JP2017123273A - Three-dimensional hot bending quenching apparatus, manufacturing method of three-dimensional hot bending quench-processed member, and manufacturing method of structural member for automobile - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress nonuniformity of mechanical properties of a 3DQ processed member and suppress fatigue failure of the 3DQ processed member without deteriorating manufacturing efficiency and shape controllability.SOLUTION: A three-dimensional hot bending quenching apparatus 10 includes: a feeding device 12, an induction heating coil 30; a cooling device 14; and a processing force applying device 15. An inner circumference 50 of the induction heating coil 30 has a plurality of straight portions 52 opposed parallel to each of a plurality of flat plate portions 62 of a steel tube 60 to be inserted and a connecting portion 54 connecting adjacent straight portions 52. The connecting portion 54 has a shape expanding outward of the coil than a corner shape S formed by extending the adjacent straight portions 52.SELECTED DRAWING: Figure 6

Description

  The present invention relates to a three-dimensional hot bending and quenching apparatus, a method for manufacturing a three-dimensional hot bending and quenching member, and a method for manufacturing a structural member for an automobile.

  A three-dimensional hot bending and quenching (3DQ) technique that can manufacture a steel pipe member having a complicated shape without using a mold is known. 3DQ is used, for example, in the manufacture of structural materials for automobiles. The structural material manufactured by 3DQ processing is characterized by light weight and high strength.

  3DQ is the following method. That is, while feeding the steel pipe that is the raw material, the steel pipe is heated locally to the Ac3 transformation point or higher by the induction heating device, and the steel pipe is cooled (quenched) quickly by the cooling device downstream of the induction heating device in the feed direction of the steel pipe. To do. At this time, a local high temperature portion is generated between the induction heating device and the cooling device in the steel pipe. The steel pipe is hot deformed by applying a processing force to the high temperature portion, and quenching and shape fixing are performed by cooling with a cooling device.

  As a processing force applying means for applying a processing force to the high temperature part, a movable roller die arranged downstream of the cooling means in the steel pipe feed direction as in Patent Document 1, and a downstream of the steel pipe feed direction as in Patent Document 2 Examples include a chuck and a manipulator attached to the steel pipe end, and a chuck and a manipulator attached to the steel pipe end on the upstream side in the steel pipe feed direction as disclosed in Patent Document 3.

  In 3DQ, in addition to bending, twisting is also possible. Patent Document 1 and Patent Document 3 disclose that twist processing is possible with 3DQ. Patent Document 4 discloses a torsion member using 3DQ.

International Publication No. 2006/093006 International Publication No. 2010/050460 International Publication No. 2011/007810 International Publication No. 2010/084898

  The present inventors used a square steel pipe as a material, and performed bending and twisting with the 3DQ apparatus of FIG.

  As a result of repeated experiments and observations, if the steel pipe is bent or twisted greatly, the corners of the steel pipe may pass through the cooling means in a red-hot state. The inventors noticed that this could happen. Moreover, even if the temperature is not so high that the steel pipe becomes red hot after passing through the cooling means, the quenched portion may be tempered or not quenched. Therefore, the inventors considered that in order to prevent unevenness in mechanical properties, it is necessary to carefully manage the temperature change at the corners of the steel pipe during bending or twisting.

  Of course, if the steel pipe comes out of the cooling means after the steel pipe is lowered after the feeding speed of the steel pipe is sufficiently cooled by the cooling means, non-uniform mechanical properties will not occur in the processed member. However, in that case, the production efficiency is lowered by the amount of the feed rate of the steel pipe being lowered.

  Of course, if the cooling range for the steel pipe by the cooling means is lengthened, it can be sufficiently cooled. However, in that case, the softened high temperature portion generated in the steel pipe becomes longer toward the inside of the cooling means, and the shape control by the processing force applying means becomes difficult.

  Inventor of the root cause of passing through the cooling means in a state where the corner of the steel pipe is red hot in order to eliminate the non-uniformity of the mechanical properties of the steel pipe processed without reducing the production efficiency and maintaining the shape controllability. Thought.

  The first cause is uneven heating of the steel pipe. When the heating means is induction heating in 3DQ, the corners are particularly heated when the square steel pipe is heated. This is a phenomenon called the edge effect. Even if the gap (clearance) between the steel pipe and the induction heating coil of the induction heating device is uniform, the corners, edges, and sharp parts (corners, etc.) are particularly heated. It is to be done. In particular, the edge effect becomes prominent when the radius of curvature of the tip such as a corner is small.

