Electric heating – Inductive heating – Specific heating application
Reexamination Certificate
1999-10-12
2001-10-09
Leung, Philip H. (Department: 3742)
Electric heating
Inductive heating
Specific heating application
C219S656000, C219S672000, C219S673000, C266S129000, C148S567000
Reexamination Certificate
active
06300608
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an induction heating coil for carrying out high frequency induction heating (heating during movement) of a hot rolled steel or the like by surrounding the hot rolled steel or the like which is placed on a plurality of conveying rollers and conveyed, and an induction heating apparatus using this induction heating coil.
BACKGROUND ART
On a hot coil production line at an electric furnace minimill, for example, high frequency induction heating using high frequency electric power of high energy density has widely been used conventionally to heat a thin slab (a kind of hot rolled steels) produced by continuous casting.
FIGS. 8 and 9
show an induction heating apparatus
20
that has been generally used to heat a thin slab on a continuous production line. This apparatus
20
has a construction such that a thin slab
21
, which is continuously cast in a continuous casting section outside the figure and is supplied to a heating section, is subjected to induction heating (high frequency heating during movement) under a moving condition.
As shown in
FIGS. 8 and 9
, the induction heating apparatus
20
is made up of a plurality of steel-made conveying rollers
23
arranged at intervals along a predetermined conveyance path, a solenoid-type induction heating coil
22
fixedly arranged between the adjacent conveying rollers
23
, and a high frequency power source
24
for supplying high frequency electric power to the induction heating coil
22
. The aforementioned induction heating coil
22
used as heating means is a solenoid-type coil wound a plurality of turns in a spiral form. Specifically, as shown in
FIGS. 8
to
10
, the induction heating coil
22
is formed by the repetition of a configuration of one turn consisting of a lower winding portion
22
a,
a side winding portion
22
b
rising upward from one end of the lower winding portion
22
a,
an upper winding portion
22
c
connecting with the upper end of the side winding portion
22
b,
and a side winding portion
22
d
falling downward from one end of the upper winding portion
22
c.
Thus, a thin slab
21
is placed on the plurality of conveying rollers
23
and conveyed so as to pass through a hollow portion (a portion surrounded by coil winding) of the solenoid-type induction heating coil
22
. More specifically, the thin slab
21
, which is supplied continuously from the continuous casting section outside the figure, is placed on the plurality of rollers
23
, which are rotated at an equal speed in the same direction, and is conveyed in a predetermined direction (in the direction of the arrow mark X in FIGS.
8
and
9
). At this time, high frequency electric power of the high frequency power source
24
is transmitted to the thin slab
21
, which is a heated body, by means of the induction heating coil
22
, whereby the thin slab
21
is heated to a predetermined temperature by high frequency induction heating during the movement. In this case, the conveying speed of the thin slab
21
, the rotational speed of the conveying roller
23
, and the high frequency electric power of the high frequency power source
24
are controlled in accordance with the type of the thin slab
21
, by which the heating temperature of the thin slab
21
is controlled.
In order to efficiently heat both of the upper and lower surfaces of the thin slab (heated body)
21
with a thickness of about 20 to 30 mm and a width of about 1000 to 1400 mm, the shape of an opening portion
25
of the induction heating coil
22
, that is, a coil shape viewed from a plane perpendicular to a coil axis S
1
is made rectangular, and the area of the opening portion
25
is determined so as to be at a necessary minimum. The axis S
1
of the induction heating coil
22
is arranged so as to be substantially in alignment with the axis S
2
of the thin slab
21
(see FIG.
9
).
The induction heating coil
22
is excited by the high frequency power source
24
, and the frequency of the high frequency power source
24
is set at about 5 to 6 KHz so that the penetration depth of induced current is not larger than a half of the thickness of the thin slab
21
. An electromagnetic field (magnetic flux) generated by the induction heating coil
22
produces an eddy current in the thin slab
21
. Taking the eddy current as I and the electric resistance of the thin slab
21
as R, Joule heat of I
2
R is produced, so that the temperature of the thin slab
21
increases. A higher heating electric power is more effective in increasing the productivity of minimill and in shortening the production line. Therefore, with the high-power high frequency power source
24
of 1000 to 2000 KW, which is the highest class that can be achieved by the present-day technology, and the induction heating coil
22
being one set, several sets to ten and over sets are arranged in series in the conveying direction of thin slab, thereby forming one heating line.
However, the induction heating coil
22
produces a slightly but non-negligible, harmful eccentric magnetic flux in addition to a magnetic flux parallel with the coil axis S
1
, which is effective in heating the thin slab
21
. This eccentric magnetic flux is generally caused by the coil winding that is wound in a spiral form while shifting in the direction along the coil axis S
1
, that is, the coil winding that is wound at a predetermined lead angle &thgr; (see
FIG. 10
) in the solenoid-type induction heating coil
22
. In this case, the lead angle is an angle formed between a line S
3
in the direction perpendicular to the coil axis S
1
(a line in the direction agreeing with the coil width direction and the width direction of the thin slab
21
) and the upper winding portion
22
c
of the induction heating coil
22
as shown in FIG.
10
. Taking the lead angle as &thgr;, cos &thgr; is an effective component, and sin &thgr; is a component that produces the eccentric magnetic flux. In an example in which the opening size of the opening portion
25
of the induction heating coil
22
is 1600 mm×110 mm, the depth size is 280 mm, and the winding material is a copper pipe of 50 mm×30 mm, the lead angle &thgr; is about 1°.
FIG. 11
shows induced current components produced on the upper surface of the thin slab
21
by electromagnetic induction caused by the induction heating coil
22
wound so as to have the lead angle &thgr;. As shown in
FIG. 11
, on the upper surface and in the vicinity thereof of the thin slab
21
, an induced current i
0
flows in the direction along the upper winding portion
22
c.
In this case, an induced current component i
1
=i
0
cos &thgr; flowing in the width direction of the thin slab
21
is produced as a component effectively contributing to the induction heating of the thin slab
21
, and on the other hand, an induced current component i
2
=i
0
sin &thgr; flowing in the direction of the axis S
2
of the thin slab
21
(or the direction of the axis S
1
of the induction heating coil
22
) is produced as a component harmful to the induction heating of the thin slab
21
. That is to say, if the eccentric magnetic flux is present, the induced current component i
2
flowing in the axial direction of the thin slab
21
is produced (see FIGS.
8
and
11
).
If the induced current component i
2
flowing in the direction of the axis S
2
of the thin slab
21
is produced in this manner, an axial current i
2
indicated by the broken line in
FIG. 8
passes through a conveying roller
23
b,
which is disposed on the downstream side in the thin slab conveying direction with respect to the induction heating coil
22
, and a ground line G, reaches a conveying roller
23
a
disposed on the upstream side in the thin slab conveying direction with respect to the induction heating coil
22
, and returns to the thin slab
21
, the axial current i
2
being a circulating current that circulates along the loop. As a result, by this circulating current, a spark (arc) is produced between the thin slab
21
and the conveying roller
23
a
and between the thin slab
Fuchigami Hiroyuki
Fujisawa Takashi
Inoh Hideo
Omura Suetaka
Alston & Bird LLP
Denki Kogyo Co., Ltd.
Leung Philip H.
LandOfFree
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