Determination Method for Flexible Mandrel Parameters of Thin-Walled Rectangular Tube Hot Bending Forming Device

Abstract

The present invention provides a hot bending forming device for thin-walled rectangular tubes and a method for determining the parameters of a flexible mandrel, belonging to the field of local hot bending plastic forming. It solves the problem that the existing methods for determining the parameters and positions of mandrels for cold bending forming of tubes are not applicable to determining the key parameters of the mandrel for local hot bending forming of thin-walled rectangular tubes and the position of the flexible mandrel head. It includes a pipe feeding device, a guiding and clamping device, a bending roll device, an inductor, a pipe and a flexible mandrel. The pipe passes through the guiding and clamping device and is arranged in clearance fit with the guiding and clamping device. The bending roll device and the pipe feeding device are respectively arranged at both ends of the guiding and clamping device. The flexible mandrel is arranged inside the pipe and is in clearance fit with the pipe. The flexible mandrel includes a mandrel shank and a flexible mandrel head. It is mainly used for local hot bending forming of thin-walled rectangular tubes.

Description

Technical Field

[0001] The invention belongs to the field of local hot bending plastic forming, and particularly relates to a method for determining the parameters of a flexible mandrel of a hot bending forming device for thin-walled rectangular tubes. Background Art

[0002] During the local hot bending forming process of thin-walled rectangular tubes, the deformation zone is not only affected by the bending moment but also by the axial thrust, which increases the amount of compressive deformation on the inner side of the bend, making the wrinkling problem of the hot bending formed pipe fittings more prominent. Therefore, wrinkling has become the main problem restricting the forming quality and forming limit of local hot bending forming of thin-walled tubes. Whether the gap between the flexible mandrel and the tube, the maximum rotation angle between the mandrel handle and the flexible mandrel head, the maximum rotation angle between the flexible mandrel heads, and the position between the flexible mandrel head and the inductor are reasonable is of great significance to the forming quality and forming limit of local hot bending forming of thin-walled rectangular tubes. Due to different forming mechanisms, the current methods for determining the parameters and positions of the mandrels for cold bending forming of tubes are not applicable to determining the key parameters of the mandrels for local hot bending forming of thin-walled rectangular tubes and the positions of the flexible mandrel heads. Summary of the Invention

[0003] In view of this, the invention aims to propose a method for determining the parameters of a flexible mandrel of a hot bending forming device for thin-walled rectangular tubes to solve the problem that the current methods for determining the parameters and positions of the mandrels for cold bending forming of tubes are not applicable to determining the key parameters of the mandrels for local hot bending forming of thin-walled rectangular tubes and the positions of the flexible mandrel heads.

[0004] To achieve the above object, the invention adopts the following technical solutions: A hot bending forming device for thin-walled rectangular tubes, which includes a pipe fitting feeding device, a guiding and clamping device, a bending roll device, an inductor, and a flexible mandrel. The pipe fitting passes through the guiding and clamping device and is arranged in clearance fit with the guiding and clamping device. The bending roll device and the pipe fitting feeding device are respectively arranged at both ends of the guiding and clamping device. The pipe fitting feeding device is connected to one end of the pipe fitting in a matching manner. The bending roll device moves up and down in the vertical direction and is in contact and cooperation with the pipe fitting. The flexible mandrel is arranged inside the pipe fitting and is in clearance fit with the pipe fitting. The flexible mandrel includes a mandrel handle and a flexible mandrel head. The flexible mandrel head is rotationally connected to one end of the mandrel handle close to the bending roll device through a positioning pin. The inductor is arranged between the guiding and clamping device and the bending roll device and is arranged around the pipe fitting.

[0005] Furthermore, an induction coil is arranged inside the inductor and is arranged around the pipe fitting.

[0006] Furthermore, a cooling device is arranged on one side of the inductor.

[0007] Furthermore, a mandrel fixing device is arranged through the pipe fitting feeding device, and the mandrel fixing device is connected to one end of the mandrel handle.

