JP5193655B2 - Induction heating apparatus and induction heating method - Google Patents

Description

  The present invention relates to an induction heating apparatus and induction heating method for induction heating a metal member.

  When heating a metal plate such as a steel plate, an induction heating method is widely used in which an induction heating coil is placed facing this metal plate and high frequency power is supplied to the induction heating coil while induction heating the metal plate. Has been. According to this induction heating method, the alternating magnetic flux generated by the alternating current flowing through the induction heating coil penetrates the metal plate to induce eddy current inside the metal plate, and the metal plate is heated by Joule heat due to this eddy current. The

  When an induction heating coil is arranged so as to surround both sides of the metal plate and this metal plate is induction heated, the inside of the metal plate is caused by an alternating current flowing in one side of the induction heating coil facing one side of the metal plate. Is induced inside the metal plate due to the direction of the eddy current induced in the coil and the alternating current flowing in the other side of the induction heating coil facing the other side of the metal plate (the surface opposite to the one side). The directions of the eddy currents are opposite to each other.

  By the way, the current penetration depth becomes shallower as the frequency of the high frequency power supplied to the induction heating coil is increased. That is, when high-frequency power having a high frequency (for example, 50 kHz or more) is supplied to the induction heating coil, the eddy current induced due to the alternating current flowing in the one-side conductive portion facing one side of the metal plate of the induction heating coil is A large amount flows on one side of the metal plate and its vicinity (one side portion). On the other hand, the eddy current induced by the alternating current flowing in the other surface side conductive portion facing the other surface of the metal plate in the induction heating coil is generated on the other surface in the metal plate and in the vicinity thereof (the other surface side portion). A lot flows.

  Contrary to the above, the current penetration depth becomes deeper as the frequency of the high frequency power supplied to the induction heating coil is lowered. That is, when high-frequency power having a low frequency (for example, 25 kHz or less) is supplied to the induction heating coil, the eddy current induced due to the alternating current flowing in the one-side conductive portion described above is a deep portion from one side inside the metal plate. Flows up. On the other hand, the eddy current induced due to the alternating current flowing through the other surface side conductive portion flows from the other surface inside the metal plate to a deep portion.

  As can be seen from the above, when an induction heating coil that surrounds both sides of a thin (for example, 5 mm or less) metal plate is arranged and this metal plate is induction heated, high frequency power of a low frequency is supplied to the induction heating coil. When this is done, eddy currents induced by the high frequency power flowing in the one-side conductive part facing one side of the metal plate and the other-side conductive part facing the other side cancel each other out inside the metal plate. For this reason, a thin metal plate is hardly heated. Therefore, the metal plate that can be heated by the induction heating method using low-frequency high-frequency power is limited to a relatively thick metal plate.

  As described above, when high frequency high frequency power is supplied to the induction heating coil, the current penetration depth becomes shallow, so even if it is a thin metal plate, eddy currents will not cancel each other out, and even if it is a thin metal plate, Induction heating is possible. However, when induction heating a thin metal plate having a thickness of about 5 mm or less, the voltage of the induction heating coil is tens of thousands of volts. For this reason, although the work which carries out induction heating of the thin metal plate by said induction heating method is theoretically possible, it is not actually performed.

  As a technique capable of induction heating a thin metal plate even when the voltage of the induction heating coil is low, a technique has been proposed in which a thin metal plate is induction heated using a deflector roll having a roll surface that contacts the metal plate as a conductor. In this technique, since the roll surface of the deflector roll is heated and this heat is conducted to the metal plate, the thin metal plate is induction-heated even when high-frequency power having a relatively low output voltage is supplied to the induction heating coil. .

  However, in the technique using the above-described deflector roll, the deflector roll is indispensable, and the metal plate is conveyed while being bent by the deflector roll. Therefore, the thin metal plate extending straight cannot be induction-heated. Moreover, in this technique, since an induction heating coil is arrange | positioned around the part currently curved by the deflector roll among thin metal plates, an induction heating apparatus becomes complicated.

  In view of the above circumstances, an induction heating method that includes a conductive member disposed between the one-side conductive portion and the one side and can easily induction-heat a thin metal plate even when using a relatively low frequency high-frequency power, and An induction heating device is provided (see, for example, Patent Document 1).

