JP2863485B2 - 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 control method for controlling the level of molten steel in a mold in continuous casting by using a stopper system or a sliding nozzle system depending on casting conditions.

[0002]

2. Description of the Related Art To stabilize the level of molten steel in a mold in continuous casting, powder is made to flow uniformly between the mold and the slab so that a powder film having a constant thickness is uniformly formed on the surface of the slab. This is an important requirement for producing a slab of good internal quality by forming a uniform solidified shell to prevent the occurrence of surface flaws and reducing slag entrainment on the molten steel surface in the mold. .

Conventionally, as a method for controlling the level of molten steel in a mold to a constant level, conventionally, the level of molten steel in the mold is detected by a level meter, and a stopper is provided so that the deviation between the detected value and the target level value becomes zero. There is a stopper (hereinafter, referred to as ST) system that moves up and down to control the flow rate of molten steel, and a sliding nozzle (hereinafter, referred to as SN) method that controls the flow rate of molten steel by opening and closing a nozzle. Due to its structure, it is possible to control the flow rate at a low flow rate at the start and end of casting, but as shown in the ST flow rate characteristic in FIG. There is a problem that the stability of the level control is inferior due to a deviation from the center of the nozzle due to a setting error or a bending at the time of ST preheating. On the other hand, in the SN system, as shown in the flow characteristics in FIG. 1, the flow controllability from a low flow rate to a high flow rate is good, and the variation in SN set and deformation due to preheating are unlikely to occur. Although excellent, at the time of low flow rate control with a small SN opening such as at the start and end of casting, solidification of molten steel occurs,
The nozzle may become clogged, and stable molten steel flow rate controllability may not be obtained.

[0004] As a method of controlling the molten steel bell in the mold,
Japanese Patent Application No. 2-262600, JP-A-6-15426,
Alternatively, Japanese Patent Application Laid-Open No. 7-88607 proposes a method of selectively using the ST method and the SN method according to casting conditions. That is, in JP-A-6-15426, S
When the level control by either T or SN is performed, the other opening is gradually changed at a constant rate of change, and when one of the opening changes accordingly reaches a set value, the control method is executed. Has been proposed. According to this method, if the rate of change of the other opening is increased, the molten steel level in the mold may fluctuate, and if the rate of change of the opening is reduced so as not to cause the molten steel level to fluctuate, the change up to switching is not possible. The time will be longer.

In order to solve the above-mentioned problem, Japanese Patent Laid-Open No.
No. 88607, when switching from the ST system to the SN system or from the SN system to the ST system,
With the level control based on one of T and SN, the other opening is changed so that the rate of change gradually decreases, and when one of the opening changes with this reaches the set value. In addition, a method of switching the control method has been proposed.

[0006]

According to the latter method of changing the opening degree of the other so that the rate of change gradually decreases while the level control is performed by one of ST and SN, the switching time is reduced. And the prevention of fluctuations in the molten steel level in the mold at the time of control switching from the SN method to the ST method could be realized. Fluctuations could be introduced. This will be described in detail below.

In the latter method, when the control is switched from the ST system to the SN system, as shown in FIG.
From the time point when the switching from the T method to the control method by the SN method is started, the SN is decreased by β _n , β _n−1 , β _n−2 / se.
c stepwise refinement, the ST opening degree increases with this detected by the opening detector, switched to level control by SN method when the detected degree of opening is increased set value delta _1% than switching start time, After switching, the ST opening is opened at a rate of increase α ₁ % / sec until the ST is fully opened, and thereafter, the level is controlled by the SN method alone when the ST is fully opened.

The ST opening during control switching from the ST system to the SN system corresponds to a portion A surrounded by a dotted line in FIG. 1 showing characteristics of ST flow control and SN flow control. After the switching is started, when the SN starts to narrow the opening from the fully open state, the ST opening gradually increases accordingly. However, when the flow rate characteristic curve of the ST used at this time is as shown in FIG. Since the amount of change in the ST opening is relatively large and the time required for the ST opening to increase by ₁ % or more is short, control switching from the ST method to the SN method occurs early, and the amount of increase in the ST opening is small. When it is large, the discharge amount of molten steel becomes unnecessarily large, and the molten steel level in the mold tends to increase. Conversely, when the flow rate characteristic line of ST is as follows, the change amount of the ST opening degree with respect to the change of the discharge flow rate is relatively small.
T opening tends less supply amount of up to _1% increase or delta, delay control switching of the SN method from the ST method,
Since molten steel is discharged in a state where the ST opening is narrowed, the supply amount of molten steel tends to decrease, and the level of molten steel in the mold tends to decrease.