[0008] Further, a coolant is provided inside the cooling device.

[0009] Further, the number of the flexible mandrel heads is one or more. A flexible mandrel head fixing seat is provided on the flexible mandrel head. The plurality of flexible mandrel heads are rotationally connected through the flexible mandrel head fixing seat. Both the mandrel shank and the flexible mandrel head and between the flexible mandrel heads are hinged connections.

[0010] Further, a plurality of ball screws are provided on the mandrel shank.

[0011] The present invention also provides a method for determining the parameters of the flexible mandrel of a thin-walled rectangular tube hot bending forming device, which includes the following steps:

[0012] Step 1: Determine the maximum rotation angle α between the mandrel shank and the flexible mandrel head, the maximum rotation angle β between the flexible mandrel heads. The pitch between the plurality of flexible mandrel heads is P, and the radius of the flexible mandrel head fixing seat is r, such that the pitch P ≤ 2r. Set the springback amount of the radius of the flexible mandrel head fixing seat to 5 mm, and calculate the maximum rotation angle α between the mandrel shank and the flexible mandrel head and the maximum rotation angle β between the flexible mandrel heads. The formula is:

[0013]

[0014] In the formula, R1 is the target bending radius of the pipe fitting, c is the gap between the inductor and the pipe fitting before bending the pipe fitting, and c1 is the gap between the inductor and the inner side of the bent pipe fitting after bending the pipe fitting;

[0015] Step 2: Determine the gap b between the flexible mandrel and the pipe fitting. The gap b between the flexible mandrel and the pipe fitting > 0.2 mm. Establish a three-dimensional elastoplastic finite element analysis model of the hot bending forming process identical to the actual working conditions to determine the maximum gap between the flexible mandrel and the pipe fitting. The rectangular pipe fitting is a four-node thermo-mechanical coupling curved thin shell element and is a deformable body. Model the external and internal rigid molds as rigid bodies. The pipe mold contact method is selected as the face-to-face contact method, and the friction form of the contact surface is selected as the Coulomb friction model. The mandrel shank and the flexible mandrel head and between the flexible mandrel heads are connected by hinges. The flexible mandrel head can only rotate around the hinge. The gap b between the flexible mandrel and the pipe fitting is sequentially subjected to simulation analysis according to the gap increment of 0.05 mm until wrinkling occurs on the pipe fitting, and the maximum gap value is determined according to the wall thickness reduction amount and the cross-sectional deformation amount. The gap b between the flexible mandrel and the pipe fitting takes the maximum gap value;

[0016] Step 3: According to the gap b between the flexible mandrel and the pipe fitting determined in Step 2, set the protrusion amount of the ball screw so that the protrusion amount of the ball screw is equal to the gap b between the flexible mandrel and the pipe fitting;

[0017] Step 4: Measure the distance k between the position of the peak temperature of the pipe fitting and the boundary of the inductor, and calculate the range of the distance a between the flexible mandrel head and the boundary of the inductor. The formula is as follows:

[0018]

[0019] In the formula is the injection angle of the coolant with respect to the axis, and L is the diameter of the pipe fitting.

[0020] Furthermore, in Step 2, the finite element ABAQUS / Temp-disp, Explicit is used to establish a three-dimensional elastoplastic finite element analysis model of the hot bending forming process identical to the actual working conditions to determine the maximum gap between the flexible mandrel and the pipe fitting.

[0021] Furthermore, in Step 4, the position of the peak temperature of the pipe fitting is determined by means of thermocouple temperature measurement.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] 1. In the present invention, a ball plunger screw is installed on the mandrel shank to avoid the contact between the flexible mandrel and the lower side of the pipe fitting, eliminating the wrinkling phenomenon caused by the frictional force generated by the contact between the flexible mandrel and the pipe fitting during the hot bending forming process, which hinders the material flow of the pipe fitting.