  The second cause is poor cooling of the steel pipe corner during bending or twisting.

  FIG. 2 is a cross-sectional view of a general cooling device (cooling means). FIG. 3 is a view of the cooling device as viewed from the downstream in the steel pipe feed direction to the upstream. In order to avoid collision of the cooling waters and impairing the collision pressure of the cooling water on the steel pipe, the sprayed cooling water is as orthogonal as possible to the surface of the steel pipe to be processed as seen from the direction in FIG. A cooling water outlet of the cooling device is provided so that the cooling waters do not collide with each other.

  FIG. 4 shows a cross section of the steel pipe passing through the cooling device. The thin line rectangle is the position of the cross section of the steel pipe when neither twisting nor bending is performed. The dotted rectangle is the position of the cross section of the steel pipe when twisted. The bold rectangle is the position of the cross section of the steel pipe when twisted and bent. The change in the position of the steel pipe corner where the uneven cooling of the steel pipe occurs is the sum of the change in position due to twisting and the change in position due to bending. In the corner corresponding to the U portion in FIG. 3, as a result of the shift of the corner portion, the cooling water interferes before the cooling water collides with the corner, and the cooling water does not collide with the corner at a predetermined pressure. Cooling capacity is low. In the corner portion corresponding to the L portion in FIG. 3, the cooling water from the side surface does not hit the corner portion, and the cooling water from the bottom hits the side surface side of the steel pipe corner portion, but the angle formed with the side surface of the steel pipe is small. However, since the steel pipe cannot be directly cooled by breaking through the water vapor film on the surface of the steel pipe, the cooling capacity is low.

  As described above, the corners of the steel pipe may not be properly exposed to the cooling water when twisted or bent, and there is a concern that cooling will be insufficient.

  That is, the corner portion of the steel pipe is excessively heated due to the first cause, and overheating of the corner portion of the steel pipe becomes obvious when bending or twisting is performed due to the second cause. The inventors were not aware of the first cause at first, but in order to investigate the second cause, a thermocouple was attached to the flat plate portion and the corner of the steel pipe, and the first cause was investigated. I noticed that the existence of and its effects can be so large that it cannot be ignored. The first cause occurs without bending or twisting in 3DQ. Therefore, take measures to prevent the corners of the steel pipe from being overheated. Reviewing the correct feed rate will raise the manufacturing efficiency of the entire 3DQ process.

  In addition, when a 3DQ-processed steel pipe is subjected to a fatigue test, fatigue failure may occur. In this case, Cu (copper) is often detected as the starting point of fatigue failure. The inventors consider this phenomenon as follows.

  That is, Cu may adhere to the surface of the steel pipe used as a raw material. When the steel pipe with Cu attached to the surface is induction-heated to about 1000 ° C., Cu melts and melts from the steel pipe surface into the crystal grain boundaries of the steel pipe. When Cu dissolves into the crystal grain boundary, the crystal grain boundary becomes weak. When a bending force is applied to a steel pipe in which Cu has melted and the crystal grain boundary has become weak, fine cracks are formed along the crystal grain boundary. When a load is repeatedly applied to a steel pipe with fine cracks, the fine cracks start and fatigue fracture occurs. The melting point of Cu is 1085 ° C., but this phenomenon occurs at a lower heating temperature because the source of Cu is brass having a melting point lower than that of Cu used in a steel pipe transfer line. Have guessed.

  In order to apply a 3DQ processed member to a member that requires fatigue resistance, such as a member applied to a place where vibration is transmitted, such as a structural member for an automobile (particularly a suspension part), it is necessary to solve this fatigue problem. .

  Therefore, as a countermeasure, it is conceivable to suppress the heating by induction heating to a temperature range in which Cu does not melt. However, as described above, when the square steel pipe is induction-heated, the corner portion is overheated and a temperature difference from the other portion (flat plate portion) is generated. For example, the heating temperature of the flat plate portion is melted by Cu. Even if it controls so that it may suppress in the temperature range which does not carry out, it will be heated to the temperature which Cu fuse | melts in a corner | angular part. As a result, there is a concern that a steel pipe that is prone to fatigue failure is produced at the corners of the square steel pipe.