The present invention makes full use of the characteristics of the molten steel level control in the mold by the ST method and the SN method so that the switching of the control method between the ST method and the SN method does not cause the fluctuation of the molten steel level in the mold. It is intended to be able to perform smoothly.

[0010]

According to a first aspect of the present invention, an ST is raised and lowered and S
The ST control method for automatically adjusting the T opening and the SN control method for automatically adjusting the SN opening are selectively used according to casting conditions.
In the method of changing the SN opening stepwise with the level control by ST performed when the control is switched from the ST method to the SN method, the time when the SN opening is switched after the casting speed reaches a constant casting speed from the start of casting. The present invention is characterized in that when the aperture is narrowed down to the SN opening calculated from the discharge flow rate of the nozzle, switching to the SN control method is performed. The SN control method for automatically adjusting the SN opening is properly used depending on the casting conditions. When the control is switched from the ST method to the SN method, the S
In the method in which the opening degree of SN is changed stepwise while the level control by T is performed, after reaching a constant discharge flow rate from the start of casting, the corrected SN opening degree is calculated from the nozzle diameter calculated from the discharge flow rate at the time of switching. Is calculated, and the SN opening is corrected S
When the aperture is narrowed down to the N opening, switching to the SN control method is performed.

In a third aspect of the present invention, the ST is moved up and down to
The ST control method for automatically adjusting the opening and the SN control method for automatically adjusting the SN opening are selectively used according to casting conditions.
In the method in which the opening degree of SN is changed stepwise while the level control by ST is performed when the control is switched from the T method to the SN method, a constant casting speed is reached after starting the casting to reach a constant discharge flow rate. Until the SN opening is switched to the SN control method when the SN opening is narrowed down to the SN opening calculated from the discharge flow at the time of switching, and after reaching a certain discharge flow, the SN opening is calculated from the discharge flow at the time of switching. A corrected SN opening is calculated from the nozzle diameter, and when the SN opening is narrowed down to the corrected SN opening, switching to the SN control method is performed.

[0012] The present inventors have estimated the causes of the fluctuations in the molten steel level in the mold when switching from the ST system to the SN system, based on the results of test investigations and analytical studies. The discharge flow rate Q _SN by SN control is generally

[0013]

(Equation 1)

## EQU1 ## Here, k: coefficient, ρ: molten steel density, d: nozzle diameter, g: gravitational acceleration, h: molten steel head height. Tundish 14 (hereinafter referred to as TD)
FIG. 2 shows a state where molten steel 12 is passing through the nozzle of FIG.
However, since it is considered that the inside of the nozzle from ST15 to SN16 has a full flow, it is assumed that the ST opening is gradually increased within the ST control range (A in FIG. 1) which is now considered and the SN opening is gradually increased. Considering the case of automatic control by
At this time, the SN opening is considered to have the same opening as that without ST because the flow rate change due to the change in the ST opening is small. That is, since the discharge flow rate is considered to follow the equation (1) regardless of the position of the stopper, the ST method shifts to S
When the control is switched to the N method, the SN opening at the time of switching to the SN control is the nozzle diameter, which is calculated back from the discharge flow rate (passing flow rate in the nozzle) at that time by equation (1), that is, the SN opening degree (Hereinafter referred to as the theoretical SN opening degree), it is considered that the level in the template fluctuates.

Specifically, when the control is switched from the ST system to the SN system, when the actual SN opening detected by the detector at the time of switching to the SN control is larger than the theoretical SN opening, Since the amount of molten steel more than necessary is injected into the mold, the level of molten steel in the mold rises. Conversely, when the molten steel level is smaller than the theoretical SN opening, the level of molten steel in the mold falls due to the influence of excessively narrowing of the SN opening. . Therefore, the timing of switching from ST control to SN control in this case may be switched to ST control when the aperture is narrowed down to the theoretical SN opening.