[0024] 2. Under the conditions of given pipe size, material, and bending radius, the present invention can quickly determine the key parameters of the mandrel, providing a scientific basis for the parameter design of the mandrel. This method improves the design efficiency and product performance of the mandrel parameters for the local hot bending forming of thin-walled rectangular pipes, shortens the production cycle of the product, and reduces the manufacturing cost of the product.

[0025] 3. Under the given induction hardening device and pipe fitting parameters, the present invention can quickly determine the reasonable range of the distance between the flexible mandrel head and the boundary of the inductor, which is of great significance for ensuring the quality of the local bending forming of thin-walled rectangular pipes, improving the forming limit, and studying the optimization of hot bending forming process parameters. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings:

[0027] Figure 1 is the schematic diagram of the overall structure of the hot bending forming device for thin-walled rectangular pipes according to the present invention and the schematic diagram of the principle for determining the parameters of the flexible mandrel;

[0028] Figure 2 is the front view schematic diagram of the structure of the mandrel shank of the hot bending forming device for thin-walled rectangular pipes according to the present invention.

[0029] Figure 3 It is a schematic top view of the structure of the mandrel shank of the thin-walled rectangular tube hot bending forming device of the present invention;

[0030] Figure 4 It is a schematic front view of the structure of the flexible mandrel head of the thin-walled rectangular tube hot bending forming device of the present invention;

[0031] Figure 5 It is a schematic top view of the structure of the flexible mandrel head of the thin-walled rectangular tube hot bending forming device of the present invention;

[0032] Figure 6 It is the finite element model of the thin-walled rectangular tube hot bending forming device of the present invention;

[0033] Figure 7 It is a table of the minimum radius of the hot-bent rectangular tube parts at different distances between the flexible mandrel head and the boundary of the inductor of the thin-walled rectangular tube hot bending forming device of the present invention.

[0034] 1 - Mandrel fixing device, 2 - Pipe feeding device, 3 - Pipe fitting, 4 - Ball screw, 5 - Guiding and clamping device, 6 - Mandrel shank, 7 - Positioning pin, 8 - Inductor, 9 - Cooling device, 10 - Flexible mandrel head, 11 - Bending roll device. Detailed implementation manners

[0035] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments can be combined with each other. The described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments.

[0036] See Figure 1-7Description of this embodiment: A hot bending forming device for thin-walled rectangular tubes, which includes a pipe feeding device 2, a guiding and clamping device 5, a bending roller device 11, an inductor 8 and a flexible mandrel. The pipe 3 passes through the guiding and clamping device 5 and is arranged in clearance fit with the guiding and clamping device 5. The bending roller device 11 and the pipe feeding device 2 are respectively arranged at both ends of the guiding and clamping device 5. The pipe feeding device 2 is connected in cooperation with one end of the pipe 3. The bending roller device 11 moves up and down in the vertical direction and is in contact and cooperation with the pipe 3. The flexible mandrel is arranged inside the pipe 3 and is in clearance fit with the pipe 3. The flexible mandrel includes a mandrel handle 6 and a flexible mandrel head 10. The flexible mandrel head 10 is rotatably connected to one end of the mandrel handle 6 close to the bending roller device 11 through a positioning pin 7. The inductor 8 is arranged between the guiding and clamping device 5 and the bending roller device 11 and surrounds the pipe 3. The number of the flexible mandrel heads 10 is one or more. A flexible mandrel head fixing seat is arranged on the flexible mandrel head 10. The multiple flexible mandrel heads 10 are rotatably connected through the flexible mandrel head fixing seat. The mandrel handle 6 and the flexible mandrel head 10, as well as between the flexible mandrel heads 10, are all hinged connections. An induction coil is arranged inside the inductor 8, and the induction coil surrounds the pipe 3. A cooling device 9 is arranged on one side of the inductor 8, and a coolant is arranged inside the cooling device 9.

