JP2006342404A - Electric heating method for steel strip - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electric heating method for a steel strip where, even in the case the cross-sectional area of a steel strip is changed at a welding point, the final arrival sheet temperature of the steel strip infiltrated into a hot dip metal bath can be controlled to a fixed range where sheet fracture and plating peeling are not generated, and further, even in the case the cut of energizing is performed before and after the welding point, the final arrival sheet temperature of the steel strip can be secured. <P>SOLUTION: In the electric heating method for a steel strip where, when a welding point connecting a preceding material and a posterior material whose cross-sectional areas are different in a steel strip passes within an electric heating range, the change of heating current is performed in such a manner that the change distance L2 from the passage of the welding point through a conductive roll to a heating current set value change point satisfies the formula: L2=L1×(1-t)×(Ja')<SP>2</SP>/((1-t)×(Ja')<SP>2</SP>+(1+t)×(Jb')<SP>2</SP>) (wherein L2: the change distance from the conductive roll to the heating current set value change point; L1: heating length in the electric heating range; Ja: the set current density of the preceding material; Jb: the set current density of the posterior material; a: the cross-sectional area of the preceding material; b: the cross-sectional area of the posterior material; Ja'=Jb×b/a; Jb'=Ja×a/b; and t: a constant). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

According to the present invention, the steel strip that is continuously fed is brought into contact with an energizing roll disposed on the entry side of the steel strip, and is brought into contact with a metal bath disposed on the exit side of the steel strip. The present invention relates to a method for electrically heating a steel strip that uses a bath as an electrode to energize and heat the steel strip between the electrodes.
Specifically, it is a method in which a steel strip to be mass-produced is continuously fed at high speed and heated by passing an electric current using an energizing roll. For example, the steel strip is quenched, annealed, and preheated for plating. The present invention relates to an electric heating method for a steel strip used for heating for various heat treatments.

Conventionally, various proposals have been made regarding the current heating method of the steel strip used when heating the steel strip for various heat treatments such as quenching, annealing, and preheating for plating.
For example, in Japanese Utility Model Publication No. 6-30844, in a continuous hot dip galvanizing facility, a conductive roll provided on the entrance side of a ring transformer that forms a space in which a steel strip travels and a hot dip zinc bath are connected by a conductive member. In a facility that configures a secondary closed circuit and heats the steel strip with a voltage induced in the secondary closed circuit by a ring transformer, the resistance of the steel strip and the resistance of the current-carrying member are within a specific range and compact. In addition, a continuous hot-dip galvanizing facility that employs an inexpensive and inexpensive atmosphere heating device has been proposed.

In this continuous hot dip galvanizing equipment, when the cross-sectional area of the steel strip changes at the welding point that connects the preceding material and the following material, the same current flows in the heating range between the energizing roll and the molten zinc bath. Since the temperature is decreased on the large area side and the temperature is increased on the small cross-sectional area side, there has been a problem in that the final reached plate temperature entering the molten zinc bath fluctuates greatly, causing troubles such as plate breakage.
In order to minimize the fluctuation of the final plate temperature, it is necessary to switch the current while the joint at the welding point is within the heating range, but determining the appropriate switching timing is a problem. It was.
For example, in the case of “the cross-sectional area of the preceding material> the cross-sectional area of the following material”, a large current necessary for heating the preceding material having a large cross-sectional area will flow until the succeeding material reaches the switching point. This current is excessive for a succeeding material having a small cross-sectional area, and if the switching timing is delayed, the succeeding material having a small cross-sectional area may be overheated to cause a plate breakage.
On the other hand, if the switching timing is too early, the preceding material having a large cross-sectional area will not be sufficiently heated, and plating peeling will occur at the portion where the plate temperature of the preceding material is lower than the predetermined temperature. In some cases, it causes damage.
In the case of “the cross-sectional area of the preceding material <the cross-sectional area of the succeeding material”, the phenomenon is reversed, but the same problem has occurred.

Moreover, in the conventional energization heating method using an energizing roll, when a joint at a welding point connecting the steel strip passes through the energizing roll, a heating current is applied to prevent generation of wrinkles on the steel strip and the roll due to sparks. When the heating current is cut, the current does not flow in the steel strip between the energizing roll and the molten zinc bath, so the problem is that the final temperature of the plate that enters the molten zinc bath decreases. there were.
No. 6-30844

    The present invention solves the above-mentioned problems of the prior art in the method of energizing and heating a steel strip having a welding point by energizing and heating the steel strip using an energizing roll electrode and a bath electrode. Even when the cutting area changes, it is possible to keep the final plate temperature entering the molten metal bath within a certain range that does not cause breakage and peeling of the plate, and when cutting the current before and after the welding point. It is an object of the present invention to provide an electric heating method for the steel strip that can secure the final temperature of the steel strip even if it exists.

