JPS5928430B2 - Breakout prevention method in continuous casting - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for preventing breakout in continuous casting, and advantageously realizes continuous casting in a safe and smooth manner.

In the continuous casting process, especially in the case of steel, there is a certain probability that unsolidified molten steel in the shell will flow out, or breakout, due to breakage of the solidified shell directly under the mold, resulting in a serious operation-disrupting accident.

This breakout has essentially a variety of causes, and although it is said that it can be divided into various cases, in reality, it often develops into a breakout due to the sudden fluctuation of force at the level of mold self-melting steel. This became clear from our operational experience.

Therefore, this invention relates to the prevention of breakout caused by such a sudden change in the molten steel level, by immediately reducing the slab drawing speed based on the drop in the level exceeding the limit,
Breakout nuclei (
It is practically effective to maintain the above-mentioned reduced drawing speed until the solidified shell thickness, including the precursor signs of the development of breakout, grows to a thickness that is safe for breakout. This is based on the knowledge that this can almost completely prevent breakouts, which are a concern during operation.

By the way, the sudden drop in the mold self-melting steel level as described above may be caused by the clogging of the tundish injection nozzle, or if the mold self-melting steel level is detected and the level is controlled based on the detection signal, the nozzle of the signal system The process by which such a level drop, caused by other erroneous signals, induces a breakout can be considered as follows.

In particular, slag bear is a process in which continuous casting flank powder, which is introduced into a mold, melts on the surface of the molten steel bath to form a slag layer that covers the molten steel bath, that is, serves as a shield from the outside air, and this slag layer comes into contact with the inner wall of the mold and is cooled. As a result, the fluidity is extremely reduced, and aggregates float around the surface of the pot bath in the mold, surrounded by the mold, as shown in FIG.

In Fig. 1, 1 is a mold, 2 is a steel bath, 3 is a solidified shell,
4 is the input flux, 5 is the molten flux or slag layer, and 6 is the slag bear.

This slag bear 6 cannot be avoided because the mold 1 is water-cooled, but if the level of the steel bath 2 remains constant or fluctuates slightly and slowly, the solidified shell 6
There is no particular impact on the production and growth of.

However, as shown in FIGS. 2a and 2b, the level of the steel bath in the mold 1 drops sharply and the slag bear 6 moves downward along with the steel bath.

At this time, if a rapid level recovery occurs, slag bears 6 will be newly generated and trapped between the solidified shell and the mold wall, as shown in FIG. A new slag bear 6' is generated and grows as shown in Fig. 4e while the level of the steel bath is restored.

The slag bear 6 that is interposed between the outer surface of the solidified shell 3 and the inner wall surface of the mold 1 exerts an insulating effect on the solidified shell 3 that is in contact with it, and hinders its smooth thickening, especially locally. Not only that, but also its horizontal shape is pointed in the direction of the thickness of the shell, and its biting causes a non-chip effect in the solidified shell 3.

If the above process is continued at a continuous drawing speed, the solidified shell will often cause breakout immediately below or below the part where it exits from the mold 1 due to stress concentration at the non-chip part. It will become.

This is due to the development of the solidified shell over time.
This is because, in addition to promoting the notch coefficient, the contact area within the mold increases toward the bottom of the mold, resulting in an increase in frictional force.

On the other hand, in conventional continuous casting technology, in the case of manual injection by an operator, it is not possible to completely guarantee level recovery measures that are slow enough not to cause slag bear entrainment under the sudden drop of one level. No 2 The presence or absence of slag bear entrainment cannot actually be confirmed, and therefore it is likely to be difficult to judge whether or not the extraction speed should be reduced to a safe speed, and appropriate measures cannot be completely guaranteed due to deposition. Therefore, it is difficult to prevent breakouts, and in this respect, even automatic injection systems equipped with an automatic level control system do not have the following breakout prevention function.

In other words, in the automatic level control method, the injection amount is generally controlled so as to follow the level fluctuation amount and keep it at the control target standard level, as shown in the chart schematic diagram in Figure 3, so the level can be adjusted within a few seconds. It will be restored.