[0037] In this embodiment, first, the pipe feeding device 2 feeds the pipe 3 axially under the constraint of the guiding and clamping device 5. At the inductor 8, the pipe 3 is locally heated by the induction coil inside the inductor 8. After heating, the pipe 3 will be advanced by the bending roller device 11, and the pipe of the pipe 3 passes through the tangent point. As the advancing distance of the bending roller device 11 increases, the required bending degree and bending radius are obtained. At the same time, the pipe body of the pipe 3 is cooled and quenched by the coolant sprayed out by the cooling device 9. The flexible mandrel is positioned inside the pipe 3 to provide rigid support for the pipe body. The positioning pin 7 is hinged to limit the horizontal movement of the flexible mandrel head 10, and it can only rotate, thus ensuring the normal bending forming of the pipe 3.

[0038] In this embodiment, a mandrel fixing device 1 is arranged through the pipe feeding device 2. The mandrel fixing device 1 is connected to one end of the mandrel handle 6, and the flexible mandrel is positioned inside the pipe 3 through the mandrel fixing device 1.

[0039] In this embodiment, a plurality of ball screws 4 are arranged on the mandrel handle 6 to prevent the mandrel from contacting the lower side of the pipe 3, eliminating the wrinkling phenomenon caused by the frictional force generated by the contact between the flexible mandrel and the pipe 3 during the hot bending forming process, which hinders the material flow of the pipe 3.

[0040] This embodiment also provides a method for determining the parameters of the flexible mandrel of a hot bending forming device for thin-walled rectangular tubes, which includes the following steps:

[0041] Step 1: Determine the maximum rotation angle α between the mandrel shank 6 and the flexible mandrel head 10, and the maximum rotation angle β between the flexible mandrel heads 10. To ensure that there is no interference during the rotation of the mandrel head, the pitch P ≤ 2r, where P is the pitch between the flexible mandrel heads 10 and r is the radius of the flexible mandrel head fixing seat. Since the temperature in the hot bending forming area is high and the springback of the formed part of the pipe fitting 3 is very small, the set radius springback amount is 5 mm. Calculate the maximum rotation angle α between the mandrel shank 6 and the flexible mandrel head 10 and the maximum rotation angle β between the flexible mandrel heads 10. The formula is

[0042]

[0043] In the formula, R1 is the target bending radius of the pipe fitting 3, c is the gap between the inductor 8 and the pipe fitting 3 before bending the pipe fitting 3, and c1 is the gap between the inductor 8 and the inner side of the bent pipe fitting 3 after bending the pipe fitting 3;

[0044] Step 2: Determine the gap b between the flexible mandrel and the pipe fitting 3. Considering the dimensional tolerance of the pipe fitting 3, if the flexible mandrel can be installed into the pipe fitting 3 without obstruction, then the gap b between the flexible mandrel and the pipe fitting 3 > 0.2 mm. Use the finite element software ABAQUS / Temp-disp, Explicit to establish a three-dimensional elastoplastic finite element analysis model of the hot bending forming process identical to the actual working conditions to determine the maximum gap between the flexible mandrel and the pipe fitting 3. The rectangular pipe fitting 3 is a four-node thermo-mechanical coupling shell element and is a deformable body. Model the external and internal rigid dies as rigid bodies. Select the face-to-face contact method for the contact between the pipe die and use the Coulomb friction model for the friction form of the contact surface. The mandrel shank 6, the flexible mandrel heads 10, and between the flexible mandrel heads 10 are connected by hinges. The flexible mandrel head 10 can only rotate around the hinge. The gap b between the flexible mandrel and the pipe fitting 3 is incremented by 0.05 mm each time for simulation analysis until wrinkling occurs on the pipe fitting 3, and determine the maximum gap value based on the wall thickness reduction and cross-sectional deformation. The gap b between the flexible mandrel and the pipe fitting 3 takes the maximum gap value;

[0045] Step 3: According to the gap b between the flexible mandrel and the pipe fitting 3 determined in Step 2, set the extension amount of the ball screw 4 so that the extension amount of the ball screw 4 is equal to the gap b between the flexible mandrel and the pipe fitting 3;