  This induction heating device is an induction heating device that induction-heats a metal plate having a predetermined thickness or less, and includes a one-sided conductive portion that faces one side of the metal plate, and another side that faces the other side opposite to the one side. An induction heating coil having a side conductive portion; and a conductive member disposed between the one side conductive portion and the one side.

In this induction heating device, a long plate-shaped metal plate is continuously conveyed with respect to the heating coil to the central portion of the heating coil with the induction heating device fixed in the induction heating. That is, the metal plate is supported outside the heating coil and is conveyed so as to continuously pass through the heating coil.
JP-A-2005-281778 (P2005-281778A)

  However, the above technique continuously heat-treats a long thin plate that is an object to be heat-treated when it is conveyed so as to enter from one side of a heating coil such as a round coil and exit from the other side. Therefore, it is difficult to apply to a member having a shape that does not continue to the outside of the coil. That is, it is difficult to hold at a desired position by electromagnetic induction force only by placing it on the pedestal. If the position of the member to be heat-treated is shifted, it becomes difficult to perform accurate heat treatment due to overheating on one side.

  In view of the above circumstances, an object of the present invention is to dispose a member to be heat-treated at a desired position and enable accurate heat treatment.

An induction heating apparatus according to an aspect of the present invention is an induction heating apparatus that induction-heats a heat-treated member that is formed of a thin metal plate and has a hole, and a pedestal on which the heat-treated member is placed; An induction heating coil having a one-sided conductive part facing one side of the member to be heat-treated, and another side-side conductive part facing the other side opposite to the one side, and disposed between the one-sided conductive part and the one side A conductive member, a first fixing portion supported on the pedestal and disposed on one side of the heat-treated member and having an inserted portion that can be inserted into the hole, and the heat-treated member supported by the pedestal And a second fixing portion having a receiving portion that can be brought into and out of contact with the inserted portion and engageable with a distal end of the inserted portion. The receiving portion is inserted on the other side through the hole portion. A fixing unit for fixing the object to be heat-treated member to the pedestal by being supported, and characterized by including the.

An induction heating method according to an aspect of the present invention is the method of induction heating a member to be heat-treated with an induction heating coil that is formed of a thin metal plate and surrounds the member to be heat-treated. A first fixing portion having an insertion portion that can be inserted into the hole portion is disposed, and a receiving portion that can be brought into and out of contact with the insertion portion and engageable with a tip of the insertion portion is disposed on the other side. 2 The fixed portion is disposed, the heat-treated member is disposed in a state where the receiving portion and the inserted portion are separated from each other, the inserted portion and the receiving portion are brought close to each other, and the inserted portion is inserted from the one side. One side of the induction heating coil that faces the one side of the member to be heat-treated by fixing the member to be heat-treated to the pedestal by inserting the hole through the hole and supporting the receiving part on the other side Conductive part between conductive part and this one side It was placed, characterized in that by supplying high-frequency power inductively heating the object to be heat-treated member to the induction heating coil.

  According to the present invention, the member to be heat-treated can be disposed at a desired position, and accurate heat treatment can be performed.

  Hereinafter, a first embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a perspective view showing an induction heating device 10 of the present embodiment, FIG. 2 is a plan view, and FIG. 3 is a side view.

  The induction heating device 10 is for induction heating of the entire thin steel plate member 70 (an example of a member to be heat treated, hereinafter referred to as a workpiece 70).

  The induction heating device 10 is disposed in the center portion, and includes a pedestal 12 that supports the work 70, an induction heating coil 20 that surrounds the pedestal 12 and the work 70, a high-frequency power source 30, a fixing mechanism 50 that fixes the work 70, and induction. And a cooling water jet cooling device 60 that cools the heated workpiece 70.

  As shown in FIGS. 4 to 6, the work 70 is formed by bending a thin steel plate having a thickness of, for example, about 3 mm into a predetermined L shape. A circular mounting hole 73 penetrating in the thickness direction is formed at the center in the width direction of the workpiece. Further, a plurality of circular hole portions 74 penetrating in the thickness direction are provided in the vicinity of the attachment hole portion and on the tip side of the workpiece. The workpiece is fixed to the pedestal 12 in the mounting hole 73 by a fixing mechanism 50 described later.

