JPH0126791B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for preventing breakout of a slab during continuous casting, and more specifically, a method for predicting and preventing breakout by measuring the temperature in the width direction of a continuous casting mold. In this process, the number of temperature measurement points in the width direction should be small and sparse, and the distance between the measurement points and the distance in the pouring direction should be adjusted to allow enough time to predict the signs of a breakout before it occurs. This relates to a prevention method that allows breakouts to be prevented without any problems by specifying the following.

Generally, the breakout of a slab in continuous casting refers to a phenomenon in which the solidified shell on the outside of the slab breaks during continuous casting, and the molten steel inside flows out. Among the troubles that occur during continuous casting operations, breakouts are one of the ones that cause the most damage, and although there has long been a strong desire to predict their occurrence and establish methods to prevent them, the causes of their occurrence remain unclear. The current situation is that it is not clearly possible to do so.

In recent years, as casting speeds have increased significantly in response to the increasing demand for improved productivity in continuous casting machines, the frequency of occurrence of breakouts has become more noticeable, and it is becoming increasingly important to predict their occurrence and establish methods to prevent them. Demand is increasing.

In view of the current situation, the present inventors first investigated a large number of slabs in which breakouts occurred in order to clarify the cause of breakouts.

As a result, the following causes became clear.

In other words, during pouring, under normal conditions, the slab descends one after another, but some parts of the slab may be in the mold for some reason, such as poor lubrication due to poor flow of mold powder between the mold and the slab. , mold variation,
When a phenomenon occurs in which the slab is restrained by the mold steel plate and stagnates due to the formation of fins in the mold corner gaps or the baking of the mold and slab, the solidified shell of the restrained stagnation part of the slab is pulled out below the mold. The slab undergoes tensile or shearing deformation at the boundary with the normal part, and eventually breaks. Even if a break occurs, at the initial stage the break is within the mold, so
The molten steel leaking from the fracture is immediately cooled in the mold and forms a thin shell, thus preventing breakout.

However, as the operation continues, the processes of fracture of the solidified shell, steel leakage, and formation of thin shell are repeated every time the mold oscillates, and as a result, the position of the fracture moves downward over time, and eventually the mold Molten steel leaks when it reaches below the bottom edge,
Breakouts occur because there is nothing outside the mold to cool the molten steel.

Therefore, prior to breakout, a part of the slab is restrained within the mold and becomes stagnant.

The present invention was established based on the above-mentioned new knowledge, and specifically, it is possible to predict the occurrence of slab breakout during continuous operation, and furthermore,
We propose a method for predicting and preventing breakouts that can prevent their occurrence.

The method of the present invention will be explained in detail below.

First, as described above, before the breakout of the slab, a part of the slab is restrained within the mold and becomes stagnant. Therefore, the thickness of the shell in the stagnant part is thicker than that in other normal parts, thereby reducing the heat flux from the shell to the mold. From this point of view, in the present invention, breakout is predicted with sufficient margin by detecting temperature changes at predetermined positions of the mold.

That is, prior to breakout, a part of the slab is restrained near the meniscus in the mold, and a phenomenon occurs in which the slab stagnates. When this phenomenon occurs, the thickness of the shell in the stagnation area naturally becomes thicker than the thickness of the shell in the continuously drawn part, even at the same distance in the pouring direction as the stagnation area. In addition, the slab in the stagnation area breaks at an angle close to the direction of maximum shear stress (e.g., 30 to 45 degrees) between the slab in the normally drawn part, so the slab in the stagnation area breaks. This shape gradually grows larger and the slab is broken into a V-shape. In other words, this phenomenon occurs in the mold prior to breakout, and if a thermocouple is embedded in the mold to detect the mold temperature,
When the above-mentioned stagnation part of the slab grows and the fractured part of the slab passes through the temperature measurement position and descends, the temperature rises rapidly when passing the fractured part, and then rapidly decreases. In other words, if this temperature change is promoted, the phenomenon of stagnation of the slab in the mold prior to breakout can be accurately known, and breakout can therefore be predicted and prevented.

However, when predicting in this way, if the temperature measurement position of the mold or the number of temperature measurement positions is not appropriate, the sudden rise or fall of the mold temperature that precedes the breakout may not be able to be promoted before the breakout occurs, or the phenomenon may be delayed. Even if it is possible to detect the breakout, it is not possible to adjust the casting speed, etc. to prevent the breakout with sufficient time, and it is not possible to actually prevent the breakout.

Regarding this point, the method of the present invention has been researched based on numerous experiments and empirically obtained knowledge, and based on this research, the change in mold temperature that occurs prior to the breakout can be detected in advance so that the breakout occurs. Forecast in advance, then adjust the casting speed with sufficient time to prevent breakout, and from this point on, measure the temperature change at the appropriate measurement location.

That is, as shown in FIGS. 1 and 2,
The temperature measurement position on the short sides 1 and 3 and the long sides 2 and 4 of the mold is at a distance l (mm) from the meniscus b ₁ in the casting direction, and at this l, the temperature is measured horizontally between the measurement positions. Gap w is 100mm
Under the above conditions, for example, a thermocouple is attached and measured.

The reason why the distance w between measurement positions is 100mm or more is as follows.
In order to prevent an increase in the heat transfer resistance of the mold due to the close proximity of the mounting points of adjacent thermocouples and the resulting insufficient heat removal, the minimum temperature measurement interval is the mold copper plate designed to minimize the heat transfer resistance. It is desirable that the distance between the bolts used to fix it to the back plate is 100 mm or more.

