Heating coil, heat treatment apparatus, and heat treatment method for elongated workpiece

A heating coil of a heat treatment apparatus is configured to inductively heat an elongated workpiece having a recessed lateral surface. The heating coil includes a base conductor and a projected conductor. A width of the projected conductor is narrower than a width of the base conductor. The projected conductor is arranged to project toward the recess from a position of the base conductor. The base conductor and the projected conductor are arranged to extend in a longitudinal direction of the workpiece. A heat treatment apparatus includes a cooling section and the heating coil. A heat treatment method uses the heating coil described above.

TECHNICAL FIELD

The invention relates to a heating coil for heating a recessed lateral surface of an elongated workpiece along a longitudinal direction of the workpiece, and also to a heat treatment apparatus and a heat treatment method for heat-treating the recessed lateral surface of the elongated workpiece.

BACKGROUND ART

Related art heating coils are configured such that, when heat-treating an elongated workpiece having a recessed lateral surface, the recessed lateral surface is heated without heating other lateral surfaces of the workpiece.

For example, JP3924084B2 discloses a heating coil having rod-shaped conductors extending in a widthwise direction of an elongated workpiece orthogonal to the longitudinal direction of the workpiece, and protrusions formed on the conductors so as to be inserted into respective recesses on the lateral surfaces, thereby inductively heating the entire recessed lateral surfaces of the workpiece while relatively moving the workpiece in the longitudinal direction. The heating coil is configured to prevent insufficient heating at longitudinal ends of the workpiece.

JP2004-204248A discloses a heating coil apparatus having coil bodies disposed on respective sides of an elongated workpiece with respect to a lateral direction orthogonal to a feeding direction of the workpiece, and protrusions formed on the coil bodies so as to protrude toward respective recesses on lateral surfaces of the workpiece. Each protrusion is designed to have a size that can inductively heat the recess of the corresponding lateral surface uniformly in the longitudinal direction, thereby improving heating efficiency.

However, the portion of the heating coil that heats the entire lateral surface of the elongated workpiece is disposed so as to extend in a direction orthogonal to the longitudinal direction of the lateral surface. Therefore, the area of the portion of the coil that faces the workpiece is limited, which makes it difficult to heat the workpiece at high speed.

To speed up the heat treatment, for example, the heating coil may be formed using a wider member to increase the area of the portion of the coil that extends across and faces the lateral surface of the elongated workpiece. In this case, however, the increased cross sectional area of the coil requires greater electric power, which deteriorates efficiency.

Alternatively, the number of portions of the coil that extend across the lateral surface of the workpiece may be increased, without using a wider member. However, this results in a complex configuration of the heating coil. As a consequence, an electric current path and a cooling fluid path through the coil also become complicated, which increases an output loss of the coil and makes it difficult to maintain a certain level of flow rate of cooling fluid.

SUMMARY OF INVENTION

It is an object of the present invention to provide a heating coil, a heat treatment apparatus, and a heat treatment method that are capable of speeding up a heat treatment on a lateral surface of an elongated workpiece with low power consumption.

According to an aspect of the present invention, a heating coil is provided. The heating coil is configured to inductively heat an elongated workpiece having a first lateral surface and a second lateral surface adjoining each other along a corner portion. The first lateral surface has a recess at a location spaced away from the corner portion. The first lateral surface, the second lateral surface and the recess extend continuously in a longitudinal direction of the workpiece. The heating coil includes a base conductor configured to face the first lateral surface, and a projected conductor configured to face the recess. A width of the projected conductor is narrower than a width of the base conductor. The projected conductor is arranged to project toward the recess from a position of the base conductor. The base conductor and the projected conductor are arranged to extend in the longitudinal direction.

According to another aspect of the present invention, a heat treatment apparatus is provided. The heat treatment apparatus includes the heating coil and a cooling section configured to cool the workpiece. The heating coil is arranged to inductively heat the workpiece that is being relatively moved in the longitudinal direction. The cooling section is arranged downstream of the heating coil in the longitudinal direction. At least a portion of the projected conductor is arranged upstream of the base conductor in the longitudinal direction.

According to another aspect of the present invention, a heat treatment method is provided. The heat treatment method includes inductively heating the workpiece by moving the workpiece in the longitudinal direction relative to the heating coil, and cooling the workpiece at a location downstream of the heating coil in the longitudinal direction. The projected conductor inductively heats the workpiece at a location upstream of the base conductor in the longitudinal direction.