In addition, when the drawing speed exceeds a certain control level range, it is temporarily accelerated or decelerated in the sense of a bank amplifier for injection volume control. The speed is accelerated or decelerated according to the amount, but here, as shown by O1■ in Figure 3,
Slag bear entrainment is most likely to occur during a sudden drop in level to the lower limit ■ followed by a rapid recovery to the upper limit ■, while the withdrawal speed is such that it recovers once the level reaches the control deadband and breaks out. Control such as casting at a safe speed (low speed) cannot be performed during a period where there is a possibility of .

In this invention, in particular, the level indicated by the dashed line in Fig. 3 is constantly monitored, so that whenever this level is passed downward, the next stage is always caused by the rapid upward level restoration as described above. In order to predict that such slag bear entrainment will occur, the extraction speed is automatically reduced to a safe range as shown in Figure 4, and the extraction speed is restored as in the conventional method, which is conditional on restoring the level. Avoid doing this.

Pulling out at such a safe speed can be continued until the solidified shell 3 has become thick enough to no longer cause breakout due to the slag bear 6, and generally, the time taken depends on the safe drawing speed. Therefore, a timer or other control means is used to cause the casting control to converge to the control target standard level of the steel bath in the mold after the above-mentioned duration has elapsed.

Therefore, in this invention, although it is possible to change the setting of the molten steel level in the mold as appropriate, in addition to detecting a control target standard level set in advance within that range, it is also possible to detect a control target standard level that is lower than this by, for example, about 50 degrees. It is first necessary to set the deceleration transmission level (the above-mentioned chain line (■), hereinafter referred to as Froe level), and to constantly monitor the (2) level, which can change the arrow tension setting appropriately at the position.

For convenience of explanation, a case will be described below in which the injection level is controlled to converge to the above-mentioned standard level by using only the swinging speed of the drawing speed (constant casting speed).

FIG. 5 shows a flow diagram of a continuous casting injection amount control system to which the present invention is applied.

In the figure, 7 is an in-mold molten steel level detecting device that uses, for example, a 60 CO gamma ray type level meter, 8 is a normal speed setting device that sets the normal drawing speed, and 9 is a safety device that sets the safe drawing speed. A speed setting device 10 is a switching device for selecting and switching the drawing speed setting signals sent from the above-mentioned normal and safety speed setting devices 8 and 9, and 11 is a drawing speed regulator, in which the output from the switching device 10 is selected. The filter drawing speed setting signal and the molten steel level signal output from the molten steel level detection device 7 are compared, and the drawing speed is adjusted according to the deviation.

A pinch roll speed voltage regulator 12 issues a command to the speed control device 14 of the motor 13 in response to a signal from the drawing speed regulator 11 to adjust the drive speed of the pinch roll 15.

In FIG. 5, the broken line indicates the path of the drawn slag.

16 is a judgment processing function according to the present invention, which inputs the in-mold molten steel novel signal, the drawing speed setting signal, and the drawing speed adjustment signal, judges and processes these signals, and switches the switching device according to each judgment. Send command signals A, B, C or D to 10.

The casting speed control by the judgment processing function 16 will be explained below.

Under normal conditions, a setting signal from the normal speed setting device 8 is sent to the drawing speed regulator 11, and the speed of the pinch rolls 15 is controlled so that the level detected by the molten steel level detector 7 is constant. ing.

Now, when the level of molten steel in the mold decreases rapidly, it is determined whether the level decrease passes downward through the level (1).

On the other hand, it is also determined whether the withdrawal speed command value from the withdrawal speed regulator 11 is higher than the breakout safe speed, and if both are yes, the command signal A is sent to the switching device 10.
, and cant the signal from the normal drawing speed setting device 8 to switch to the speed command from the safe drawing speed setting device 9.

Thus, the speed of the pinch roll 15 will be controlled to the safe speed setting.

Then, after confirming with the drawing speed detector 17 that the drawing speed control system is set to a safe speed and that the pinch roll drawing speed is being driven at a safe speed, a command signal B is sent after a preset time has elapsed. is sent to the switching device 10, and the drawing speed is returned to the setting value from the normal speed setting device 8, that is, the normal drawing speed.

