JP2003073746A - Heat treatment method for steel sheet and apparatus therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heat treatment method for homogenizing the whole steel sheet, by reducing the non-steady part generating at a head part and a tail part of the steel sheet when heating it with an induction heating device, and an apparatus therefor. SOLUTION: This apparatus includes arranging several induction heating devices 5-1 to 5-6 at intervals, and conveyance roller groups 8a, 8b, and 8c, each having an independently variable conveyance speed, for passing a steel sheet 1 through the induction heating devices. This method includes sequentially heating them, while approaching the rear end of the precedent sheet 1-1 to the top end of the following sheet 1-2. The method alternatively includes sequentially heating them, while arranging a dummy material 14 ahead of the precedent steel sheet 1-1 and/or behind the following steel sheet 1-2, so that they can come close.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for heat treatment of steel sheet using an induction heating device.

[0002]

2. Description of the Related Art Generally, a steel material is subjected to a heat treatment process of obtaining high strength and high toughness by quenching and subsequent tempering. In particular, the tempering treatment is generally a heat treatment in a furnace using combustion of gas or the like as an energy source. Recently, some ideas have been proposed to increase the heat treatment efficiency.
For example, Japanese Laid-Open Patent Publication No. 9-260553 proposes a technique for improving efficiency by devising a temperature pattern. In this technology, when steel is continuously conveyed and heat-treated in the furnace, the set temperature of the furnace is changed in accordance with the method of advancing the steel, and the inlet side of the furnace is set to high temperature and the outlet side is set to low temperature. . Further, in this technique, the temperature set at the inlet side of the furnace is set to be 200 ° C. or more higher than the intended heat treatment temperature, and the set furnace temperature is gradually decreased toward the outlet side of the furnace to set the temperature of the furnace before the outlet of the furnace. The target heat treatment temperature is within ± 20 ° C.

There is also a method of increasing the efficiency by increasing the temperature rising rate as in the technique described in Japanese Patent Laid-Open No. 4-358022. This technology prevents the recovery of dislocations, the coarsening of the microstructure / precipitate, the precipitation of solute carbon atoms during the temperature rise, and the strength and toughness by setting the heating rate during tempering to 1 ° C / sec or more. It is possible to increase.

On the other hand, a heating method using induction heating has also been proposed as a method for heating a hot rolled steel sheet. For example, Japanese Unexamined Patent Publication No. 9-225517 proposes a method of heating a roughly-rolled rough bar by induction heating on the entry side of a finish rolling mill in the process of manufacturing a hot-rolled steel sheet. This technique heats the coarse bar passing at a constant speed so that the temperature of the coarse bar becomes uniform in the longitudinal direction.

Further, Japanese Patent Application Laid-Open No. 51-148611 proposes a heat treatment method for a steel material by induction heating. In this technique, an induction hardening apparatus used for heat treatment of a steel pipe is also applied to tempering, and a surface hardened layer produced by quenching of a steel pipe is tempered at a high temperature to soften it.

[0006]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
As in the technique described in Japanese Patent No. 56053, in the heating method by gas combustion, rapid heat transfer cannot be performed because heat is transferred by radiation or convection during heat treatment of steel material in a gas combustion furnace. Further, in the gas combustion furnace, it is actually difficult to heat only the surface layer portion, and the temperature of the surface layer portion and the central portion of the plate thickness rises while there is almost no difference between the steel plates having a plate thickness of 50 mm or less. .

On the other hand, in the solenoid type induction heating method in which the steel sheet is heated while passing through a coil that covers the entire circumference of the steel sheet, when the tip of the steel sheet enters the induction heating device, the steel sheet which is the object to be heated is placed in the coil. Until the target is fully filled, that is, until the heated object is filled in the coil, and when the rear end of the steel plate reaches the coil and the heated object no longer fills the coil, the current flowing through the coil decreases. Then, there is a problem that power is reduced. The relationship is shown in FIGS. FIG. 6 shows a positional relationship when the steel plate 1 conveyed on the table roller 8 passes through the solenoid type induction heating device 5, and FIG. 7 schematically shows a change in the amount of electric power input at that time.