The above-mentioned control switching can be applied at a stage where almost no inclusions are found in the nozzle, such as when switching after a constant casting speed is reached after the start of casting, and after the start of casting, the inside of the nozzle is changed. When the control switching is performed after the molten steel amount of several tens tons or more has passed, the inclusion of the inclusion 10 has occurred in the nozzle (see FIG. 3), so that the SN control flow rate characteristic shown in FIG. The relationship of the SN opening does not hold. Therefore, the actual SN opening at the time of switching is
If the opening degree is N, the theoretical SN opening degree is smaller than the actual nozzle diameter, so that the supply amount of molten steel is insufficient, and the molten steel level in the mold is lowered.

On the other hand, as described above, the flow rate characteristics of the ST control vary depending on the ST setting error, bending at the time of ST preheating, and the like. However, the discharge flow rate Q _ST is expressed as follows. Q _ST = f (A) (2) A = πl _sin (α / 2) · (l _tan (α / 2) + D + R (1- _cos (α / 2))) (3) Here, A: opening area between ST and nozzle, l: ST opening, α: angle of ST head, D: nozzle diameter, R: part where ST is in contact with the upper part of nozzle when fully closed (circle) The discharge flow rate Q _ST at the time of ST control is represented by ST opening degree 1 and nozzle diameter D
Since there is a relationship as shown in equations (2) and (3),
After the start of casting, the molten steel amount of several tens tons or more has passed through the nozzle, and then ST
When the control is switched from the method to the SN method, the nozzle diameter at that time is back calculated from the ST opening and the discharge flow rate before the start of the switching, that is, how much the nozzle diameter is reduced due to nozzle narrowing is calculated. The nozzle diameter corrected in this case is referred to as a corrected nozzle diameter. By calculating the relationship between the SN opening degree and the discharge flow rate with respect to the corrected nozzle diameter by the equation (1), it is possible to know the SN opening degree when the SN is independently controlled in the case of the discharge flow rate at the time of switching. Then, the SN opening degree is set so that the level of the molten steel in the mold does not fluctuate when switching from the ST control to the SN control.
The SN opening at this time is referred to as a corrected theoretical SN opening.

In summary, when performing control switching from the ST system to the SN system, the timing of the switching in the method of changing the SN opening stepwise in a state where the level control by the ST is performed is based on the casting start. Immediately after, the SN opening is narrowed down to the theoretical SN opening and then switched. After the casting is started, the state of the nozzle narrowing at that time is estimated, and the SN opening is narrowed down to the corrected theoretical SN opening and then switched.

FIG. 4 shows a continuous casting facility 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 long nozzle 13, a control device 1
The molten steel flow rate is adjusted by ST15 or SN16 driven by control signals from 9 and 23, and the molten steel is injected into the mold 17. The control of the molten steel level in the mold by ST15 is performed as follows.

The molten steel level in the mold detected by the level sensor 18 is compared with a set value of the molten steel level in the mold previously input to the control device 19 via the converter 20, and 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 16 is performed by inputting the molten steel level value in the mold detected by the level sensor 18 into the control device 23 via the converter 20 in advance. Compare with the level setting value,
If there is a deviation, the deviation is output to the SN opening controller 25, and SN1
The controller 25 drives the cylinder 26 according to the deviation from the SN opening detected by the SN opening detector 27 of No. 6;
The molten steel level is controlled by adjusting the SN opening. As described above, the molten steel level in the mold is controlled by both the ST method and the SN method, and one of them is selected according to the casting conditions.

Next, an example of a switching method when 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. From the state where ST is fully closed and SN is fully open, molten steel is poured into the TD from the ladle in FIG. 4 and casting is started when the TD weight reaches the set weight. 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, S
The molten steel level control in the mold is performed by the T method.

Immediately after the start of casting, the casting speed is equal to the set casting speed V.
c, which is stable and is detected by the weight detector
Weight reached the set weight W _2, after the opening degree variation of ST is reduced, switching of the control method by SN method from the ST method is initiated, as shown the SN opening, beta _n, beta
_N-1 and β _n-2 % / sec. Then, using the relationship between the SN opening degree and the SN control flow rate characteristic in FIG. 1, the theoretical SN opening degree is back-calculated by a calculator from the discharge flow rate before the start of switching (the amount of molten steel passing through the nozzle), and the theoretical SN opening degree At the point when the aperture is narrowed down, the control is switched to ST control.

After the start of casting, the method of switching after casting several tens of tons is such that the casting speed reaches the set casting speed Vc and is stable, and the TD weight detected by the weight detector reaches the set weight W _2. in the place where, after opening variation becomes smaller set time ST, the switching of the control method by SN method from the ST method is started, β _n, β _n-1 as shown the SN opening, beta _{n Narrow} down gradually to _-2 % / sec. The arithmetic unit first calculates the nozzle diameter (corrected nozzle diameter) at that time from the ST opening before switching and the discharge flow rate using equations (2) and (3). The relationship between the SN opening and the discharge flow rate is calculated, and in the case of the discharge flow rate at the time of switching, the SN opening (correction theoretical SN opening) when SN is controlled alone is calculated. Then, when the aperture is narrowed down to the correction theoretical SN opening, the control is switched to the ST control.

According to the present method, immediately after the start of casting, or after casting several tens tons after the start of casting, the molten steel level in the mold is maintained without changing the metal surface level.
Control switching from the T system to the SN system can be performed. Example 1 Using the continuous casting equipment shown in FIG. 4, after the start of casting, the SN opening was fully opened (100%), and the level of molten steel in the mold was controlled to S.
The switching to the SN system was performed under the following conditions while the system was automatically performed by the T system.

Mold thickness 200 mm Cast width 965 mm Casting speed at the time of switching 0.7 m / min Nozzle diameter 50 mm TD molten steel head height 1000 mm The discharge flow rate at the time of switching to the SN method is 0.96 Ton / min.
The SN opening movement amount detected by the opening detector 27 is 21 mm from the fully closed state, and the theoretical SN opening at the above-mentioned discharge flow rate calculated from the equation (1) is 68%. Therefore, the SN should be shifted by 32% from the fully opened state. Switching the start of the SN method were carried out by the automatic after casting speed and TD weight has passed 10 seconds to reach the set value W _2. As the switching procedure, after the switching is started, the change rate of the SN opening is narrowed down to 0.4% / sec for 75 seconds in the first stage and 0.06% / sec for 33 seconds in the second stage.
Switched to the system. After the switching, the rate of change of the ST opening was raised to 0.06% / sec for 33 seconds in the first stage, and to 0.4% / sec for 75 seconds in the second stage, until it was fully opened. FIG. 6 shows a time chart at the time of implementation.

Example 2 After the start of casting, after 200 Ton was already cast, the SN opening was fully opened (100%) and the level of molten steel in the mold was controlled by S.
The switching to the SN method was performed under the following conditions while the T method was used. Mold thickness 200mm Casting width 925mm Casting speed when switching 0.7m / min Nozzle diameter 50mm TD molten steel head height 1000mm Discharge flow rate when switching to SN system is 0.92 Ton / min. The detected movement amount of the ST from the fully closed state during the automatic control was 15 mm, but when the discharge flow rate is 0.92 Ton / min, the movement amount of the ST from the fully closed state is considered from the flow rate characteristics of the ST. And 10 mm, which is equivalent to a change in the nozzle diameter from 50 mm to 40 mm according to the ST flow characteristics of the present equipment. That is, it means that the molten steel passage area is reduced due to the inclusion of the inclusion in the nozzle. When the nozzle diameter is 40 mm, the SN movement amount obtained from Expression (1) is 22
mm, and the theoretical SN opening is 82% in this facility. Therefore, the SN should be moved by 18% from the fully opened state. Switching to the SN system was performed automatically after 10 seconds had elapsed after the casting speed and the TD weight reached the set values. As a switching procedure, the change rate of the SN opening is set to 0.4% / sec for 42 seconds in the first stage and 20% in the second stage.
The aperture was narrowed down to 0.06% / sec, and switched to the SN system. After the switching, the change rate of the ST opening is set to 20 in the first stage.
0.06% / sec for second, 0.4% for 42 second in second stage
/ Sec to fully open. FIG. 7 shows a time chart when the operation is performed.