  In view of the above circumstances, the problem to be solved by the present invention is to eliminate non-uniformity in the mechanical characteristics of the 3DQ processed member without reducing the manufacturing efficiency and shape controllability by eliminating the first cause. Moreover, it is suppressing the fatigue failure of a 3DQ process member.

  A 3DQ device according to the present invention includes a feeding device that feeds a steel pipe having a corner portion and a plurality of flat plate portions in an axial direction, and a steel pipe fed by the feeding device is inserted, and induction heating that heats the inserted portion of the steel pipe. A three-dimensional heat comprising a coil, a cooling device that is disposed downstream of the induction heating coil in the feeding direction and that cools a portion heated by the induction heating coil, and a processing force applying device that applies a processing force to the steel pipe. In the inter-bending quenching device, the inner circumference of the induction heating coil is a plurality of straight portions facing in parallel to each of a plurality of flat plate portions of the steel pipe to be inserted, and a connecting portion connecting adjacent straight portions, The connecting portion has a shape that spreads outside the coil rather than a square shape formed by extending the adjacent straight portions.

  In this 3DQ device, the connecting portion on the inner periphery of the induction heating coil has a shape that extends to the outside of the coil rather than a rectangular shape that is formed by extending adjacent straight portions. For this reason, the clearance between the steel pipe inserted through the induction heating coil and the induction heating coil is increased at the corners of the steel pipe. As a result, overheating of the corner portion of the steel pipe by the induction heating coil can be suppressed.

  Moreover, the manufacturing method of the three-dimensional hot bending-quenching member according to the present invention is a method of manufacturing a three-dimensional hot bending-quenching member using the three-dimensional hot bending-quenching device described above. The material sent by the apparatus is a steel pipe having a cross-sectional shape substantially similar to a polygon formed by extending the plurality of linear portions on the inner periphery of the induction heating coil.

  Moreover, the manufacturing method of the three-dimensional hot bending quenching process member which concerns on the other aspect of this invention heats the flat plate part and corner | angular part of a steel pipe to the temperature more than Ac3 transformation point and 950 degrees C or less by the said induction heating coil. .

  The 3DQ apparatus according to the present invention is excellent in that it can suppress non-uniformity of mechanical properties of a 3DQ processed member without reducing manufacturing efficiency and shape controllability, and can suppress fatigue failure of the 3DQ processed member. Has an effect.

It is a perspective view which shows the whole structure of 3DQ apparatus. It is sectional drawing of the common cooling device for cooling a square steel pipe. It is the figure which looked at the cooling device of FIG. 2 toward the upstream from the steel pipe feed direction. It is a figure for demonstrating the position of the steel pipe cross section which passes a cooling device, when bending and twisting are carried out. It is a cross-sectional view of the square steel pipe used as a raw material. It is a figure which shows a mode that the induction heating coil of this embodiment was seen toward the upstream from the steel pipe feed direction with the cross section of the steel pipe penetrated. It is a figure which shows a mode that the conventional induction heating coil was seen toward the upstream from the steel pipe feed direction with the cross section of the steel pipe penetrated. It is a graph which shows the relationship between the corner | angular part outer radius of curvature at the time of heating a steel pipe using the conventional induction heating coil, and the temperature difference of a corner | angular part and a flat plate part. 7 is a graph showing a relationship between a clearance difference C2 (coil gap difference C2) in FIG. 6 and a temperature difference between a corner portion and a flat plate portion. (A) shows a part of the inner periphery of the induction heating coil according to Modification 1, and (B) shows a part of the inner periphery of the induction heating coil according to Modification 2.

  Hereinafter, an embodiment of a three-dimensional hot bending quenching apparatus (hereinafter abbreviated as a 3DQ apparatus) according to the present invention will be described with reference to the drawings.

<3DQ equipment>
FIG. 1 shows the overall configuration of the 3DQ apparatus 10. As shown in this figure, the 3DQ device 10 of this embodiment includes a feeding device 12, an induction heating coil 30, a cooling device 14, a movable roller die 16 and a support roller 18 as a processing force applying device 15, and It has.

(Feeding device)
The feeding device 12 includes a chuck 13 that holds the rear end (upstream end portion in the feeding direction) of the steel pipe 60, and the steel pipe 60 is fed in the axial direction when the chuck 13 is pushed out. The chuck 13 has a structure corresponding to the cross-sectional shape of the steel pipe 60, and the chuck 13 according to the present embodiment has a structure capable of gripping the end of a rectangular steel pipe (steel pipe 60) having a rectangular cross section to be described later. .