FIG. 6 (a) shows the front end of the steel plate before it enters the induction heating device 5. At this time, as shown by A in FIG. 7, the applied power is still in a low load state of about coil loss. is there. Then, when the tip of the steel plate reaches the inlet end of the coil 6 of the induction heating device 5 as shown in FIG. 6B, the power gradually increases as shown in B of FIG. As shown in FIG. 7C, the input power increases as the steel plate 1 fills the inside, and when the steel plate tip reaches the coil 6 outlet end of the induction heating device 5 as shown in FIG. 6D, the input power is full. After the power (D in FIG. 7) is reached, steady power is supplied to the central portion of the steel plate as shown in FIG. 6 (e), and as a result, the temperature rise is constant (E in FIG. 7).

As described above, the amount of temperature rise at the tip of the steel plate is smaller than that of the central portion after that, from the leading edge to the length of the coil of the induction heating device 5. Similarly,
Similar unsteady heating occurs when the rear end of the steel plate 1 comes out of the induction heating device 5. As a result, the leading end and the trailing end of the steel sheet are unsteadyly heated, and there is a problem that the heating is not sufficient. Therefore, the leading end and the trailing end of the steel sheet are less heated than the central part and the temperature is non-uniform, and it is difficult to obtain a homogeneous material.

The present invention has been made to solve the above problems, and when heating with an induction heating device, the unsteady portions of the leading end and the trailing end of the steel sheet are reduced so that the entire steel sheet. It is an object of the present invention to provide a heat treatment method and an apparatus for the same so as to obtain a homogeneous material.

[0011]

A heat treatment method for a steel sheet according to claim 1 of the present invention is a heat treatment method for a steel sheet, wherein the steel sheet is passed through an induction heating device to be heated, and It is characterized in that the front end of the row steel plate is brought close to and continuously heated.

According to the first aspect of the invention, by bringing the trailing end of the preceding steel plate and the leading end of the following steel plate close to each other, the inside of the coil of the induction heating device is always moved with the steel plate as the object to be heated being filled. Therefore, the unsteady portion does not occur at the front end portion and the rear end portion of the steel sheet, and the entire steel sheet can be heat-treated at a uniform temperature. The induction heating device includes a transverse type and a solenoid type, but the present invention can be applied to any type.

According to a second aspect of the present invention, there is provided a heat treatment method for a steel sheet, which comprises heating the steel sheet through an induction heating device to heat the steel sheet. It is characterized in that the dummy materials are arranged close to each other before and / or after and continuously heated.

In the second aspect of the invention, when heat-treating a single steel plate, the dummy material is placed close to the front and / or rear of the steel plate and continuously heated. As a result, it is possible to reduce the unsteady portion of the front end portion and / or the rear end portion of the steel sheet, and it is possible to heat treat one steel sheet substantially uniformly. The dummy material does not have to be the same size as the steel plate of the object to be heated, but it is desirable that the material, especially the electromagnetic characteristics, be the same.

According to a third aspect of the present invention, there is provided a heat treatment method for a steel sheet, wherein the steel sheet is passed through an induction heating device to be heated, and a dummy is provided before the leading steel sheet and after the trailing steel sheet. It is characterized in that the materials are arranged close to each other, and the steel sheets in the middle are brought close to each other to be continuously heated.

In the first aspect of the invention, since there is no object to be heated before the front steel plate or after the last steel plate, after heating by the induction heating device, the non-heated material is not applied to the front end of the front steel plate or the rear end of the last steel plate. A stationary part may occur. In order to prevent this, in the third aspect of the invention, the dummy material is arranged close to the front steel plate and / or the rear steel plate. In addition, the intermediate steel plates are conveyed close to each other and are continuously heated as in the first invention.

Further, the present invention is characterized in that the steel sheet is heated by a plurality of induction heating devices installed at intervals.

As a result, the output of each induction heating device can be adjusted, and since the coil length is shortened, insufficient heating and overheating can be reduced, and the unsteady portions at the leading and trailing ends of the steel sheet can be reduced. The length of can be shortened.

The steel plate heat treatment apparatus of the present invention comprises a plurality of induction heating devices installed at intervals, and a group of conveying rollers for conveying the steel plate. And an intermediate roller group between the induction heating devices and a delivery roller group on the downstream side of the final stage induction heating device are configured such that the transport speeds thereof can be independently changed.