DESCRIPTION OF EMBODIMENTS

As shown inFIG. 1, a workpiece10to be heat treated in an embodiment of the present invention is an elongated workpiece having a substantially constant cross section taken along a plane orthogonal to a longitudinal direction of the workpiece10. An axial line of the workpiece10described below is straight, but may have an arc shape or an annular shape.

The workpiece10has a substantially rectangular cross section. InFIG. 1, first lateral surfaces of the workpiece10on the right and left sides of the workpiece10have an irregular shape, and second lateral surfaces of the workpiece10on the top and bottom sides of the workpiece10are flat. The first lateral surface11and the second lateral surface12adjoin each other along a corner portion13.

At the corner portion13the first lateral surface11and the second lateral surface12are connected to each other such that the first lateral surface11and the second lateral surface12form an angle. Non-limiting examples of the corner portion13include an angular apex, a rounded surface, an inclined surface, and a constricted surface. Heat transfer rate is higher at the corner portions13than at the first lateral surfaces11and the second lateral surfaces12.

Each of the first lateral surfaces11is formed with a recess14at a location spaced away from the corner portions13, at a central location in the present embodiment. The recess14is formed to extend continuously along the longitudinal direction of the workpiece10. The profile of the recess14is not limited, in so far as the recess14is recessed with respect to its adjoining sides. In the present embodiment, the recess14is formed in each of the first lateral surfaces11provided on respective sides of the second lateral surfaces12.

The first lateral surfaces11, including the recesses14, and the second lateral surfaces12of the workpiece10extend continuously in the longitudinal direction of the workpiece10over the entire length of the workpiece10in a substantially constant form.

Heat Treatment Apparatus

A heat treatment apparatus20is configured to perform heat treatment, such as quenching, on the workpiece10. More specifically, the heat treatment apparatus20is configured to perform quenching on a surface layer of each of the first lateral surfaces11. As shown inFIG. 1, the heat treatment apparatus20includes a heating coil21configured to inductively heat the first lateral surfaces11and a cooling section22disposed downstream of the heating coil21to cool the workpiece10by spraying a cooling fluid onto the workpiece10. The heat treatment apparatus20is has a feed mechanism configured to move the workpiece10at a given speed toward one side along the longitudinal direction.

Heating Coil

As shown inFIGS. 1 to 4, the heating coil21has a plurality of projected conductors23and base conductors24that are arranged to face the first lateral surfaces11, including the respective recesses14, at the respective sides of the second lateral surfaces of the workpiece10. The projected conductors23and the base conductors24are joined together to form a single piece structure. Each of the base conductors24has a width that corresponds to the width of the first lateral surface11in a first direction orthogonal to the longitudinal direction of the workpiece10, and is arranged to face the corresponding first lateral surface11with a gap therebetween. Each of the projected conductors23has a width that is narrower than the width of each of the base conductors24in the first direction, and is arranged to face the corresponding recess14with a gap therebetween.

Each of the base conductors24has a prismatic shape or a block shape, and a side of the base conductor24that faces the first lateral surface11has a substantially flat surface. The surface of each of the base conductors24that faces the first lateral surface11has a width that can heat the entire first lateral surface11. Each of the projected conductors23has an oval shape, and a side of the projected conductor23that faces the recess14projects from the base conductor24and has a curved surface corresponding to a shape of the recess14. That is, the projected conductor23is arranged to project toward the recess14from a position of the base conductor24in a second direction orthogonal to both the longitudinal direction and the first direction. The projected part of the projected conductor23has a size that can enter in the recess14of the workpiece10.

The base conductor24and the projected conductor23are arranged to extend along a longitudinal axis of the workpiece10. It is preferable that the projected conductor23be longer than the base conductor24in the longitudinal direction. Further, it is preferable that the base conductor24be arranged to overlap the projected conductor23, at least partially, preferably entirely.

One end of the base conductor24and one end of the corresponding projected conductor23are arranged at the same position in the longitudinal direction. A portion of the projected conductors23is arranged to protrude toward one side from the corresponding base conductor24in the longitudinal direction, that is, toward an upstream of the base conductor24.

A magnetic flux concentrating member25may be provided at a portion of the projected conductor23that does not face the workpiece10, and a magnetic flux concentrating member26may provided at a portion of the base conductor24that does not face the workpiece10, so that the magnetic flux is more concentrated on the first lateral surfaces11and the recesses14.