(Induction heating coil)
The induction heating coil 30 is provided so that the steel pipe 60 fed by the feeding device 12 is inserted. When the steel pipe 60 is inserted into the induction heating coil 30, the portion of the steel pipe 60 inserted through the induction heating coil 30 is rapidly heated by the induction heating coil 30. The detailed structure of the induction heating coil 30 of this embodiment will be described in detail later.

(Cooling system)
A cooling device 14 is provided in the vicinity of the induction heating coil 30 downstream of the induction heating coil 30 in the feeding direction. The cooling device 14 rapidly cools the steel pipe 60 rapidly heated by the induction heating coil 30. Thereby, the steel pipe 60 is quenched to improve the strength. As a specific structure of the cooling device 14, for example, the cooling device shown in FIGS. 2 and 3 already described can be used.

(Processing force application device)
The processing force application device 15 includes a movable roller die 16 and a support roller 18. The support roller 18 is provided in the vicinity of the induction heating coil 30 upstream of the induction heating coil 30 in the feed direction, and supports the steel pipe 60 so as to be movable in the axial direction. On the other hand, the movable roller die 16 is provided downstream in the feed direction of the cooling device 14 and is configured to be movable while holding the steel pipe 60. When the movable roller die 16 moves to apply a processing force to the steel pipe 60 and the support roller 18 receives a processing reaction force from the steel pipe 60, the softened high temperature portion generated in the steel pipe 60 is deformed.

(Steel pipe used as material)
FIG. 5 shows a cross-sectional shape of a steel pipe 60 that is a material. As shown in this figure, the steel pipe 60 is a rectangular steel pipe having a rectangular cross-sectional shape, and includes four flat plate portions 62 and four corner portions 64 that connect the flat plate portions 62 to each other. The angle formed by the adjacent flat plate portions 62 is 90 degrees. As the steel pipe 60, for example, an electric resistance steel pipe (electric resistance welding steel pipe) is used.

(Induction heating coil)
Next, the induction heating coil 30 of this embodiment will be described.

  FIG. 6 shows a state in which the induction heating coil 30 of the present embodiment is viewed from the downstream side in the steel pipe feeding direction to the upstream side along with the cross section of the steel pipe 60. As shown in this figure, the induction heating coil 30 is provided in a rectangular coil body 32 having a shape substantially similar to a cross section of a steel pipe 60 as a material, and a winding start portion and a winding end portion of the coil body 32. And a pair of terminals 34. An insulator (not shown) is provided between the pair of terminals 34. The coil body 32 illustrated in FIG. 6 is a one-turn coil, but may be a multiple-turn coil.

(Principal part: shape of inner circumference: shape that suppresses edge effect)
The induction heating coil 30 is characterized by the shape of its inner periphery 50. That is, the shape of the inner periphery 50 of the induction heating coil 30 is determined so that the clearance C between the outer surface 60 </ b> A of the steel pipe 60 and the inner periphery 50 of the induction heating coil 30 is wide at the corner 64 of the steel pipe 60. In addition, the inner periphery 50 here refers to the innermost edge of the induction heating coil 30 when the induction heating coil 30 is viewed from the steel pipe feeding direction (the direction shown in FIG. 6).

  Specifically, the inner periphery 50 of the induction heating coil 30 includes four straight portions 52 and four connecting portions 54 that connect the adjacent straight portions 52 to each other. Among these, the linear part 52 is a part which opposes in parallel with the flat plate part 62 of the steel pipe 60 penetrated. Therefore, assuming a shape in which the four straight portions 52 are extended, the shape is substantially similar to the cross-sectional shape of the steel pipe 60 to be inserted. Further, the length of the straight portion 52 is substantially equal to the length of the flat plate portion 62 of the steel pipe 60 facing each other (the length when viewed in the cross section shown in FIG. 6), or the length of the flat plate portion 62. Is a little short (about 80% to 100% length).

  On the other hand, the connecting portion 54 that connects the adjacent straight portions 52 is parallel to two right-angle portions 56 extending perpendicularly from the end portion of the straight-line portion 52 to the outside of the coil, and parallel to the extension line of the straight-line portion 52 from the coil outer end of the right-angle portion 56. And two parallel portions 58 extending in the direction.