With such a device configuration,
The heat treatment method of the present invention described above can be efficiently carried out.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an explanatory view of a steel plate production line according to the present invention. The thick steel plate 1 hot-rolled by the hot rolling mill 2 is subjected to quenching treatment by the water cooling device 3. After that, the straightening device 4 corrects the distortion, and the induction heating device 5 performs online heat treatment. In FIG.
10 is a cooling bed. The present invention is not limited to online heat treatment, and an induction heating device may be installed offline. As the induction heating device 5, there are a transverse type and a solenoid type. Here, a solenoid type induction heating device is used, but the effects of the present invention can be similarly obtained in the transverse type. The transverse type has a pair of upper and lower coils 6 as shown in FIG.
Are arranged substantially parallel to the front and back surfaces of the steel plate 1, and in the solenoid type, as shown in FIG. 5B, the coil 6 is wound so as to surround the entire circumference of the steel plate 1 in the plate width direction. It will be. In the figure, 7 is a power supply. Since the solenoid type is heated from the surface layer portion, there is a temperature difference between the surface layer portion and the inside. The transverse type is characterized in that the temperature rises in a state where the temperature distribution in the plate thickness direction is uniform. Both are intended for
It is selected in consideration of purpose, cost, etc., or used in combination.

As shown in FIG. 2, the induction heating device 5 includes, for example, six solenoid type induction heating devices 5-1 to 5-1.
The device configuration is such that the thick steel plate 1, which is an object to be heated, passes through the inside of 5-6. Between the upstream side of the first solenoid-type induction heating device 5-1 and the downstream side of the last-stage solenoid-type induction heating device 5-6 and each solenoid-type induction heating device, the object to be heated is transported. Transport roller group 8
a, 8b, and 8c are arranged to configure a roller table (conveyance table) whose conveyance speeds can be independently changed. The roller group of each transport table can finely control the number of rotations of the roller for each transport table. Thereby, the input power of each solenoid type induction heating device 5-1 to 5-6 and the time for which the steel plate 1 passes are finely controlled according to the plate width, the plate thickness, and the processing amount of the steel plate 1 which is the object to be heated. However, fine temperature control is possible. Hereinafter, the upstream roller group 8a of the first-stage solenoid-type induction heating device 5-1 is referred to as the "inlet roller group 8".
a ", roller group 8b between each solenoid type induction heating device
Is the "intermediate roller group 8b" and the downstream roller group 8c of the final stage solenoid type induction heating device 5-6 is the "outgoing roller group 8".
c ”. In the intermediate roller group 8b, one or two rollers immediately before the first-stage solenoid-type induction heating device 5-1 and the final-stage solenoid-type induction heating device 5-6.
One or two rollers immediately after may be included.
Reference numeral 12 is a thermometer arranged between each solenoid type induction heating device and on the first inlet side and the last outlet side.

There are two reasons for dividing the induction heating device into a multi-stage heating device. Firstly, when the induction heating device is divided, it is possible to monitor the temperature distribution in the longitudinal direction of the steel sheet during heating as in the present embodiment, so that the following solenoid type induction heating device can be monitored while checking the heating condition. This is because it is possible to adjust the output of. As a result, it becomes possible to further reduce insufficient heating and overheating. Secondly, the unsteady heating portion at the front and rear ends of the steel sheet is reduced. That is, as described with reference to FIGS. 6 and 7, the induction heating becomes unsteady in the region corresponding to the coil length from the front and rear ends of the steel sheet. Therefore, the shorter the coil length, the shorter the steel plate front end length and the steel plate rear end length that are subject to load fluctuations. That is, the induction heating device is
When the coil is divided into a plurality of induction heating devices each having a short coil length and capable of independent control, it means that the unsteady part of the steel plate front end and the steel plate rear end is shorter.

The induction heating method of the present invention will be described below with the above equipment configuration. In the hot rolling mill 2, a thick steel plate is usually rolled by the reverse rolling, which is 1-2.
One thick steel plate is rolled at a minute pitch. The rolled thick steel plate 1 is water-cooled by a passage-type water cooling device 3 and then straightened by a straightener 4.
Correct the shape with. At this stage, the thick steel plates that have been straightened are individually conveyed to the rolling line one by one. The movement of the thick steel plate 1 and the positional relationship of the solenoid type induction heating device 5 will be schematically described with reference to FIG. Here, when heating with the induction heating apparatus 5, the entrance side roller group 8a on the entrance side of the induction heating apparatus 5-1 and the intermediate roller group 8b between the induction heating apparatus and the exit side roller group 8c on the induction heating apparatus exit side. The roller rotation speed can be changed independently.