An end conductor27is joined to one end of each of the projected conductors23. A lead28connected to a power source is provided on each of the end conductors27. The ends of the base conductors24are joined to a C-shaped coupling conductor29, whereby the base conductors24are coupled to each other. As shown inFIG. 5, electric current is supplied from one of the leads28and flows through the heating coil21in the order of the end conductor27, the projected conductor23and the base conductor24on one side, and via the coupling conductor29, the base conductor24, the projected conductor23, the end conductor27and the lead28on the other side.

Cooling fluid inlet portion31is provided on the coupling conductor29, and a cooling fluid outlet portion32is provided on a contact portion28aof each of the leads28. Each of the conductors23,24,27,29is formed from a hollow member, such as a steel pipe. These conductors23,24,27,29, the cooling fluid inlet portion31, and the cooling fluid outlet portion32are brought into mutual communication with each other, whereby a cooling fluid flow path is formed. According to the present embodiment, an intermediate portion of the coupling conductor29is blocked so that separate lines of cooling fluid flow path is provided on respective sides of the first lateral surfaces11.

Heat Treatment Method

To perform heat treatment on the workpiece10using the heat treatment apparatus20having the heating coil21, as shown inFIG. 1, the workpiece10is relatively moved at given speed towards one side along the longitudinal direction such that the workpiece10passes through the heating coil21to which high frequency power is fed and the cooling section22to which cooling fluid is fed.

The workpiece10illustrated inFIG. 6Afirstly passes by the projected conductors23, the recesses14and the portions near the recesses14of the first lateral surfaces11are inductively heated as shown inFIG. 6B. Subsequently, the workpiece10passes by the base conductors24, whereby the entire first lateral surfaces11are inductively heated as shown inFIG. 6C.

In this state, at a location immediately after passing by the coupling conductor29of the heating coil21, the cooling section22sprays a cooling fluid onto the workpiece10to rapidly cool the workpiece10, whereby the surface layer of each of the first lateral surfaces11is quenched along the shape of the first lateral surface11, and the heat treatment ends. After the heat treatment, the workpiece10may be cut from the second lateral surface12, which are not quenched, such as tapping of screw holes, as shown inFIG. 6D.

According to the heating coil21described above, the base conductor24that faces the first lateral surface11and the projected conductor23that faces the recess14are arranged to extend along the longitudinal direction of the workpiece10such that electric current flows through the base conductor and the projected conductor in the longitudinal direction. Therefore, a length along which the heating coil21faces the workpiece10can be extended, so that the first lateral surfaces11can be heated without increasing electric power.

Further, because the base conductors24and the projected conductors23are arranged to extend along the longitudinal direction of the workpiece10, the shape of the heating coil21can be made to conform to the shape of the workpiece10with a simple configuration. Therefore, an electric current flowing distance can be made short to suppress loss, and the coil cooling fluid flow path can be made simple and easy to cool.

As a result, heat treatment of the first lateral surfaces11of the workpiece10can be speeded up with small electric power.

When the base conductor24is arranged to overlap the projected conductor23, it is advantageous in that the length along which the heating coil21faces the workpiece10can be shortened, so that the heating coil21can be made compact.

By making the projected conductor23longer than the base conductor24in the longitudinal direction, when heating the first lateral surface11, the recess14and the portion of the first lateral surface11near the recess14are sufficiently heated by the projected conductor23, and the base conductor24can heat the overall first lateral surface11with a small amount of heat. Accordingly, the first lateral surface11can be efficiently heated while preventing excessive heating of the second lateral surface12which otherwise may be caused by heat transfer.

That is, fast heat transfer occurs at the corner portions13, so that the second lateral surfaces12are likely to be widely heated by heat from the first lateral surfaces11. In view of this, the base conductors24heat the overall first lateral surfaces11with a small amount of heat, thereby minimizing heating of the second lateral surfaces12.

The heating coil21heats the workpiece10that is relatively moved from the projected conductors23toward the base conductors24. That is, the projected conductors23firstly heat the recesses14and the portions near the recesses14, and the base conductors24subsequently heat the overall first lateral surfaces11. Therefore, it is possible to minimize a risk of the second lateral surfaces12being excessively heated by heat transfer at the time when the heating is finished.

