JPH07331340A - Method for electrically heating metal strip - Google Patents

Abstract

PURPOSE:To provide an electric heating method for metal strip executing the stable operation by reducing the variation of temp. in the electric heating to the metal strips having different sizes. CONSTITUTION:At the time of electrically heating the welded part 7 of the metal strips having different cross-sectional areas by a continuous running electric heating device for metal strip, this electric heating method for metal strip, by which the loading voltage 6 at the front and the rear parts containing the welded part 7 is lower than that at the stationary part having the constant cross-sectional area so as to restrain the abnormal raising of the temp. in the metal strip 1b at the small cross-sectional area side, is used. By this method, the overheat and the thermal expansion caused by the sharp variation of the strip temp. near the welded part is prevented and the deterioration of quality of the metal strip is not brought about, and further, the threading is smoothly executed and the stable continuous heat treatment is executed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric heating method for continuously heat treating a metal strip.

[0002]

2. Description of the Related Art Conventionally, as a heating method in a continuous annealing furnace for metal strips, a direct fire type non-oxidizing heating method, a radiation tube heating method and the like have been used. Since it is a heating method by the above, the heating time per unit time must be long, and the heating time is inevitably long.

In response to such a heating method using heat transfer, in recent years, a method of directly energizing a metal strip to heat it has been proposed. For example, Japanese Patent Publication No. 60-26818 proposes a method in which a metal strip is directly energized and heated through an energizing roll.

Further, in Japanese Patent Laid-Open No. 01-142032, an energizing roll is provided in front of and behind a metal band passage penetrating an annular transformer, and both energizing rolls are connected by a conductive member so that a metal band, an energizing roll and a conductive member are used. A method is disclosed in which an induction current is generated by using a closed circuit as a secondary coil, and heating is performed by Joule heat generated by the induction current.

[0005]

In these energization heating methods, the temperature of the metal strip abruptly changes when welding parts having different cross-sectional areas pass through the energizing rolls, which causes the steel sheet to be instantaneously produced. Due to thermal expansion, unstable factors such as tension fluctuations occur.

Since this fluctuation in tension leads to unstable operation, conventionally, when the welded portion passes through the energizing roll, the power source is temporarily cut off or the strip running speed is extremely reduced.

However, in the method of temporarily cutting off the power supply, the constant length of the different cross-section area is not annealed at all, and the non-annealed part must be cut off. Further, since the material of the non-annealed portion is remarkably different from the other portions, the meandering will occur in the subsequent threading section.

Further, in the method of extremely reducing the plate passing speed when the welded part passes through, not only the efficiency is lowered but also the soaking time and the cooling time are different from those of the normal heating part, so that the material of the same part is changed. Will occur.

The present invention provides an electric heating method for electrically heating metallic strips of different sizes, which minimizes temperature fluctuations that occur when heating the vicinity of the welded portion.

[0010]

DISCLOSURE OF THE INVENTION The present invention suppresses an abnormal rise in the temperature of a metal strip on the side of a small cross-sectional area when electrically heating a welded portion of a metal strip having a different cross-sectional area by a continuous running electric current heating device for a metal strip. As described above, the load voltage before and after including the welded portion is reduced as compared with the steady portion having a constant cross-sectional area.

The present invention is also a method for electrically heating a metal strip, characterized in that the load voltage before and after including the welded portion is V'represented by the following equation (2).

[0012]

## EQU2 ## V '= [T' / (axT)] ^1/2 xV (2) where V ': load voltage to the weld, T': target temperature of the weld V: Load voltage to the constant cross-section area, T: Temperature of the constant cross-section area a: Cross-section area ratio of the metal band of the weld zone (large cross-section area / small cross-section area)

[0013]

EXAMPLES The present invention will be described below by way of examples along with its operation.

FIG. 2 shows the temperature 6 of the metal strip 1 at the point of the high temperature side energizing electrode 3 when heating the metal strip having a constant cross-sectional area.
And voltage 4 between the high temperature side energizing electrode 3 and the low temperature side energizing electrode 2
Is shown with the longitudinal direction position of the metal strip 1 as the horizontal axis. The heating is in a steady state, so that neither the voltage 4 nor the temperature 6 changes in the longitudinal direction of the metal strip 1.

FIG. 1 shows the temperature 6 of the metal strip 1 and the high temperature side energization electrode 3 at the point of the high temperature side energization electrode 3 when heating the metal strip 1 when the welded portions 7 having different cross-sectional areas pass through the energization electrode. The voltage 4 between the low temperature side energizing electrode 2 and the low temperature side energizing electrode 2 is shown in the same manner as in the case of FIG. 2 with the longitudinal direction position of the metal strip 1 as the horizontal axis.

Reference numeral 1a denotes a large-section metal band, and 1b denotes a small-section metal band which is connected to the large-section metal band 1a and is subsequently processed. The size of the metal strip 1a is 1 width
m, thickness 0.65 mm. The size of the metal strip 1b is
The width is 1 m and the thickness is 0.40 mm.