First, one thick steel plate 1 which is an object to be heated.
-1 is passed through the first solenoid type induction heating device 5-1 (FIG. 3 (a)). Next, when the rear end of the thick steel plate 1-1 reaches the first solenoid-type induction heating device 5-1, the second thick steel plate 1-2 of the object to be heated is changed to the preceding steel plate 1-1.
Is conveyed to the entrance side of the induction heating device 5-1 at a speed faster than the conveyance speed of (Fig. 3 (b)), and the leading end of the trailing steel plate 1-2 is brought close to the trailing end of the preceding steel plate 1-1, It is continuously passed through the preceding steel plate 1-1 (FIG. 3 (c)). At this time, since the trailing end of the preceding steel plate 1-1 and the leading end of the following steel plate 1-2 passing through the coils of the induction heating devices 5-1 to 5-6 are close to each other, the coil of the induction heating device is Is always supplied with electric power while the steel sheet, which is the object to be heated, is filled in the coil. Normally, such an induction heating device controls the voltage of the coil terminal so that the current flowing in the coil becomes constant. For example, when a steel sheet passes independently, the tip of the steel sheet enters. During the process and the process of the trailing edge of the steel plate coming out, the state where the object to be heated is not filled in the coil occurs. This time,
When the induction heating device was not supplied with power and was sent at a constant speed, the temperature rise at the front and rear ends of the steel plate was a steady part, that is, the coil was constantly heated with the object to be heated filled. Compared to the part, the amount of heat rise is small and the temperature drops. However, when the rear end of the preceding steel plate 1-1 and the front end of the following steel plate 1-2 are continuously brought close to each other and passed through the coil as in the present invention, the inside of the coil is always filled with the object to be heated. Since the power is supplied in this state, the load is always constant and the rear end of the leading steel plate 1-1 and the leading end of the trailing steel plate 1-2 are constantly heated. Therefore, in this method, constant power is supplied to any portion in the longitudinal direction of the steel sheet, and the amount of heat rise becomes constant. Therefore, the temperature distribution in the longitudinal direction of the steel sheet after heating is uniform and uniform heating is performed.

The preceding steel sheet 1-1 is the final stage induction heating device 5
At the stage of exiting -6, the output side roller group 8c after the induction heating device
The roller rotation speed is increased to quickly move the preceding steel plate 1-1 away from the coil (FIG. 3 (e)). As described above, the induction heating apparatus of the present invention can continuously heat-treat a steel sheet, and the inside of the induction heating apparatus can be efficiently heat-treated by passing the steel sheets in close proximity to each other. is there. By independently varying the rotational speed of each of the entrance side roller group 8a, the intermediate roller group 8b, and the exit side roller group 8c before and after the induction heating device, the preceding steel plate and the following steel plate are arbitrarily brought close to each other. It can be separated or separated, and stable induction heating can be applied from the front end to the rear end of the steel sheet.

[0027]

(First Embodiment) As a first embodiment of the present invention, an example in which the solenoid type induction heating apparatus shown in FIG. 2 is applied to a steel plate production line will be described. In addition, the coil length (dimension corresponding to the longitudinal direction of the steel sheet) of the six solenoid-type induction heating devices 5-1 to 5-6 is 80 cm, and the interval between the induction heating devices is 1 m. The maximum plate width that can pass is 4,600 mm, and the maximum plate thickness is 100 mm. Further, a thermometer 12 is provided between the induction heating devices for the purpose of monitoring the temperature of the steel sheet.

A hot-rolled thick steel plate having a plate thickness of 25 mm, a plate width of 3000 mm, and a length of 10 m was subjected to a quenching treatment by water cooling to accelerate it to 30 ° C., and subsequently, to this induction heating device. The generated distortion was removed by a straightening machine to flatten it. After that, heat treatment was performed through six solenoid type induction heating devices 5-1 to 5-6 so that the temperature of the central portion of the plate was raised to 650 ° C. which is a tempering temperature.
At this time, the input power of the solenoid-type induction heating devices 5-1 to 5-6 and the plate-passing speed of the thick steel plate were set as shown in Table 1. The frequency of each solenoid induction heating device is 1
At a constant 000 Hz, the steel plate was passed through the induction heating device at a constant speed.