  By being formed in this way, the connecting portion 54 has a shape that extends to the outside of the coil rather than the rectangular shape S that is formed by extending the adjacent linear portions 52 (shown in the lower right of FIG. 6). (Refer to the connection part 54 and the dashed line S).

<Action and effect>
Next, the operation and effect of this embodiment will be described.

  First, in order to investigate the relationship between the edge effect and the corner outer radius of curvature R (hereinafter abbreviated as corner R), the material steel pipe 60 shown in FIG. 5 is heated using the conventional induction heating coil 300 shown in FIG. And the temperature difference of the corner | angular part 64 and the flat plate part 62 was measured.

  FIG. 7 shows the clearance between the conventional induction heating coil 300 and the steel pipe 60. The clearance between the induction heating coil 300 facing the side of the steel pipe cross section is set to a predetermined value (for example, 3 mm), and the corner 64 is extended by extending the inner peripheral side of the induction heating coil facing the cross section of the square steel pipe. Yes. The width W of the steel pipe is 50 mm, the height H is 40 mm, the wall thickness t is 1.4 mm, and the clearance between the induction heating coil 300 and the steel pipe 60 is 3 mm. The heating temperature was set so that the temperature of the flat plate portion 62 would be 1100 ° C.

  FIG. 8 shows the relationship between the corner R and the temperature difference between the corner portion 64 and the flat plate portion 62. As shown in this figure, the smaller the angle R of the steel pipe 60, the higher the temperature of the corner 64, and the edge effect increased. When the angle R was 5 mm, the temperature difference was about 150 ° C., and overheating was about 15% with respect to the heating target temperature (1100 ° C.).

  Next, in the case of a condition (edge R is small) that the edge effect cannot be ignored, a material (steel pipe) having an angle R of 5 mm is used to verify how much the coil at the corner should be escaped. Heating was performed using an induction heating coil 30 according to the present invention shown in FIG.

  Specifically, the clearance difference C2 (coil gap difference C2) of the portion (connecting portion 54) facing the position of the corner 64 is changed with respect to the portion (straight portion 52) facing the flat plate portion 62 of the steel pipe 60. Then, the temperature difference between the corner portion 64 and the flat plate portion 62 was measured. The results are shown in FIG.

  As shown in this figure, the temperature difference was about 110 ° C. when C2 = 1 mm, the temperature difference was about 80 ° C. when C2 = 2 mm, and about 60 ° C. when C3 = 3 mm. . Thus, the temperature difference between the corner portion 64 and the flat plate portion 62 is reduced by increasing the clearance of the portion facing the corner portion 64.

  As can be seen from the above experimental results, according to the 3DQ device 10 of the present embodiment, by appropriately setting the shape of the connecting portion 54 of the inner periphery 50 of the induction heating coil 30, the flat plate portion 62 of the steel pipe 60 and A temperature difference from the corner 64 can be reduced. Further, since the Ac3 transformation point is approximately 800 to 900 ° C. depending on the composition of the steel material, the heating temperature of the steel pipe 60 by the induction heating coil 30 is set to the Ac3 transformation point or more 950 for both the flat plate portion 62 and the corner portion 64. It is also possible to set the temperature below ℃. According to such a heating temperature, since the Cu adhering to the surface of the steel pipe 60 is suppressed from melting, it is possible to manufacture a 3DQ processed member that hardly causes fatigue failure.

(Control device)
The 3DQ device 10 includes a control device (not shown), and the control device is a steel pipe feed speed by the feed device 12, a high-frequency current flowing through the induction heating coil 30, and a machining force applied to the steel pipe 60 by the machining force application device 15. Control power and so on. The heating temperature of the steel pipe can be set by appropriately setting the shape of the inner periphery 50 of the induction heating coil 30 and controlling the high-frequency current, the feed rate, etc. by the control device.

[Modification of induction heating coil]
In the induction heating coil 30 of the above embodiment, the shape formed by extending the plurality of straight portions 52 is rectangular, and the connecting portion 54 is composed of a right angle portion 56 and a parallel portion 58. The “induction heating coil” of the present invention is not limited to this. For example, induction heating coils 130 and 230 according to modifications described below may be used.

  FIG. 10A shows a part of the inner periphery 50 of the induction heating coil 130 according to the first modification. In the first modification, the angle formed by the adjacent linear portions 52 is not 90 degrees, but is about 120 degrees. In the steel pipe 60 to be inserted, the angle formed by the adjacent flat plate portions 62 is 120 degrees, and the straight portion 52 and the flat plate portion 62 are opposed in parallel. Moreover, the connection part 54 in the modified example 1 is not a square shape as in the above embodiment, but a rounded shape.