When the projected conductors23and the base conductors24are formed as a single piece structure, attachment and positioning of the heating coil can be easily performed. Besides, the electric current flow distance and the cooling fluid flow path can be made short and simple, whereby the heat treatment of the workpiece10can be further speeded up.

When the heating coil21has a plurality of sets of the projected conductor23and the base conductor24and the sets are coupled to each other at the projected conductor23or the base conductor24of each of the sets, heat treatment of the plurality of first lateral surfaces11and the recesses14of the first lateral surfaces11of the workpiece10can be speeded up with smaller power. In particular, by suppressing excessive heating of the second lateral surfaces12at the time when the heating is finished, change in composition of the second lateral surfaces12between the first lateral surfaces11is prevented, whereby processability of the second lateral surfaces12is ensured.

According to the heat treatment apparatus20and the heat treatment method using the heating coil21, it is possible to speed up heat treatment of the first lateral surfaces11of the workpiece10with small electric power. The projected conductors23heat the recesses14at a location upstream of the base conductors24, and the workpiece10is quenched immediately after heating the overall lateral surfaces11. Therefore, the first lateral surfaces11can be heat treated while suppressing the second lateral surfaces12from being heat treated.

While the present invention has been described with reference to a certain embodiment thereof, various changes and modifications can be made therein within the scope of the present invention.

For example, in the above embodiment, at least a portion of the base conductor24is arranged to overlap the projected conductor23. However, the base conductor24and the projected conductor23may be arranged at different locations in the longitudinal direction of the workpiece10. In this case also, it is preferable that the projected conductor23be arranged upstream of the base conductor24, so that the overall first lateral surface11is heated by the base conductor24after the recess14has been heated by the projected conductor23, and a temperature of the recesses14is prevented from excessively dropping before reaching the cooling section22.

EXAMPLES

First Example

Induction heating and cooling were performed in the same manner as in a first reference example except that the heating coil21shown inFIG. 1was used and electric power was supplied at a frequency of 10 kHz. As a result, the first lateral surfaces11and the recesses14were quenched in a suitable manner. An increase in the temperature of the cooling fluid for the heating coil21from an inlet side to an outlet side was 8° C.

Second Example

Induction heating and cooling were performed in the same manner as in a second reference example except that the heating coil21shown inFIG. 1was used and a feed rate of the workpiece10was increased by 30%. As a result, the first lateral surfaces11and the recesses14were quenched in a suitable manner. An increase in the temperature of the cooling fluid for the heating coil21from an inlet side to an outlet side was 12.5° C.

First Reference Example

A workpiece10having a substantially rectangular cross section with recesses14provided on the pair of first lateral surfaces11was prepared, an angled heating coil was prepared and disposed in a heating region as shown inFIG. 7, and induction heating and cooling were carried out by feeding electric power to the heating coil at a frequency of 25 kHz. As a result, the first lateral surfaces11and the recesses14were quenched in a suitable manner. An increase in the temperature of the cooling fluid for the heating coil from an inlet side to an outlet side was 21° C.

Second Reference Example

Induction heating and cooling were performed in the same manner as in the first reference example except that electric power was higher by 20% and was fed at a frequency of 10 kHz. The first lateral surfaces11and the recesses14were not quenched in a suitable manner, and sufficient hardness was not provided.

Results of the examples show that the heating coils21of the first and second examples could suitably perform quenching at smaller electric power while speeding up heat treatment. As for the heating coil shown inFIG. 7, the heat treatment speed was improved as compared with the related art by ensuring a sufficient area where the heating coil faces the workpiece10while suppressing electrical interactions. However, when compared with the heating coils21of the first and second examples, the heat treatment speed with the heating coil shown inFIG. 7was lower. Further, the heating coil shown inFIG. 7entailed a great increase in temperature of cooling fluid and a greater loss than that caused by the heating coils21of the examples. Thus, it was found that the heat treatment efficiency with the heating coil shown inFIG. 7is lower that with heating coils21of the examples.

INDUSTRIAL APPLICABILITY

One or more embodiments of the invention provide a heating coil, a heat treatment apparatus, and a heat treatment method that are capable of speeding up a heat treatment on a lateral surface of an elongated workpiece with low power consumption.

This application is based on Japanese Patent Application No. 2012-047435 filed on Mar. 2, 2012, the entire content of which is incorporated herein by reference.