  The pedestal 12 shown in FIGS. 1 to 3 is made of an electrically insulative material such as ceramic, for example, and is arranged in a lower portion of a region surrounded by the induction heating coil 20. The pedestal 12 has a predetermined shape corresponding to the shape of the workpiece 70 as the heat-treated member, and has a wide portion 13 having a wide width on one side and a narrow portion 14 having a narrow width on the other side in a plan view. Configured. The distal end side is placed on the wide portion 13, and the proximal end side is placed on the narrow portion 14. Here, the case where the upper surface of the base 12 is planar is shown as an example. The pedestal 12 can be moved up and down, and the work 70 is installed in a state of being positioned above, and the induction heating process is performed in a state of being disposed inside the lower coil 20. FIG. 2 shows a state where the pedestal 12 is raised and positioned above.

  The fixing mechanism 50 includes a first fixing portion 51 and a second fixing portion 55 that are disposed on one side and the other side of the work 70, respectively.

  As shown in FIGS. 7 to 11, the first fixing portion 51 is fixed to the pedestal 12 by screws and extends upward, and is supported by the first support member 52 and extends to the other side. An inserted member (inserted portion) 53 is provided.

  The first support member 52 is made of a ceramic-based or zirconia-based insulating material, has a semicircular concave portion, and is composed of two plate-like members 52a and 52b disposed so as to face each other. By the combination of the two plate members 52a and 52b facing each other, the first support member 52 is formed in a rectangular plate shape having a circular support hole 52c and a slit continuous therewith. The inserted member 53 is supported by being inserted into and fixed to the circular support hole 52c.

  The inserted member 53 is made of a ceramic-based or zirconia-based insulating material, and has a stepped cylindrical member having a large-diameter portion 53a and a small-diameter portion 53b whose axial direction is the first direction from one side to the other side. Consists of. The base end side of the inserted member 53 is inserted into the support hole 52 c, and the small-diameter portion 53 b on the distal end side forming the convex portion is inserted into the mounting hole 73. A step portion between the large diameter portion 53 a and the small diameter portion 53 b constitutes a first support edge 53 c that engages with the peripheral edge portion of the attachment hole 73.

  As shown in FIG. 7 and FIGS. 12 to 15, the second fixing portion 55 is disposed on the other side of the work 70, that is, the other surface side here. The second fixing portion 55 includes a second support member 56 and a receiving member 57 supported by the second support member 56.

  The second support member 56 is a plate-like member made of cast metal, and is fixed to a movable shaft 58a of an air piston 58 as a mechanism through a plurality of attachment holes formed on the base end side thereof. The second support member 56 is shaped to have a step so that the proximal end side is wide and the distal end side is narrow, and a receiving member 57 described later is fixed to the narrow distal end side, that is, near the upper end. A fixing hole 56a is formed.

  The receiving member 57 is made of, for example, a conductive member such as copper, and is formed of a two-stage cylindrical member including a proximal-side small-diameter portion and a distal-end-side large-diameter portion. The receiving member 57 is supported with the first direction as the axial direction by inserting and supporting the small-diameter portion on the proximal end side through the fixing hole 56a.

  A receiving portion 57a that is recessed toward the proximal end is formed on the end face on the distal end side. The receiving portion 57a has a circular shape and is configured such that the tip of the inserted member 53 is fitted therein.

  The end surface on the base end side of the receiving member 57 is movable in the axial direction, that is, in the X direction in the figure by an adjusting mechanism 58 that is configured by a cylinder or the like and supported by the pedestal 12. Due to the air pressure from the adjusting mechanism 58, the base end side of the receiving member 57 is pressed in the first direction, or the pressing force is released. With this pressing force, the receiving member 57 can reciprocate in the first direction. The proximity state is indicated by a solid line, and the separation state is indicated by a broken line.

  With the movement of the receiving member 57 by the adjustment mechanism 58, the receiving portion 57a can be brought into contact with and separated from the tip of the inserted member 53. The peripheral portion of the receiving portion 57 a constitutes a second support edge 57 b that supports the peripheral portion of the attachment hole 73 of the workpiece 70.