In order to prevent breakout according to this invention, it is necessary to determine the relationship between the drop in the level of molten steel in the mold passing downward through the above-mentioned level, the drawing speed at that time, and the safe speed for the risk of breakout. It is desirable to give an actual deceleration command.For example, if the safe speed is selected to be 0.55mΔ based on experience, it corresponds to a deceleration holding release speed range that is slightly lower than this, for example, 5mAll1n or less. At such a low drawing speed, it is more advantageous to be insensitive to the command of the above-mentioned ○ level detection signal.

By the way, in addition to controlling the standard target level mentioned above, the drawing speed should be adjusted at a speed lower than the safe speed mentioned above to avoid the phenomenon of sudden downward changes in the steel bath surface than would normally be expected. If this is severe, it may be necessary to deal with it advantageously.

In other words, if a sudden drop in the level of the pot bath level in the mold occurs during operation, the above-mentioned drawing speed control system will work to reduce the drawing speed to a safe speed, but even at this safe speed a breakout may still occur. In this case, the drawing speed must be lowered even further to avoid breakout.

In this regard, if the safe speed setting device 9 has multiple set values and a sudden drop in the level of the pot bath surface in the mold occurs as described above, the speed is switched to a speed lower than the preset safe speed. Accidents can be prevented by doing this.

Also, when the operator notices a sudden drop in the steel bath level,
Normally, the setting knob of the speed setting device 8 may be immediately operated to change the speed to the lowest speed allowed by the setting device 8 (in extreme cases, the drawing may be stopped as 0), and this is the judgment processing function. 16, after confirming that the set pulling speed is below the safe speed,
As shown by command C in the figure, the drawing speed setting signal is switched to the normal speed setting device 8 to instruct it to perform drawing at the lowest speed, and after the set time has elapsed, the setting value of the setting device 8 is restored. It is also possible to issue an alarm command to inform the operator of the operation, which is advantageous in ensuring that there is no problem in the event that there is still doubt about continuing operation at a safe speed.

In addition, if there is a risk that the steel bath level in the mold will rise suddenly by suddenly changing the drawing speed to a safe speed,
A gate inhibiting signal is sent to a casting amount control device (not shown), and the casting nozzle 1 provided in the tundish 18 is activated.
By controlling the pouring amount by operating, for example, the sliding gate 20 attached to the mold 1, overflow from the mold 1 can be suppressed.

Generally, in continuous casting of steel, rapid fluctuations in the level of molten steel in the mold are mainly caused by looseness in the pin portion of the dummy bar link when starting up the continuous casting machine.

However, by reducing the casting speed to about 0.777'Z/72in, breakout can be almost completely avoided. By taking the measures according to the invention, breakouts, which are a concern in the continuous casting process, can be advantageously and reliably prevented.

[Brief explanation of the drawing]

Figure 1 is a sectional view of the main part of the mold during the continuous casting process, Figure 2a
-e is an explanatory diagram of the slag bear generation process, Fig. 3 is an explanatory diagram of the injection amount control procedure, Fig. 4 is an explanatory diagram of the breakout prevention control procedure according to the present invention, and Fig. 5 is an explanatory diagram of the injection amount according to the present invention. This is a flow diagram of the control system. 1...mold, 2...steel bath, meter...solidified shell, 4...
... Input flux, 5... Slag layer, 6... Slag bear, 7... In-mold molten steel level detector, 8... Normal speed setting device, 9... Safety speed setting device, 10... - Switching device, 11... Pulling speed regulator, 12... Pinch roll speed voltage regulator, 13... Motor, 14...
・Speed control device, 15...pinch roll, 16...
Judgment processing function, 17... Pulling speed detector, 18...
・Tanditshu, 19... Casting nozzle, 20...
・Sliding gate.

Claims (1)

[Claims] 1. During the continuous casting process, constantly monitor whether the mold self-molten steel level passes downwardly below a preset control target level, and adjust the mold self-molten steel level according to the detection result obtained from this monitoring. In continuous casting, casting speed control consists of cutting off the response operation of the casting speed control system that follows fluctuations in the molten steel level, and switching the casting speed setting to a safe value that does not cause breakout. Breakout prevention method.