[0046] Step 4: Measure the position of the temperature peak of the pipe fitting (3) by means of thermocouple temperature measurement, and measure the distance k between the position of the temperature peak of the pipe fitting 3 and the boundary of the inductor 8. Calculate the range of the distance a between the center of the pin hole of the flexible mandrel head 10 and the boundary of the inductor 8. The formula is

[0047]

[0048] In the formula is the injection angle of the coolant with respect to the axis, and L is the diameter of the pipe fitting 3.

[0049] In this embodiment, a B1500HS rectangular pipe fitting with specifications of 30 (L) × 40 × 1.5 mm is taken as an example. Given a bending radius R1 = 70 mm, and within the given distance range between the flexible mandrel head 10 and the induction boundary, the bending radius is less than the bending radius without a flexible mandrel, which is 291.1 mm. The feeding speed of the pipe fitting 3 is 600 mm / min, the advancing speed of the bending roll device 11 is 150 mm / min, the inductor 8 is made of a copper pipe with specifications of 10 × 10 × 1 mm, and the gap between it and the pipe fitting 3 is 8 mm. The injection angle of the coolant with respect to the axis is 50°, and the gap c1 between the inductor 8 and the inner side of the bent pipe fitting 3 after bending is 2.8 mm.

[0050] This embodiment includes the following steps:

[0051] Step 1: Determine the maximum rotation angle α between the mandrel shank 6 and the flexible mandrel head 10, and the maximum rotation angle β between the flexible mandrel heads 10. The radius r of the flexible mandrel head fixing seat is 8 mm, and the pitch P between the flexible mandrel heads 10 is taken as 18.25 mm. The maximum rotation angle α between the mandrel shank 6 and the flexible mandrel head 10, and the maximum rotation angle β between the flexible mandrel heads 10 are calculated by the formula to obtain

[0052]

[0053] Step 2: Determine the gap b between the flexible mandrel and the pipe fitting 3. Considering the dimensional tolerance of the pipe fitting 3, if the flexible mandrel can be installed into the pipe fitting 3 without obstacles, then the gap b between the flexible mandrel and the pipe fitting 3 > 0.2 mm. Use the finite element software ABAQUS / Temp-disp, Explicit to establish a three-dimensional elastoplastic finite element analysis model of the hot bending forming process that is the same as the actual working condition (as Figure 6 shown) to determine the maximum gap between the flexible mandrel and the pipe fitting 3. The rectangular pipe fitting is a four-node thermomechanical coupling curved thin shell element and is a deformable body. The external and internal rigid molds are modeled as rigid bodies. The contact between the pipe mold is a face-to-face contact method and the Coulomb friction model. The mandrel shank 6, the flexible mandrel heads 10, and between the flexible mandrel heads 10 are connected by hinges. The flexible mandrel head 10 can only rotate around the hinge. Select the gap between the inductor 8 and the flexible mandrel head 10 to be 0.5 mm. The number of flexible mandrel heads 10 is one. The gap b between the flexible mandrel and the pipe fitting 3 is simulated and analyzed successively with a gap increment of 0.05 mm. According to the analysis results: when the gap is 0.35 mm, the rectangular pipe fitting has an obvious wrinkling trend. The cross-sectional deformation rate is between (5.5%, 11.5%), which is within an acceptable range. Select the gap of 0.35 mm between the flexible mandrel and the pipe fitting 3 for research.

[0054] Step 3: According to the gap b between the flexible mandrel and the pipe fitting 3 determined in Step 2, set the extension of the ball screw 4 to be 0.35 mm.

[0055] Step 4: The position of the temperature peak of the pipe fitting 3 is determined by means of thermocouple temperature measurement. The distance k between the position of the temperature peak of the pipe fitting 3 and the boundary of the inductor 8 is measured to be 0.5 mm. According to the formula

[0056]

[0057] Obtain the range (-1.94, 4.89) of the distance a between the center of the pin hole of the flexible mandrel head 10 and the boundary of the inductor 8.