[0029]

[Table 1]

First, the first steel plate is rapidly conveyed to the entrance side of the induction heating device 5-1 by the entrance roller group 8a, and then the intermediate roller group 8b is rotated so that the steel plate speed becomes a predetermined 2.4 mpm. , Sent the steel plate. After that, the steel sheet was heated while passing through the induction heating devices 5-1 to 5-6. In addition, the thermometer 1 arranged between each induction heating device
The subsequent output of the induction heating device was automatically controlled by the output of 2, and at the time of leaving the induction heating device, the central portion of the steel sheet was heated to a predetermined temperature of 650 ° C. (In this case, for this steel sheet, there was no previous material at the tip, so the tip became unsteady heating, and 650 ° C
It was not heated up to about 50 ° C and was insufficiently heated. )

When the rear end of this steel plate reaches the inlet of the induction heating device 5-1, the next steel plate is quickly conveyed by the entrance side roller group 8a, and the rear end of the preceding steel plate and the front end of the following steel plate. When the distance between and was 25 cm, the trailing steel plate was fed into the induction heating device at the same speed as the preceding steel plate, that is, 2.4 mpm. At this time, the distance between the leading steel plate and the trailing steel plate was set to 25 cm in the present embodiment, but if this distance is too short, discharge due to sparks occurs between the leading end of the leading steel plate and the trailing end of the trailing steel plate, In some cases, it melts and becomes welded or becomes hot. Here, 5 cm or more is desirable. Further, if it is too far away, the amount of steel plate filled in the coil will decrease and the load power of the coil will change, so it is 2/3 or less of the coil length, preferably half or less of the coil length.
In the case of this embodiment, since the coil length is 80 cm, 40c
m or less is desirable.

As described above, with the trailing edge of the preceding steel sheet and the leading edge of the following steel sheet brought close to each other, they are passed through the induction heating device,
When the trailing end of the preceding steel plate reaches the coil outlet of the final stage induction heating device, the conveyance speed of the exit side roller group 8c is increased,
The preceding steel sheet that had undergone the heat treatment was promptly transported to the next step, the cooling floor. By repeating such conveyance, it is possible to continuously heat the steel sheet. The temperature of the vicinity of the rear end of the preceding steel sheet after heating was heated to 650 ° C., which is the same as that of the central portion. (Second Embodiment) In the second embodiment, a dummy material 14 having a length of about half or more of the coil as shown in FIG. 4 is brought close to the leading end and / or the trailing end of the steel sheet to be heat treated. It is a case of passing through. In addition, this example is expected to have an effect of complementing the portion where the temperature of the tip portion of the first steel sheet is lowered when it is continuously heated. When the same steel plate is heat-treated with the same equipment configuration as in the first embodiment, the same width 3000 mm and thickness 25 mm as the material to be heated is provided before the steel plate 1-1.
A dummy material of a steel plate having a length of 2 m was placed and continuously passed through to continuously heat the dummy material and the steel plate 1-1. When the dummy material and the steel plate 1-1 are brought close to each other, the induction heating device 5 is passed through and the trailing end of the dummy material reaches the coil outlet of the final stage induction heating device 5-6. The conveyance speed of 8c was increased to promptly convey the dummy material to the cooling floor which is the next step. The temperature in the vicinity of the tip of the steel sheet 1-1 after heating was heated to 650 ° C., which is the same as that in the center, and the insufficient heating of the tip was eliminated.

The size of this dummy material is preferably the same in width and thickness as the material to be heated, but it is not necessary to make the sizes the same. Further, the length of the dummy material may be equal to or longer than the coil length, and considering the prevention of dropping from the transport table, it is necessary to have three times or more the interval between the roller tables, but any longer length may be used. However, if it is too long, unnecessary power is consumed, so it is desirable to make it as short as possible. Further, the electromagnetic characteristics of the dummy material are preferably materials having the same characteristics as the steel sheet to be heated. Figure 4
In, since the length of the dummy material 14 is shortened,
In order to prevent falling from the transport table, a steel plate having the same size and the same material as the dummy material 14 is used as the counterweight 15, and both are connected by a support material 16 to form a frame-shaped dummy material.