  With the distal end of the inserted member 53 and the receiving portion 57a separated from each other, and with the pedestal 12 disposed above, the inserted member 53 is inserted into the mounting hole 73 and between the receiving portion 57a and the receiving portion 57a. The work 70 is disposed on the surface. Thereafter, the receiving member 57 is moved in the first direction by the adjusting mechanism 58 and brought into a proximity state, whereby the convex portion at the tip of the inserted portion 53 is engaged with the concave portion of the receiving portion 57 a of the receiving member 57.

  In this proximity state, the inserted member 53 is inserted into the mounting hole 73 from one side, and the tip thereof is supported by the receiving portion 57a on the other side, and the first support edge 53c and the second support edge 57b. Thus, the work 70 is fixed to the pedestal 12 by sandwiching both surfaces of the peripheral edge of the attachment hole 73 of the work 70. After fixing, the work 70 is held inside the induction heating coil 20 by lowering the base 12.

  As shown in FIGS. 1 to 3, the induction heating coil 20 is a two-turn box coil and surrounds an electrically insulating base 12. The induction heating coil 20 faces the one surface W1 of the work 70 and extends in the first direction (the Y direction in the drawing) along the length direction thereof, and the other surface W2 of the work 70 (opposite of the one surface W1). And the other-surface-side conductive portion 22 extending in the width direction thereof. A cooling liquid path (not shown) through which a cooling liquid for cooling the induction heating coil 20 flows is formed inside the induction heating coil 20.

  The one-side conductive part 21 and the other-side conductive part 22 are electrically connected by a first connection conductive part 23. The first connection conductive portion 23 extends in a second direction (X direction in the drawing) intersecting the first direction at a position away from the one end W3 of the work 70. The end portion of the first connection conductive portion 23 is electrically connected to the one end portion 22 a of the other surface side conductive portion 22.

  A second connection conductive portion 24 connected to the high frequency power supply 30 is bent and extends from the other end portion 22 b of the other surface side conductive portion 22. Further, the third connection conductive portion 25 connected to the high frequency power supply 30 is bent and extended from the end portion of the one-side conductive portion 21. The second connection conductive portion 24 and the third connection conductive portion 25 are electrically insulated by an electrical insulating member 31.

  Of the induction heating coil 20, the part that substantially contributes to the heating of the work 70 includes the one-side conductive part 21 and the other-side conductive part 22, but as will be described later, the one-sided conductive part 21 and one side of the work 70. Since the copper plate 40 functioning as a magnetic flux reducing member and a conductive member is disposed between W1 and AC, the alternating current flowing through the single-sided conductive portion 21 heats the surface layer portion of the workpiece 70 via the copper plate 40. It will be.

  As described above, the induction heating coil 20 surrounds the workpiece 70, but the copper plate 40 is disposed between the one-side conductive portion 21 and the one-side W <b> 1 of the workpiece 70. The width and length of the copper plate 40 are substantially the same as the width and length of the single-sided conductive portion 21. Cooling liquid paths through which the cooling liquid flows are formed inside the copper plate 40, and cooling liquid inlets 42, 42 for supplying and discharging the cooling liquid to the cooling liquid path at both ends in the longitudinal direction of the copper plate 40. Is formed. The copper plate 40 is cooled by supplying the coolant from one of the coolant inlets 42 and 42 to the coolant path and discharging the coolant from the other. The copper plate 40 is fixed to the one-side conductive portion 21 via the electrical insulating member 32.

  Here, an example of the thickness and arrangement interval of each member is given.

  The thickness t1 of the one-side conductive portion 21 is 10 mm, the thickness t2 of the electrically insulating member 32 is 2 mm, the thickness t3 of the copper plate 40 is 10 mm, and the thickness t4 of the workpiece 70 is 3 mm. The thickness t5 of the other surface side conductive portion is 10 mm. Further, the distance L1 from the copper plate 40 to the one surface W1 of the work 70 is 60 mm, and the distance L2 from the other surface W2 of the work 70 to the other surface side conductive portion 22 is 70 mm.

  The cooling device 60 is configured by arranging a cooling jacket so as to surround the pedestal below the heating coil.

  After the heating is completed, the pedestal 12 is lowered to the cooling jacket position by lifting means (not shown). The workpiece 70 is cooled by spraying the cooling water from the cooling jacket while the workpiece 70 is surrounded by the cooling jacket.

  16 and 17 show the hardness of each part of the work 70 after induction heating. In the figure, P1, P2, P3, P4, P5, P6 and P7 indicate each part in the workpiece, and the corresponding symbols are shown in FIG. In FIGS. 16 and 17, the hardness at the curved outer side surface 70a on which the burr is not formed by the pressing process during manufacturing, the curved inner side surface 70b on which the burr is formed, and the central portion 70c in the thickness direction. Respectively. The measurement load for hardness was 5 kg. Regarding the hardness, it can be seen that the hardness after induction hardening satisfied the specification of the upper limit of 577 HV. FIG. 16 shows a case where it is arranged in the RH direction, and FIG. 17 shows a case where it is arranged in the opposite LH direction. The arrangement shown in FIG. 2 is the direction of RH.

  In any case, the metal structure after induction hardening was a martensite structure.

  According to the induction heating apparatus 10 of the present embodiment, since the conductive copper plate 40 is disposed between the one-side conductive portion facing one side of the workpiece and this direction, the alternating current flowing in the one-side conductive portion is The resulting alternating magnetic flux is attenuated by the copper plate 40. For this reason, the work 70 having a thickness of about 3 mm can be easily induction-heated.

  According to this embodiment, the following effects can be obtained. That is, by inserting the inserted member 53 into the mounting hole 73 and engaging and fixing the tip of the inserted member 53 to the recessed portion of the receiving portion 57a, the contact portion of the fixing mechanism 50 to the workpiece is reduced and the influence on the magnetic flux is reduced. Can be prevented. Therefore, the workpiece 70 having a shape arranged inside the box-shaped induction heating coil 20 can be easily and accurately held.

  Moreover, since the member 53 to be inserted functions as a reducing member similarly to the copper plate 40 by configuring the member 53 to be inserted, the member 53 to be inserted is also heated uniformly. The influence on the heat treatment characteristics can be reduced. That is, the temperature of the heat treatment member may vary in temperature between the portion in contact with the jig and the contacted portion, but according to the present embodiment, the entire temperature can be equalized. As a result, it is possible to install the holding jig in a portion that is exposed (heated) to the magnetic flux in the heating coil. Moreover, while being able to ensure the low (constant) deformation | transformation of to-be-processed material, uniform hardness can be ensured.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage.

  In the above-described example, the induction heating coil 20 is used for induction heating of a thin plate material, but the induction heating coil 20 can also be applied when induction heating the entire thin linear member.

  The inserted member 53 is made of copper, but other conductive materials such as copper alloy, aluminum, aluminum alloy, silver, and stainless steel may be used instead.

  Moreover, in the said 1st Embodiment, although the one side and the single side | surface were made into the same side, you may reverse, for example, arrange | position a receiving part on the single side and comprise this receiving part with a conductive member. Good. In this case, the same effect as that of the above embodiment can be obtained.

  In the example of the above embodiment, the upper surface of the pedestal 12 has a planar shape, but may have a stepped shape. By forming the pedestal so as to conform to the shape of the workpiece and appropriately adding a stepped portion, it is possible to process a heat-treated member having a plurality of types of shapes with the same fixing mechanism.

  In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

The perspective view which shows the induction heating apparatus concerning 1st Embodiment of this invention. The top view of the induction heating apparatus. The top view of the induction heating apparatus. Side view of the workpiece according to the embodiment Plan view of the workpiece Front view of the work Side view of fixing mechanism according to embodiment Side view of first support member according to embodiment Front view of the first support member Side view of inserted member according to the embodiment Front view of the inserted member Side view of second support member according to embodiment Plan view of the second support member The front view of the receiving member which concerns on the same embodiment. The side view of the receiving member which concerns on the same embodiment. The table | surface which shows the hardness at the time of arrange | positioning the workpiece | work in the same embodiment in RH direction. The table | surface which shows the hardness at the time of arrange | positioning the workpiece | work in the same embodiment in the LH direction.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Induction heating apparatus, 12 ... Pedestal, 12 ... Pedestal, 20 ... Induction heating coil, 21 ... Single side | surface side conductive part, 22 ... Other side side conductive part, 23 ... 1st connection conductive part, 24 ... Connection conductive part,
DESCRIPTION OF SYMBOLS 25 ... Connection electroconductive part, 31 ... Electrical insulation member, 30 ... High frequency heat source, 32 ... Electrical insulation member, 40 ... Copper plate, 42 ... Coolant inlet / outlet, 50 ... Fixing mechanism (fixing part), 51 ... 1st fixing part 52 ... support members, 52a. 52b ... Plate-like member, 52c ... Support hole, 53 ... Inserted member (inserted portion), 53a ... Large diameter portion, 53b ... Small diameter portion, 53c ... Support edge, 55 ... Second fixing portion, 56 ... Support member, 56a ... fixing hole, 57 ... receiving member (receiving portion), 57b ... support edge, 58 ... air piston (adjusting mechanism), 60 ... cooling device, 70 ... workpiece, 73 ... mounting hole portion (hole portion), 74 ... hole Department.

