawing for explaining the principle of the present invention, FIG. 2 is a schematic sectional view of an embodiment of the present invention in which heating and forced cooling can be performed simultaneously, and FIGS. 3 and 4 perform forced cooling. FIG. 5 is a layout diagram showing the second embodiment of the present invention in which heating and forced cooling can be performed alternately, and FIG. 6 is a graph showing the effect of the second embodiment. FIG. 7 is a block diagram showing a control method of the present invention, FIG. 8 is a graph for obtaining a high frequency output, and FIG. 9 is a graph for obtaining an optimum cooling amount. 1 ... Induction heater, 2 ... Induction heating coil, 3 ... Refrigerant injector, 4 ... Refrigerant, 6 ... Metal tube

Claims (3)

[Claims] 1. A temperature of a metal tube is detected in the process of alternately performing induction heating and forced cooling of the metal tube while the metal tube is running longitudinally, and the uneven heat of the metal tube is reduced based on the detected temperature. An induction heating method for a metal tube, comprising controlling an induction heating output for the metal tube and an amount of a refrigerant injected into the metal tube. 2. A refrigerant is jetted onto the surface of a metal pipe through a plurality of annular induction heating coils which are arranged with a predetermined gap in the longitudinal direction of the metal pipe and surround each of the carrying regions, and the induction heating coils. At least one induction heater having a refrigerant injector, and the induction heating coil output and the refrigerant injector for reducing the unbalanced heat of the metal tube based on the temperature of the metal tube measured at the outlet side of the induction heater. And a control device for controlling the amount of the refrigerant injected from the metal pipe into the metal pipe. 3. An annular induction heating coil surrounding a plurality of sets of metal pipes arranged alternately along the longitudinal direction of the metal pipe, and downstream of the induction heating coil in the longitudinal direction of the metal pipe from the induction heating coil. A refrigerant injector that is located and injects a refrigerant into a metal tube, and controls the temperature of the metal tube, and controls the output of the induction heating coil based on the detected temperature and the amount of refrigerant injected from the refrigerant injector into the metal tube. An induction heating device for a metal tube, comprising: