JP2856305B2 - Control method of molten steel level in mold in continuous casting - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for controlling the level of molten steel in a mold in continuous casting, and more particularly to a method for accurately controlling the level of molten steel in a mold so that a deviation from a target level value becomes zero. It relates to a control method.

[0002]

2. Description of the Related Art In continuous casting, the level of molten steel in a mold is stabilized by uniformly flowing powder between the mold and the slab to prevent the occurrence of surface flaws on the slab. This is an important requirement for preventing slag from being entrained and producing a slab with good internal quality.

Conventionally, as a method for controlling the level of molten steel in a mold to a target level, the level of molten steel in the mold is detected by a level meter, and when there is a deviation from the target level, the detected value and the target level value are detected. So that the deviation from
There are a stopper (hereinafter, referred to as ST) system for controlling the flow rate of molten steel by raising and lowering a stopper, and a sliding nozzle (hereinafter, referred to as SN) system for controlling the flow rate of molten steel by opening and closing a nozzle.

As can be seen from the flow characteristics of ST and SN in FIG. 1, the ST system has poor flow control characteristics in a high flow region, whereas the SN system has a flow control characteristic ranging from a low flow region to a high flow region. It has good control characteristics and is excellent in controlling the level of molten steel in a mold. However, during low flow rate control, the solidification of molten steel may occur, resulting in nozzle clogging and repeated reuse may not be possible.

In order to avoid the above problem, Japanese Patent Laid-Open No.
No. 38859 includes an ST method and an SN method according to casting conditions.
There has been proposed a method for selectively using different methods. In this method, when switching from the ST method to the SN method or vice versa, switching from the SN method to the ST method performs level control by the SN method while gradually increasing the ST opening degree as a function of time. When the degree is fully opened, level control by the SN method alone is performed, or level control by the ST method is performed while gradually increasing the SN degree as a function of time, and when the SN degree is fully opened, Level control is performed by the ST method alone.

[0006]

When switching from the above-mentioned conventional ST system to the SN system or to the reverse control system is performed, the opening degree of either ST or SN is expanded as a function of time, or When the aperture is narrowed down, if the other opening is not changed correspondingly, the other opening changes abruptly when the control method is switched, resulting in a change in the level of molten steel in the mold.

In order to solve this problem, the present applicant has previously disclosed in Japanese Patent Application No. 4-175696 a ST or S
In the state where the level control is performed by one of N, the other opening is gradually changed at a constant rate of change, and when the one opening that changes accordingly reaches a set value, the control method is switched. The proposed control method is proposed. Also in this method, if the rate of change of the other opening is increased, the level of molten steel in the mold may fluctuate, and if the rate of change of the degree of opening is reduced so as not to cause the level of molten steel, switching will occur. Time will be long.

[0008] The present invention takes full advantage of the characteristics of molten steel level control in a mold by the ST and SN methods to achieve ST and SN.
The switching of the control system between the systems can be performed more smoothly without causing a change in the level of molten steel in the mold.

[0009]

In order to solve the above-mentioned problems, the present invention detects the molten steel level in a mold and automatically adjusts the ST opening by raising and lowering the ST so that the deviation from a target level value becomes zero. By selectively using the ST method and the SN method that automatically adjusts the SN opening so that the deviation between the detected value of the molten steel level in the mold and the target level value becomes 0 according to casting conditions, In the method of controlling the molten steel level,
Switching from the ST method to the SN method or the reverse control method is performed in a multi-step manner so that the rate of change of the other is gradually reduced while the level control is performed by either ST or SN. , And the opening is changed when one of the degrees of opening that changes with this reaches a set value.

The control method is switched from the ST method to the SN method or vice versa when the fluctuation of the ST or SN opening degree is small, that is, under the condition that the casting speed is constant and stable.
It is desirable to avoid the case where the fluctuation of the molten steel level in the mold becomes large, and to carry out under the condition that the fluctuation of the molten steel level in the mold is 20 mm or less. The switching of the control method can be performed manually, but is desirably performed automatically, for example, when the casting speed reaches a set casting speed, or when the casting speed is poured from a ladle. It is preferable to perform the process when the tundish (hereinafter referred to as TD) weight reaches a set weight due to the molten steel, and more preferably, after the set time has elapsed since the set casting speed and the TD weight have reached the set weight. Good.

[0011] Even when the above-mentioned one opening degree reaches the set value, it is desirable that the switching is performed after a lapse of a predetermined time from the reaching of the set value.

[0012]

FIG. 2 shows a continuous casting equipment used for carrying out the method of the present invention. After molten steel 12 in a ladle 11 is injected into a TD 14 through a nozzle 13, control devices 19 and 23 are provided.
ST15 or SN16 driven by a control signal from
The flow rate of molten steel is adjusted by the above method and the molten steel is injected into the mold 17. S
The control of the molten steel level in the mold by T15 is performed as follows.

The molten steel level value in the mold detected by the level sensor 18 is compared with a molten steel level set value in the mold previously input to the control device 19 through the converter 20. If there is a deviation, Output to the ST opening adjuster 21,
ST detected by ST opening detector 24 in ST15
The controller 21 drives the cylinder 22 in accordance with the deviation from the opening to adjust the ST opening to control the level of molten steel in the mold.

Similarly, the control of the molten steel level in the mold by the SN is performed by inputting the molten steel level value in the mold detected by the level sensor 18 into the control device 23 through the converter 20 in advance. The deviation is compared with the set value, and if there is a deviation, it is output to the SN opening degree controller 25, and the S
The controller 25 drives the cylinder 26 in accordance with the deviation from the SN opening detected by the N opening detector 24 to adjust the SN opening to control the molten steel level. ST as described above
The control of the molten steel level in the mold is performed in both the system and the SN system, and one of them is selected according to the casting conditions.

Next, an example of a switching method for switching from the ST method to the SN method or the reverse control method during continuous casting using the above equipment will be described with reference to a time chart of FIG. ST is fully closed, SN is poured molten steel into the TD14 from the ladle 11 in FIG. 2 from the state of full opening, when the TD weight reaches a set weight W _1, to start casting. The injection of molten steel into the TD gradually increases, and the casting speed also gradually increases. When the molten steel level in the mold reaches the target level, the molten steel level in the mold is controlled by the ST method.

After the casting speed reaches the set casting speed Vc and stabilizes, the TD weight detected by the weight detector reaches the set weight W _2, and after a lapse of time t _{s at} which the variation of the opening degree of the ST is reduced, the ST method is used. Switching to the control method based on the SN method is started, and β _m , β _m−1 , β
We narrow down step by step to _m-2 % / sec. S that rises with this
The T opening is detected by the opening detector 24, and the detected opening increases by ₁ % from the switching start time, and the variation of the ST opening decreases while maintaining the SN opening at the time of the ₁ % increase. after the time t ₁ has elapsed, switched to the molten steel level control in the mold by SN method, after switching, ST opening degree index increased to fully open α
Open at ₁ % / sec. After the ST is fully opened, the molten steel level in the mold is controlled by the SN method alone.

After the lapse of the time t _E at which the molten steel in the TD decreases and the set weight W ₃ is interrupted, and the SN opening fluctuation fluctuates, switching from the SN system to the ST system is started, and
The rate of decrease of the T opening is narrowed down step by step to α _n , α _n-1 and α _n-2 % / sec. The SN opening to be opened with this is determined by the detector 24.
After the time t _{2 at} which the variation of the SN opening becomes small, while the detected SN opening keeps the ST opening at the time of Δ ₂ % opening from the switching start time, the molten steel in the mold by the ST method is detected. The mode is switched to the level control, and after the switching, the SN opening is opened at a rate of increase of β ₂ % / sec until fully opened.

According to this method, the ST and SN opening degrees are narrowed down stepwise after the start of switching so that the rate of change gradually decreases, and immediately after the start of switching, the rate of change of the opening degree is large. As shown in FIG. 5, the rate of change of the flow rate in ST is small from the fully open state to the time when the throttling is performed to a considerable extent. Therefore, even if the pressure is rapidly reduced from the fully opened state, the fluctuation of the flow rate is small, and the fluctuation of the molten steel level in the mold is also small. In addition, the time until switching is shorter than that of a constant and low change rate.

[0019]

【Example】

Example 1 After the start of casting, the level of molten steel in the mold was automatically controlled by the ST method with the SN opening fully opened. Thereafter, switching to the SN system was automatically performed after 10 seconds had elapsed since the casting speed and the TD weight reached the set values. As a switching procedure, the change rate of the SN opening degree is 0.1% / sec for 3 minutes in the first stage, and 0.07% / se for 3 minutes in the second stage.
The aperture was narrowed down as c, then the ST opening increased by 4%, the SN opening at that time was maintained for 10 seconds, and then the control was switched to the SN system control. After the switching, ST was raised to a fully opened state at a rate of 0.5% / sec.

As a result, as shown in FIG. 4, when the control system is automatically switched from the ST system to the SN system, the change of the molten steel level in the mold at the time of the change is small, and the surface and internal quality of the slab are small. Both are good, and the switching has conventionally required 8 minutes, but the present invention has reduced the time to about 6 minutes. Example 2 After the start of casting, the level of molten steel in the mold was automatically controlled by the SN method with the ST opening fully opened.

After that, switching to the ST method was performed automatically after 10 seconds had elapsed since the casting speed and the TD weight reached the set values. As a switching procedure, the rate of change of the ST opening is narrowed down to 0.1% / sec for 3 minutes in the first stage and 0.07% / sec for 3 minutes in the second stage, and then the SN opening is 4%. After rising, and maintaining the SN opening at that time for 10 seconds, the control was switched to the SN system control. After switching, the SN was opened up to the fully opened state with a hole opening rate of 0.5% / sec.

As a result, as shown in FIG. 5, when the control system is automatically switched from the SN system to the ST system,
Fluctuation of molten steel level in the mold when switching is small,
The surface and internal quality of the cast slab were both good, and the switching required eight minutes in the past, but the present invention has reduced it to about six minutes.

[0023]

The present invention is configured as described above and has the following effects. According to the control method of the first aspect,
Switching from the ST method to the SN method or the reverse control method is performed in a multi-step manner so that the rate of change of the other is gradually reduced while the level control is performed by either ST or SN. , The switching time can be shortened, and the level of molten steel in the mold at the time of switching does not fluctuate, so that a slab with no surface flaws and good internal quality can be manufactured.

According to a second aspect of the present invention, the control method is switched under a condition that the casting speed is constant and stable, or a condition that the fluctuation of the molten steel level in the mold is 20 mm or less. For example, the switching can be performed in a state where the fluctuation of the ST or SN opening is small. According to the method of claim 4, 5 or 6, the changeover is started at a casting speed, a TD weight,
Alternatively, it can be performed automatically under a stable flow rate of molten steel.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between ST opening and SN opening and molten steel discharge amount.

FIG. 2 is a schematic diagram of a molten steel level control device used in the method of the present invention.

FIG. 3 is a time chart according to the method of the present invention.

FIG. 4 is a time chart when switching from the ST system to the SN system.

FIG. 5 is a time chart at the time of switching from the SN system to the ST system.

[Explanation of symbols]

15 ... ST 16 ... SN 17 ... Mold 18 ... Level sensor 19,23 ... Control device 20 ... Converter 21 ... ST opening degree adjuster 22,26 ...
Cylinder 24 ・ ・ ・ Opening degree detector 25 ・ ・ ・ SN opening degree adjuster

Continuation of the front page (56) References JP-A-6-15426 (JP, A) JP-A-4-13859 (JP, A) JP-A-3-142051 (JP, A) JP-A-62-179859 (JP) , A) (58) Fields investigated (Int. Cl. ⁶ , DB name) B22D 11/00 B22D 11/18 B22D 39/00

Claims (6)

(57) [Claims] 1. A stopper system for detecting a molten steel level in a mold and automatically adjusting a stopper opening by raising and lowering a stopper so that a deviation from a target level value becomes zero, and detecting a molten steel level in the mold. The deviation between the value and the target level value is 0
In order to control the molten steel level in the mold by using the sliding nozzle system that automatically adjusts the opening of the sliding nozzle according to the casting conditions, the sliding nozzle system is used instead of the stopper system, or vice versa. When switching to the control method, while the level control is performed by one of the stopper and the sliding nozzle, the other opening is changed in multiple steps so that the rate of change decreases stepwise, and the change is caused by this. A method for controlling the level of molten steel in a mold in continuous casting, wherein the method is performed when the opening degree of the mold reaches a set value. 2. The method for controlling the level of molten steel in a mold in continuous casting according to claim 1, wherein the switching of the control method is performed under the condition that the casting speed is constant and stable. 3. The switching of the control method is performed under the condition that the level of molten steel in the mold has a fluctuation of 20 mm or less.
A method for controlling the level of molten steel in a mold in the continuous casting described above. 4. The switching of the control system is started after the casting time reaches a set casting speed and a set time elapses.
A method for controlling the level of molten steel in a mold in the continuous casting described above. 5. The method for controlling the level of molten steel in a mold in continuous casting according to claim 1, wherein the switching of the control method is started after the tundish weight reaches a set weight and a set time has elapsed. 6. The method for controlling the level of molten steel in a mold in continuous casting according to claim 1, wherein the switching of the control method is started after a lapse of a set time from when the one opening degree reaches a set value.