[0058] This embodiment was tested under the conditions that the power heating power is 45 KW, the heating frequency is 14250 Hz, the number of flexible mandrel heads 10 is one, and the gap b between the flexible mandrel and the pipe fitting 3 is 0.35 mm. The minimum radius of the thermally bent rectangular pipe fitting 3 at different distances between the flexible mandrel head 10 and the boundary of the inductor 8 was obtained (as Figure 7 shown). The cross-sectional deformation rate of all the thermally bent rectangular pipe fittings 3 is within 14.3%. When the distance between the flexible mandrel head 10 and the boundary of the inductor 8 is 0.5 mm, the minimum bending radius of the rectangular pipe is obtained as 69.6 mm, reaching the given bending radius. When the distance between the flexible mandrel head 10 and the boundary of the inductor 8 is -2.5 mm, the minimum bending radius of the rectangular pipe is obtained as 263 mm, and the role of the flexible mandrel in improving the forming limit is limited. When the distance between the flexible mandrel head 10 and the boundary of the inductor 8 is 5 mm, the minimum bending radius of the rectangular pipe is obtained as 308 mm. The flexible mandrel not only fails to play a role in improving the forming limit, but instead reduces the forming limit, verifying the effectiveness of the method for determining the key parameters and positions of the flexible mandrel for local thermal bending forming of thin-walled rectangular pipes.

[0059] The embodiments of the present invention disclosed above are only used to help illustrate the present invention. The embodiments do not describe all the details in detail, nor do they limit the invention to the specific embodiments described. According to the content of this specification, many modifications and variations can be made. These embodiments are selected and specifically described in this specification to better explain the principle and practical application of the present invention, so that those skilled in the art can understand and utilize the present invention well.

Claims

1. A method for determining the parameters of a flexible mandrel of a hot bending forming device for thin-walled rectangular tubes, characterized in that: The hot bending forming device for thin-walled rectangular tubes includes a pipe feeding device (2), a guiding and clamping device (5), a bending roll device (11), an inductor (8) and a flexible mandrel. The pipe (3) passes through the guiding and clamping device (5) and is arranged in clearance fit with the guiding and clamping device (5). The bending roll device (11) and the pipe feeding device (2) are respectively arranged at both ends of the guiding and clamping device (5). The pipe feeding device (2) is connected with one end of the pipe (3) in a matching manner. The bending roll device (11) moves up and down in the vertical direction and is in contact and cooperation with the pipe (3). The flexible mandrel is arranged inside the pipe (3) and is in clearance fit with the pipe (3). The flexible mandrel includes a mandrel handle (6) and a flexible mandrel head (10). The flexible mandrel head (10) is rotationally connected to one end of the mandrel handle (6) close to the bending roll device (11) through a positioning pin (7). The inductor (8) is arranged between the guiding and clamping device (5) and the bending roll device (11) and is arranged around the pipe (3). The method for determining the parameters of the flexible mandrel of the hot bending forming device for thin-walled rectangular tubes includes the following steps: Step 1: Determine the maximum rotation angle α between the mandrel handle (6) and the flexible mandrel head (10), the maximum rotation angle β between the flexible mandrel heads (10). The pitch between multiple flexible mandrel heads (10) is P, and the radius of the flexible mandrel head fixing seat is r, such that P ≤ 2r. Set the springback amount of the radius of the flexible mandrel head fixing seat to 5 mm, and calculate the maximum rotation angle α between the mandrel handle (6) and the flexible mandrel head (10) and the maximum rotation angle β between the flexible mandrel heads (10). The formula is: In the formula, R1 is the target bending radius of the pipe (3), c is the gap between the inductor (8) and the pipe (3) before bending of the pipe (3), and c1 is the gap between the inductor (8) and the inner side of the bent part of the pipe (3) after bending of the pipe (3). Step 2: Determine the gap b between the flexible mandrel and the pipe (3). The gap b between the flexible mandrel and the pipe (3) is > 0.2 mm. Establish a three-dimensional elastoplastic finite element analysis model of the hot bending forming process identical to the actual working conditions to determine the maximum gap between the flexible mandrel and the pipe (3). The rectangular pipe (3) is a four-node thermomechanical coupling curved thin shell element and is a deformable body. Model the external and internal rigid dies as rigid bodies. The pipe-die contact method is selected as the face-to-face contact method, and the friction form of the contact surface is selected as the Coulomb friction model. The mandrel handle (6) and the flexible mandrel head (10) and between the flexible mandrel heads (10) are connected by hinges. The flexible mandrel head (10) can only rotate around the hinge. The gap b between the flexible mandrel and the pipe (3) is successively simulated and analyzed with a gap increment of 0.05 mm until wrinkling occurs in the pipe (3), and the maximum gap value is determined according to the wall thickness reduction amount and the cross-section deformation amount. The gap b between the flexible mandrel and the pipe (3) takes the maximum gap value. Step 3: Set the extension of the ball plunger screw (4) according to the gap b between the flexible mandrel and the pipe fitting (3) determined in Step 2, so that the extension of the ball plunger screw (4) is equal to the gap b between the flexible mandrel and the pipe fitting (3). Step 4: Measure the distance k between the position of the temperature peak of the pipe fitting (3) and the boundary of the inductor (8), and calculate the range of the distance a between the flexible mandrel head (10) and the boundary of the inductor (8). The formula is: In the formula is the injection angle of the coolant with the axis, and L is the diameter of the pipe fitting (3).