The ratio of the cross-sectional areas of the metal strips (cross-sectional area of 1a / cross-sectional area of 1b) is 1.6. The plate temperature of the stationary part is 750 ° C. The metal strip is heated by the electric power from the power source 4 via the current-carrying electrode 2 and the electrode 3 on the high temperature side. The distance between the energizing electrode 2 and the high temperature side electrode 3 is 15 m.

23 m from the weld 7 of the large cross-section metal strip 1a
When the point (3) reached the electrode 3 on the high temperature side, the plate temperature started to decrease to 470 ° C. As a result of various tests by changing the distance between the electrodes, the plate temperature starts to decrease at the high temperature side at a point 1.5 times the distance L between the electrodes from the welded portion 7 of the large cross-section metal band 1a. It was found that it was the time when the electrode 3 was reached. The reached temperature is 1 / a times the plate temperature of the stationary part.

After that, the plate temperature rises at the welded portion 7 as a boundary,
It reached 1200 ° C. This is a times the plate temperature of the stationary part. After that, the plate temperature decreased, and the point 23 m from the weld 7 of the small cross-section metal band 1b reached the temperature of the steady part (750 ° C.) when it reached the electrode 3 on the high temperature side.

In this way, when the load voltage is constant,
A large plate temperature variation of 730 ° C occurs before and after the welded portion 7, resulting in poor quality of the overheated portion, and tension variation caused by this temperature variation causes throttling, meandering, breakage, etc. This will greatly impair the sheet passing property.

In order to prevent this temperature fluctuation as much as possible, the present inventors have found that it is only necessary to set a target value (T ') of the maximum temperature and take a load voltage pattern based on this target value. The load voltage pattern will be described in detail below.

In order to prevent overheating, the load voltage may be lowered before the welded portion 7 reaches the high temperature side electrode 3.

The timing for starting the decrease is not particularly limited, but the point at which the plate temperature starts to increase when the load voltage is constant, that is, between the weld 7 of the large cross-section metal strip 1a and the electrodes. It is preferable that the point at a distance 1.5 times the distance L reaches the electrode 3 on the high temperature side.

As a result of an enormous amount of tests, the minimum value of the load voltage was found to be V'using the previously set maximum target temperature T '.
= [T ′ / (a × T)] ^1/2 × V (2) Equation (2), and it has been found that it is desirable to set L / (1 + a) from the point where the voltage starts to be reduced.

FIG. 3 shows the plate temperature when the maximum attainable target temperature T ′ is 750 ° C. in comparison with the case where the load voltage is constant. The plate temperature at the time of the lowest temperature was 290 ° C. This can prevent overheating of the steel sheet and
The plate temperature fluctuation which was 0 ° C could be reduced to 460 ° C.

The restored pattern of the reduced load voltage is not particularly limited, but if the welding point is returned to the original level at the time of passing through the high temperature side electrode, the metal strip 1b is overheated. Can be annealed without.

[0027]

As described above, according to the current-carrying heating method of the present invention, when the metal strips of different sizes are heated by the current-carrying electrodes, welding is performed so as to suppress an abnormal rise in the temperature of the metal strip on the small cross-sectional area side. By reducing the load voltage before and after including the part compared to the steady part where the cross-sectional area is constant, overheating, thermal expansion, etc. due to sudden changes in the plate temperature near the welded part are prevented, and the quality of the metal strip does not deteriorate. Further, the plate passing is smoothly performed, and stable continuous heat treatment can be performed.

[Brief description of drawings]

FIG. 1 is a diagram showing a voltage between electrodes and a plate temperature in a conventional case in which a welded portion of a metal strip having a different cross-sectional area is electrically heated.

FIG. 2 is a diagram showing a voltage between electrodes and a plate temperature in a general case in which a metal strip having a constant cross-sectional area is electrically heated.

FIG. 3 is a diagram showing an interelectrode voltage and a plate temperature in the case of conducting heating according to an example of the present invention.

[Explanation of symbols]

 1 metal strip 1a metal strip with large cross-sectional area 1b metal strip with small cross-sectional area 2 energizing electrode 3 high temperature side electrode 4 metal strip temperature 6 inter-electrode voltage 7 weld

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masami Onoda 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Co., Ltd.Technical Development Division (72) Inventor Yoshiaki Hirota 20-1 Shintomi, Futtsu-shi, Chiba Made in Shinnihon Steel Co., Ltd. Technology Development Division

Claims (2)

[Claims] 1. A method for electrically heating a welded portion of a metal strip having a different cross-sectional area by means of a continuous running electrification heating device for a metal strip,
In order to suppress the abnormal rise in the temperature of the metal strip on the small cross section side,
A method for energizing and heating a metal band, characterized in that a load voltage before and after including a welded portion is reduced as compared with a steady portion having a constant cross-sectional area. 2. A method for electrically heating a metal strip, wherein a load voltage before and after including a welded portion is V ′ represented by the following formula (1). ## EQU1 ## V '= [T' / (axT)] ^1/2 xV (1) where V ': load voltage to the weld, T': target temperature of the weld V: Load voltage to the constant cross-section area, T: Temperature of the constant cross-section area a: Cross-section area ratio of the metal band of the weld zone (large cross-section area / small cross-section area)