(Comparative Example) As a comparative example, a heating state of a steel sheet when only one steel sheet is heat-treated under the induction heating conditions shown in Table 1 will be described. When the temperature in the longitudinal direction of the steel sheet was continuously measured by the thermometer 12 on the outlet side of each induction heating device,
The front and rear ends of the steel sheet at the time of passing through the induction heating device are
The length of the coil, that is, the tip 80 cm and the rear end 80c
m was about 50 ° C. lower than the surface temperatures of the central portion in the longitudinal direction and the central portion in the width direction of the steel sheet. Compared to the portion that was subjected to the normal heat treatment, this portion did not have the initially expected heat treatment effect, and the hardness of the front end portion and the rear end portion was high. When the microstructure of this portion was observed, the as-quenched structure was observed. As described above, there were regions where the tempering effect was insufficient at the leading end and the trailing end, and the required quality could not be obtained. In this case, the hardness was the same as in the quenched state, and the tempering effect was not obtained. Therefore, the front end 100 cm and the rear end 1 of this steel plate
00 cm was cut off with a cutting machine.

The unsteady portion of the plate edge can be somewhat reduced by changing the heat treatment pattern and the passing speed. For example,
Insufficient heating of the front and rear edges of the plate may be improved by slowing down the passage speed between the front and rear edges or by stopping the coil in the coil, but from the front to the center or at the rear edge. The heating of a wide steel plate region from the central part to the central part is unsteady heating, which is not a desirable method.

[0036]

As described above, according to the present invention, since the rear end of the leading steel plate and the front end of the trailing steel plate are continuously heated by the induction heating device, the front end portion and the rear end portion of the steel plate are heated. It is possible to reduce the non-stationary portion of the steel sheet and heat treat the entire steel sheet into a homogeneous material.

[Brief description of drawings]

FIG. 1 is a schematic view of a steel sheet production line according to the present invention.

FIG. 2 is a schematic diagram of a heat treatment apparatus of the present invention.

FIG. 3 is an explanatory diagram showing movements of a leading steel plate and a trailing steel plate in the first embodiment.

FIG. 4 is a configuration diagram of a dummy material used in the second embodiment.

FIG. 5 is an explanatory diagram of a transverse type and a solenoid type of the induction heating device.

FIG. 6 is a diagram showing an aspect of heating a steel sheet while passing it through a solenoid type induction heating device.

7 is a diagram showing a transition of the amount of electric power supplied to a coil when a steel sheet reaches each position in the solenoid type induction heating apparatus shown in FIG.

[Explanation of symbols]

1 Steel plate (thick steel plate) 1-1 Leading steel plate 1-2 Trailing steel plate 2 hot rolling mill 3 Water cooling device 4 straightening machine 5 induction heating device 5-1 to 5-6 Solenoid type induction heating device 6 coils 8 Transport table 8a Entry side roller group 8b Intermediate roller group 8c Delivery side roller group 10 cooling floor 12 Thermometer 14 Dummy material 15 counterweight 16 Support material

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Claims (5)

[Claims] 1. A heat treatment method for a steel sheet, which comprises heating the steel sheet by passing it through an induction heating device, wherein the trailing edge of the preceding steel sheet and the leading edge of the following steel sheet are brought into close proximity and continuously heated. Steel plate heat treatment method. 2. A method for heat treating a steel sheet, which comprises heating the steel sheet by passing it through an induction heating device, wherein in the case of a single steel sheet,
A heat treatment method for a steel sheet, characterized in that a dummy material is arranged in proximity to and continuously heated before and / or after the one steel sheet. 3. A method for heat treating a steel sheet, which comprises heating the steel sheet through an induction heating device, wherein dummy materials are arranged in proximity to each other before the leading steel sheet and after the trailing steel sheet, and the intermediate steel sheets are mutually adjacent. A method for heat treating a steel sheet, which comprises heating the steel sheet continuously in close proximity to 4. The steel sheet is heated by a plurality of induction heating devices installed at intervals.
The heat treatment method for a steel sheet according to any one of 1. 5. An induction heating device provided with a plurality of induction heating devices installed at a distance from each other, a conveyance roller group for conveying a steel plate, and an inlet side roller group upstream of the first stage induction heating device and each induction heating device. 2. The heat treatment apparatus for steel plate, wherein the intermediate roller group and the output roller group on the downstream side of the final stage induction heating device are configured such that the transport speeds thereof can be independently changed.