Claims (9)

An induction heating device that induction-heats a heat-treated member composed of a thin metal plate and having a hole,
A pedestal on which the heat-treated member is placed;
An induction heating coil having a one-sided conductive part facing one side of the heat-treated member, and another side-side conductive part facing the other side opposite to the one-side;
A conductive member disposed between the one-side conductive portion and the one-side,
A first fixing portion that is disposed on one side of the member to be heat treated and has a portion to be inserted that can be inserted into the hole, and disposed on the other side of the member to be heat treated and capable of contacting and separating from the portion to be inserted. A second fixing portion having a receiving portion with which the distal end of the inserted portion can be engaged, and the inserted portion is inserted into the hole portion from one side and supported by the receiving portion on the other side. A fixing portion for fixing the heat-treated member to the pedestal,
An induction heating apparatus comprising: The first fixing portion is formed of a cylindrical member with a step having a large diameter portion and a small diameter portion whose axial direction is a first direction from the one side to the other side, and the step portion is a peripheral edge of the hole portion. A first support edge that engages the portion and supports the peripheral edge of the hole,
The second fixing portion has a receiving portion formed at a tip of a cylindrical member having the first direction as an axial direction, and a second support edge that supports a peripheral portion of the hole portion around the receiving portion. Is configured,
In the proximity state, the inserted portion is inserted into the hole portion from the one side, and the tip thereof is supported by the receiving portion on the other side, and the heat treatment is performed by the first support edge and the second support edge. The induction heating apparatus according to claim 1, wherein the heat-treated member is fixed to the pedestal by sandwiching a peripheral edge portion of the hole portion of the member.   The induction heating device according to claim 1 or 2, wherein a member disposed on the other surface side of the inserted portion and the receiving portion is made of a conductive material.   The induction heating device according to any one of claims 1 to 3, wherein among the inserted portion and the receiving portion, the member disposed on the one side is made of an insulating material. The induction heating device according to any one of claims 1 to 4, wherein the inserted portion is made of copper, aluminum, silver, or stainless steel. Cooling means for injecting a cooling liquid;
A moving means for moving the heat-treated member to a cooling position of the cooling means after the induction heating treatment;
An induction heating apparatus according to any one of claims 1 to 5, further comprising: In the method of induction heating this heat-treated member with an induction heating coil surrounding the heat-treated member having a hole formed of a thin metal plate ,
On one side of the pedestal, a first fixing part having an inserted part that can be inserted into the hole part is arranged,
On the other side, a second fixing part having a receiving part that can be brought into and out of contact with the inserted part and engageable with a tip of the inserted part,
In a state where the receiving portion and the inserted portion are separated from each other, the heat-treated member is disposed,
The inserted portion and the receiving portion are brought close to each other, and the inserted portion is inserted through the hole portion from the one side and supported by the receiving portion on the other side, whereby the heat-treated member is placed on the pedestal. Fixed,
A conductive member is disposed between the one side of the induction heating coil and the one side of the conductive member facing the one side of the heat-treated member,
An induction heating method, wherein high frequency power is supplied to the induction heating coil to inductively heat the member to be heat treated. Induction heating method according to claim 7, in said conductive member, characterized by the use of plate-like members through which cooling liquid flows therein. The induction heating method according to claim 7, further comprising a step of cooling the heat-treated member after induction-heating the heat-treated member.

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