[0028]

The present invention is configured as described above and has the following effects. According to the control method of the first aspect,
By switching from the ST method to the SN method after narrowing the SN opening to the theoretical SN opening, fluctuations in the molten steel level in the mold at the time of switching are prevented, and control switching to the SN method can be performed smoothly. it can.

According to the control method of the present invention, the SN opening is corrected by taking into account the degree of narrowing of the nozzle after the start of casting.
By switching after narrowing to N opening,
Fluctuation in the molten steel level in the mold at the time of switching can be prevented, and control switching to the SN system can be performed smoothly. According to the control method of the third aspect, the SN opening is reduced to the theoretical SN opening until a constant discharge flow rate is reached, and after reaching the constant discharge flow rate, the SN opening is reduced to the corrected SN opening degree before switching. By doing so, a change in the molten steel level in the mold at the time of switching at the time of switching is prevented, and control switching to the SN system can be performed smoothly.

[Brief description of the drawings]

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

FIG. 2 is a diagram showing the flow of molten steel in the vicinity of an SN plate.

FIG. 3 is a diagram showing a closed state near a nozzle;

FIG. 4 is a schematic view of a molten steel level control device used in the method of the present invention.

FIG. 5 is a time chart according to the method of the present invention.

FIG. 6 is a time chart when switching from the ST method to the SN method.

FIG. 7 is a time chart when switching from the ST method to the SN method.

[Explanation of symbols]

11. Ladle 12, molten steel 13, long nozzle 14, tundish TD 15, stopper 16, SN 17, mold 18, molten metal level sensor 19, 23, control device 20, conversion Unit 21 ST opening adjuster 22, 26 Cylinder 24 ST opening detector 25 SN opening adjuster 27 SN opening detector

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI B22D 41/22 B22D 41/22 (56) References JP-A-7-88607 (JP, A) JP-A-6-15426 (JP) JP-A-4-138859 (JP, A) JP-A-3-142051 (JP, A) JP-A-60-102252 (JP, A) JP-A-56-141953 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) B22D 11/18 B22D 37/00 B22D 41/16 B22D 41/22

Claims (3)

(57) [Claims] 1. An ST control method for automatically adjusting an ST opening by raising and lowering an ST and an SN control method for automatically adjusting an SN opening are selectively used according to casting conditions, and control is switched from the ST method to the SN method. Sometimes, in a method in which the opening degree of the SN is changed stepwise while the level control by the ST is performed, the SN calculated from the discharge flow rate of the nozzle at the time when the SN opening degree is switched after the casting speed becomes constant after the start of casting. A method for controlling the level of molten steel in a mold in continuous casting, characterized by switching to an SN control method when the aperture is narrowed down to an opening. 2. An ST control method for automatically adjusting the ST opening by raising and lowering the ST and an SN control method for automatically adjusting the SN opening depending on casting conditions, and control switching from the ST method to the SN method. Sometimes, in a method in which the opening degree of SN is changed stepwise while the level control by ST is performed, after a certain discharge flow rate is reached from the start of casting, the correction SN is calculated from the nozzle diameter calculated from the discharge flow rate at the time of switching. A method for controlling the level of molten steel in a mold in continuous casting, comprising calculating an opening and switching to an SN control method when the SN opening is narrowed down to a corrected SN opening. 3. An ST control method for automatically adjusting the ST opening by raising and lowering the ST and an SN control method for automatically adjusting the SN opening depending on casting conditions, and control switching from the ST method to the SN method. Sometimes, in a method in which the opening degree of SN is changed stepwise while the level control by ST is performed, after the start of casting, the SN opening degree becomes constant at a constant casting speed and reaches a constant discharge flow rate. When the aperture is narrowed down to the SN opening calculated from the discharge flow rate at the time of switching, the mode is switched to the SN control method. After reaching a certain discharge flow rate, the corrected SN opening is calculated from the nozzle diameter calculated from the discharge flow rate at the time of switching. A method for controlling the level of molten steel in a mold in continuous casting, wherein the SN control method is switched when the SN opening is narrowed down to the corrected SN opening.