  FIG. 10B shows a part of the inner periphery 50 of the induction heating coil 230 according to the second modification. In the modified example 2, although a part of the connecting portion 54 (point P in the figure) overlaps with the rectangular shape S formed by extending the adjacent straight portion 52, the connecting portion 54 as a whole is adjacent to the straight line. The rectangular shape S formed by extending the portion 52 is wider than the coil. What the connection part 54 was made into such a shape has the same effect, and is contained in this invention.

[Supplementary explanation of the above embodiment]
In the feeding device 12 of the above embodiment, the rear end of the steel pipe 60 is pushed out by the chuck 13 and the steel pipe 60 is inserted through the induction heating coil 30 whose position is fixed. It is not limited to. The feeding device may be a device that moves (relatively moves) the steel pipe 60 in the axial direction with respect to the induction heating coil 30. For example, the induction heating coil 30 without moving the chuck 13 that holds the rear end of the steel pipe 60. May move the steel pipe 60 in the axial direction with respect to the induction heating coil 30.

  Moreover, the processing force provision apparatus for giving a processing force to a steel pipe is not specifically limited. The processing force imparting device may be a chuck and a manipulator attached to a steel pipe end on the downstream side in the steel pipe feed direction, or may be a chuck and manipulator attached to the steel pipe rear end on the upstream side in the steel pipe feed direction. .

  Further, the feeding device and the processing force applying device of the present invention may be configured by three manipulators that are controlled in cooperation with each other. Specifically, there is one manipulator that holds the rear end of the steel pipe, one manipulator that holds the induction heating coil and the cooling device, and one manipulator that holds the tip of the steel pipe. May be configured.

  Moreover, if the 3DQ device of the present invention is used, various structural members for automobiles can be manufactured. For example, bumper reinforcing members provided along the vehicle width direction at the front end portion and the rear end portion of the vehicle, a front pillar, a door beam disposed in a door, and a seat reinforcing material are exemplified. In particular, according to the 3DQ apparatus according to the present invention, a 3DQ processed member with reduced occurrence of fatigue failure can be manufactured. Therefore, the 3DQ device can be preferably used for manufacturing a vehicle structural member such as a suspension lower arm. Can do.

10 3DQ equipment (3D hot bending quenching equipment)
DESCRIPTION OF SYMBOLS 12 Feed device 14 Cooling device 15 Processing force imparting device 30 Induction heating coil 50 Inner circumference 52 Straight line portion 54 Connection portion 60 Steel pipe 62 Flat plate portion 64 Corner portion S Square shape 230 formed by extending adjacent straight portions 230 Induction heating coil 300 Induction Heating coil

Claims (4)

A feeding device for feeding a steel pipe having a corner portion and a plurality of flat plate portions in the axial direction;
The steel pipe sent by the feeding device is inserted, and an induction heating coil that heats the inserted part of the steel pipe,
A cooling device that is disposed downstream of the induction heating coil in the feeding direction and cools a portion heated by the induction heating coil;
A processing force applying device for applying a processing force to the steel pipe;
A three-dimensional hot bending quenching apparatus comprising:
The inner circumference of the induction heating coil has a plurality of straight portions facing each other in parallel to the plurality of flat plate portions of the steel pipe to be inserted, and a connecting portion connecting adjacent straight portions,
The connecting portion has a shape that spreads outside the coil rather than a rectangular shape that is formed by extending the adjacent straight portions.
3D hot bending quenching equipment. A method for producing a three-dimensional hot bending and quenching member using the three-dimensional hot bending and quenching apparatus according to claim 1,
The material to be sent by the feeding device is a steel pipe having a cross-sectional shape substantially similar to a polygon formed by extending the plurality of linear portions on the inner periphery of the induction heating coil.
A method for producing a three-dimensional hot bending and hardening member. By the induction heating coil, both the flat plate portion and the corner portion of the steel pipe are heated to a temperature not lower than Ac3 transformation point and not higher than 950 ° C.,
The manufacturing method of the three-dimensional hot bending hardening member of Claim 2.   The manufacturing method of the structural member for motor vehicles by the manufacturing method of the three-dimensional hot bending hardening member of Claim 2 or Claim 3.