The present invention has been made as a result of intensive studies in order to solve the above-mentioned problems, and the gist of the present invention is the following contents as described in the claims.
(1) A steel strip that is continuously fed is brought into contact with an energizing roll disposed on the entry side of the steel strip, and is brought into contact with a metal bath disposed on the exit side of the steel strip. An electric heating method for a steel strip that uses a bath as an electrode to energize and heat the steel strip between the electrodes, and a welding point that connects a preceding material and a succeeding material having different cross-sectional areas of the steel strip within the current heating range. When passing, the heating current is switched so that the switching distance L2 from the welding point passing through the energizing roll to the heating current set value switching point satisfies the following formula (A): Band heating method.
L2 = L1 × (1-t) × (Ja ′) ² / ((1-t) × (Ja ′) ² + (1 + t) × (Jb ′) ² ) (A)
Where L2: switching distance from energizing roll to heating current set value switching point
L1: Heating length in the current heating range
Ja: Set current density of the preceding material
Jb: set current density of the following material
a: Cross-sectional area of the preceding material
b: Cross-sectional area of following material
Ja '= Jb × b / a
Jb '= Ja x a / b
t: Constant (2) The current shortage with respect to the current corresponding to the final temperature of the steel strip due to the energization cut that stops the current when the welding point of the steel strip passes through the energizing roll, The method for energizing and heating a steel strip according to (1), wherein a final ultimate plate temperature of the steel strip is secured by supplying in addition to a normal set current before and after.

According to the present invention, even when the cross-sectional area of the steel strip changes at the welding point, it is possible to keep the final reached plate temperature entering the molten metal bath within a certain range. There is no occurrence of plate breakage, plating peeling, etc. at the part, and heating can be performed by energization and stable production becomes possible.
In addition, even when performing energization cut before and after the welding point, the heating current is cut in the vicinity of the energizing roll by supplying more current before and after the energization cut by the amount that the final ultimate plate temperature decreases by energization cut. Even in this case, it is possible to ensure the plating quality by ensuring the final temperature of the steel strip, and to achieve remarkable industrially useful effects, such as improving the yield of the steel strip.

Hereinafter, an embodiment of an electric heating method to which the present invention is applied will be described with reference to FIG. In addition, this embodiment is an apparatus which preheats in order to hot-plate a steel strip.
FIG. 1 is a diagram illustrating an embodiment of a method for electrically heating a steel strip in the present invention.
The voltage induced in the steel strip W by the ring transformer 20 flows through the conductive roll 3 and the metal bath 30 with the conductive member 14 as a retrace line, and the steel strip W is heated. The length of the steel strip W from the center of the energizing roll 3 to the immersion point in the bath 32 is the heating length L1.
The metal bath 30 is configured by filling a bath 31 with a molten metal 32, and the end of the conductive member 14 is immersed in the molten metal 32. The steel strip W that is sent by being electrically heated is changed in direction by the direction changing roll R5, immersed in the molten metal 32, changed in direction by the direction changing roll R6, and discharged. In addition, the conductive member 14 is divided into an upper portion 14a and a lower portion 14b immediately after the energizing roll 3, and is united immediately before the direction changing roll R5. The upper part 14c and the lower part 14d are immersed in the molten metal 32 by being divided into side parts 14d. As the molten metal, for example, molten zinc or a molten zinc-based alloy can be employed.

FIG. 2 is a detailed view around the energizing roll in FIG. 1.
In FIG. 2, 1 is a preceding material, 2 is a following material, 3 is an energizing roll (CDR), L1 is a heating length from the energizing roll to the metal bath intrusion position, and L2 is an energizing roll to a heating current set value switching point. Indicates the switching distance.
As shown in FIG. 2, in the steel strip to be heated by the present invention, the preceding material 1 and the following material 2 are joined at the welding point, and from the center of the energizing roll (CDR) 3 to the metal bath intrusion position. A current is supplied up to the distance of the heating length L1, and it is heated from room temperature to a final temperature of about 450 ° C., for example.
Here, when the welding point connecting the preceding material 1 and the succeeding material 2 passes through the electric heating range, the final reached plate temperature of the preceding material 1 and the succeeding material 2 is “below the temperature at which the plate breaks, and The switching control is performed by obtaining an optimum switching point that is “above the temperature at which plating peeling does not occur”.
At this time, the point at which the current heating range is reached is changed to the heating setting of the next material (= energization amount setting) from the viewpoints of preventing overheating of the plate and preventing non-plating due to temperature drop. It becomes a very important factor.