2. The method for determining the parameters of the flexible mandrel of the thin-walled rectangular tube hot bending forming device according to claim 1, characterized in that: An induction coil is provided inside the inductor (8), and the induction coil is arranged around the pipe fitting (3).

3. The method for determining the parameters of the flexible mandrel of the thin-walled rectangular tube hot bending forming device according to claim 1, characterized in that: A cooling device (9) is provided on one side of the inductor (8).

4. The method for determining the parameters of the flexible mandrel of the thin-walled rectangular tube hot bending forming device according to claim 1, wherein: A mandrel fixing device (1) is penetrated inside the pipe feeding device (2), and the mandrel fixing device (1) is connected to one end of the mandrel shank (6).

5. The method for determining the flexible mandrel parameters of the thin-walled rectangular tube hot bending forming device according to claim 3, wherein: Coolant is provided inside the cooling device (9).

6. The method for determining the parameters of the flexible mandrel of the thin-walled rectangular tube hot bending forming device according to claim 1, wherein: The number of the flexible mandrel heads (10) is one or more. A flexible mandrel head fixing seat is provided on the flexible mandrel head (10). The multiple flexible mandrel heads (10) are rotationally connected through the flexible mandrel head fixing seat. The mandrel shank (6) and the flexible mandrel head (10) as well as between the flexible mandrel heads (10) are all hinged connections.

7. The method for determining the parameters of the flexible mandrel of the thin-walled rectangular tube hot bending forming device according to claim 1, wherein: A plurality of ball plunger screws (4) are arranged on the mandrel shank (6).

8. The method for determining the parameters of the flexible mandrel of the thin-walled rectangular tube hot bending forming device according to claim 1, characterized in that: In Step 2, the finite element ABAQUS / Temp-disp, Explicit is used to establish a three-dimensional elastoplastic finite element analysis model of the hot bending forming process identical to the actual working conditions to determine the maximum gap between the flexible mandrel and the pipe fitting (3).

9. The method for determining the parameters of the flexible mandrel of the thin-walled rectangular tube hot bending forming device according to claim 1, wherein: In Step 4, the position of the temperature peak of the pipe fitting (3) is determined by the method of thermocouple temperature measurement.

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