  Then, the current heating method of the steel strip according to the present invention is such that the continuously fed steel strip is brought into contact with the current-carrying roll 3 disposed on the entry side of the steel strip and disposed on the exit side of the steel strip. An electric heating method for a steel strip that is brought into contact with a metal bath and energizes and heats the steel strip between the electrodes using the current roll 3 and the metal bath as an electrode, and the preceding material 1 and the rear material having different cross-sectional areas of the steel strip When the welding point connecting the row material 2 passes through the energization heating range, the switching distance L2 from the welding point passing through the energization roll to the heating current set value switching point is calculated by the following method. The heating current is switched.

<Concept of switching distance setting method>
Since the amount of heat for heating the steel strip is proportional to the square of the current, the method of setting the switching distance L2 is to set the current switching point by the ratio of the square of the required secondary current of the preceding material 1 and the following material 2. The way to find it can be considered.
However, with this method, it is difficult to prevent overheating and insufficient heating when the cross-sectional area difference is large because the current density of the own material when the counterpart material of the preceding material 1 and the following material 2 is set cannot be considered. I found out that
Therefore, in the present invention, the target temperature of the counterpart material is basically set on the assumption that the target temperature of the leading material 1 and the following material 2 does not change (= the required current density at the same speed does not change). In this case, the current density of the own material was calculated, and the switching distance L2 was determined by apportioning the heating length L1 according to the ratio.

That is, if the cross-sectional area of the preceding material: a (mm2), the cross-sectional area of the following material: b (mm2), the set current density of the preceding material: Ja (= setting of the following material: Jb) (A / mm2) The current density Jb ′ of the succeeding material 2 when the succeeding material 2 enters the heating portion while being controlled by the current setting of the preceding material 1 is expressed by the following equation.
Jb '= Ja x a / b (A / mm2)
Further, the current density Ja ′ of the preceding material 1 in the heating section when the current setting of the succeeding material 2 is switched is expressed by the following equation.
Ja '= Jb × b / a (A / mm2)
Further, since the ratio of Ja ′ and Jb ′ increases as the cross-sectional area difference increases, the switching distance L2 is determined by the following equation (A) that apportions the heating length L1 with this ratio.
By using this equation (A), the switching distance L2 can be obtained in consideration of the current density of the own material when the preceding material 1 and the succeeding material 2 are set as the mating materials. Even when is large, overheating and insufficient heating can be prevented.
L2 = L1 × (1-t) × (Ja ′) ² / ((1-t) × (Ja ′) ² + (1 + t) × (Jb ′) ² ) (A)
Where L2: switching distance from energizing roll to heating current set value switching point
L1: Heating length in the current heating range
Ja: Set current density of the preceding material
Jb: set current density of the following material
a: Cross-sectional area of the preceding material
b: Cross-sectional area of following material
Ja '= Jb × b / a
Jb '= Ja x a / b
t: Constant The constant t is a correction coefficient for adjusting the range of the final ultimate plate temperature, and may be set as appropriate according to the target upper and lower limits of the final ultimate plate temperature.

The heated steel strip is immersed in a plating bath to be plated. However, if the steel strip temperature deviates from the target final temperature, non-plating or non-uniform plating occurs.
On the other hand, when the welding point passes through the energizing roll (CDR) 3, the energization of the energizing roll 3 or the steel strip may be flawed due to the unevenness of the welding point, so that energization is stopped to prevent sparks. However, there is a problem of a decrease in the plate temperature at this time.
Therefore, a preferred embodiment of the electric heating method of the present invention is an electric heating method for a steel strip in which a steel strip having a welding point is energized and heated using an energizing roll (CDR) 3 and a metal bath 30 as electrodes. The current that is insufficient for the current corresponding to the final temperature of the steel strip is added to the normal set current before and after the energization cut by the energization cut that stops the current when the welding point passes through the energization roll 3. By supplying, it is characterized by ensuring the final temperature of the steel strip.
The present inventors have intensively studied the compensation amount and the compensation distance of the insufficient current by the current cut that stops the current when the welding point for joining the leading material 1 and the following material 2 passes through the energizing roll (CDR) 3. As a result, the final ultimate plate temperature of the steel strip is supplied by supplying more current before and after the energization cut by the amount that the final ultimate plate temperature is reduced by the energization cut that stops the current when the welding point passes the energization roll. It was found that the plating quality can be ensured.

An embodiment in which a hot-dip zinc bath is employed as the metal bath shown in FIG. 1 and the steel strip having the welding points shown in FIG. 1 is heated using the electric heating method of the present invention under the following conditions is shown in FIGS. .
FIG. 3 is a diagram showing the intrusion temperature into the metal bath pot when the cross section of the steel strip is changed from a thick material to a thin material using the method for energizing and heating the steel strip of the present invention.
FIG. 4 is a diagram showing the intrusion temperature into the metal bath pot when the cross section of the steel strip is changed from a thin material to a thick material using the method for energizing and heating the steel strip of the present invention.
<Conditions for implementation>
・ Heating length L1: 18m
・ Final temperature: 450 ℃

Figure 3 shows the current heating range of the leading material 1 with a plate thickness of 0.5 mm and a plate width of 1000 mm at a current of 9.8 KA, and the welded part with the subsequent material 2 with a plate thickness of 0.4 mm and a plate width of 1000 mm. It shows the change of the pot penetration temperature when conducting heating through the inside.
At this time, when the heating current switching point of the preceding material 1 and the succeeding material 2 was substituted by the heating length of 18 m and t = 0.1 in the following formula (A), the switching distance was 4.5 m. As a result of changing the setting of the heating current at this point, the minimum temperature of the leading material on the thick material side is 300 ° C, and the maximum temperature of the following material on the thin material side is 600 ° C. There was no occurrence, and energization switching was possible at the weld joint.
L2 = L1 × (1-t) × (Ja ′) ² / ((1-t) × (Ja ′) ² + (1 + t) × (Jb ′) ² ) (A)
Where L2: switching distance from energizing roll to heating current set value switching point
L1: Heating length in the current heating range
Ja: Set current density of the preceding material
Jb: set current density of the following material
a: Cross-sectional area of the preceding material
b: Cross-sectional area of following material
Ja '= Jb × b / a
Jb '= Ja x a / b
t: Constant

Also, in FIG. 4, contrary to FIG. 3, the preceding material 1 having a plate thickness of 0.4 mm and a plate width of 1000 mm is energized and heated at a current of 7.84 KA, and the succeeding material 2 continuously has a plate thickness of 0.5 mm and a plate width of 1000 mm. This shows the change in the pot intrusion temperature when the energization heating is performed by passing the welded portion in the energization heating range, and the result of setting switching at the switching distance L2 obtained by the above-described equation (A). There was no occurrence of breakage of the plate, peeling of the plating, etc., and the energization could be switched at the weld joint.
In both FIG. 3 and FIG. 4, the current is cut in the range of 50 cm before and after the welded portion, and the current that is insufficient due to the current cut is applied by passing the heating current at 6 m before and after the welded portion over the normal current. Compensation could prevent temperature drop.

It is a figure which illustrates embodiment of the current heating method of the steel strip in this invention. It is a figure which illustrates embodiment of the current heating method of the steel strip in this invention, Comprising: It is detail drawing around the electricity supply roll in FIG. It is a figure which shows the penetration | invasion temperature to a metal bath pot in case the cross section of a steel strip is changed from a thick preserve to a thin thing using the current heating method of the steel strip of this invention. It is a figure which shows the permeation temperature to a metal bath pot in case the cross section of a steel strip is changed into a thick thing from the thin thing using the current heating method of the steel strip of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Leading material 2 Subsequent material 3 Current supply roll 14 Conductive member 20 Ring transformer 30 Metal bath 31 Bath tub 32 Molten metal R5, R6 Direction change roll W Steel strip L1 Heating length L2 Switching from current supply roll to heating current set value switching point distance

Claims (2)

The steel strip that is continuously fed is brought into contact with an energizing roll disposed on the entry side of the steel strip, and is brought into contact with a metal bath disposed on the exit side of the steel strip, and the energizing roll and the metal bath are connected to electrodes. In this method, the steel strip between the electrodes is energized and heated, and the welding point connecting the preceding material and the subsequent material having different cross-sectional areas of the steel strip passes through the current heating range. The heating current is switched so that the switching distance L2 from the welding point passing through the energizing roll to the heating current set value switching point satisfies the following expression (A): Heating method.
L2 = L1 × (1-t) × (Ja ′) ² / ((1-t) × (Ja ′) ² + (1 + t) × (Jb ′) ² ) (A)
Where L2: switching distance from energizing roll to heating current set value switching point
L1: Heating length in the current heating range
Ja: Set current density of the preceding material
Jb: set current density of the following material
a: Cross-sectional area of the preceding material
b: Cross-sectional area of following material
Ja '= Jb × b / a
Jb '= Ja x a / b
t: Constant When the welding point of the steel strip passes through the energizing roll, a current that is insufficient with respect to the current corresponding to the final temperature of the steel strip due to the energization cut that stops the current is set to a normal set current before and after the energization cut. The method for energizing and heating a steel strip according to claim 1, wherein a final ultimate plate temperature of the steel strip is ensured by supplying in addition to the above.

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