In this case, it is necessary that the distance l and the interval w satisfy the relationship expressed by the following equation (1).

0.6 {(L-l)-k ₂・V _R・tr}×1/k ₁ <w<{(
L−l)−k ₂・V _R・tr}×1/k ₁ ……(1) However, L: Distance mm from the molten metal meniscus b ₁ in the mold to the mold outlet, V _R : Casting speed (mm /
minutes), _k1 , _k2 : constants determined by casting conditions such as molten metal to be cast, mold cooling rate, casting speed, etc. _k1 is 0.29 to 0.5, _k2 is 0.008 to 0.017. The range is the preferred value, and tr is the time (in seconds) from when a sudden change in mold temperature before breakout is detected until breakout occurs.

Furthermore, when determining the interval w using equation (1), it is desirable to set the interval w to a value as close to the upper limit as possible. In other words, measuring temperatures at as few positions as possible provides good accuracy, is economical, and allows accurate prediction of breakouts. On the contrary, if
If the distance w is shortened to make the measurement locations closer together, the heat transfer resistance will increase at the temperature sensor mounting point, resulting in insufficient heat removal, which will slow down the temperature response and make it difficult to predict breakouts. become certain. Furthermore, maintenance work for the temperature sensor also increases, which is undesirable. Note that although it is desirable that the interval w be close to the upper limit as described above, the lower limit is kept at 0.6 times the upper limit. The reason for this is that once the casting conditions are determined, k ₁ has a large influence on the interval w, and the ratio of the minimum value to the maximum value of the interval w is k ₁ = 0.29 to 0.50, so 0.29/0.50≒0.58
Therefore, in terms of safety, it is 0.6.

In addition, in normal casting, the meniscus fluctuates to some extent, and the mold temperature in the vicinity of this fluctuates widely, so it may be very difficult to clearly grasp the phenomenon of slab stagnation. Therefore, it is desirable that the distance l is usually 30 mm or more. Further, L is a factor that is almost fixed depending on the length of the mold. In addition, tr is the time from when the mold temperature changes before breakout until breakout actually occurs, and depending on how you take this tr, even if you predict breakout, it may actually be too short. breakout cannot be prevented. Therefore,
In practice, it is sufficient to determine how long it takes for mold temperature changes to occur and the casting speed to be adjusted to address breakout prevention. In short, under each condition of equation (1) above, if the measurement position of the mold, that is, the values of w and l, are determined in relation to tr, then the casting speed should be set with sufficient margin after predicting the mold temperature change. can be adjusted and breakouts can be prevented.

Next, examples will be described.

First, in the conditional expression (1) above, V _R :1000
By setting mm/min, L = 600 mm, tr = 25 sec, and l = 200 mm and finding the measurement position, it was found that it is preferable to measure at l = 200 mm with an interval w = approximately 300 mm. Therefore, we embedded thermocouples at points a ₁ to a ₁₀ as shown in Figure 2, and adjusted the casting size.
A 200 x 1300 mm slab was cast and the temperature changes at each location were measured, and the results were as shown in Figure 3.
Figure 3 shows the mold temperature changes up to breakout at each measurement position, and especially, prior to breakout, the change in temperature between the slab stagnation due to mold restraint and the normally drawn slab. fracture position,
In other words, when the V-shape mentioned above descends and this part passes through each measurement position, a characteristic phenomenon of sudden rise and fall in temperature occurs.
Seen in a ₂ and a ₃ . The breakout at this time is b ₂
Occurs in In this case, measurement positions a ₃ and a ₄ are positioned to detect temperature changes approximately 30 seconds before the breakout occurs, and after predicting the temperature change, casting is interrupted within 15 seconds and the solidified shell is removed. After recovering the shell thickness in the thin part, especially after the shell strength became stronger than the restraint strength in the stagnation part, we were able to restore the casting speed to the original value, so the restraint in the stagnation part was released. , it was possible to return to normal conditions and prevent a breakout. For comparison, the mold l=300mm,
Assuming that w = 1500 mm (however, equation (1) is not satisfied at this position), mold long side 2,
When a thermocouple was embedded in the center of the widthwise direction of 4 and cast under the same conditions as above, the temperature change was as shown in FIG. In FIG. 4, A indicates long side 2 and B indicates long side 4, which shows the characteristic change in mold temperature when breakout occurs. In this case, it took only 5 seconds from the temperature change in the mold to breakout, and there was no time to adjust the casting speed to prevent breakout, so it was foreseeable that breakout would occur. However, it could not be prevented.

As described above, the method of the present invention detects changes in mold temperature by determining the interval between measurement positions in the mold width direction and the distance in the casting direction so that there is sufficient time from the prediction of breakout to the occurrence of breakout. It predicts and prevents breakouts. Therefore,
By detecting the phenomenon of restraint and stagnation of the slab in the mold by the method of the present invention, breakout can be prevented with sufficient margin, and when using a fixed mold other than one with an oscillation mechanism, the so-called horizontal connection It can also be applied to casting.

[Brief explanation of drawings]

Figure 1 is a perspective view showing an example of a continuous casting mold, Figure 2 is an explanatory diagram of the temperature measurement position showing the mold in an expanded state, and Figure 3 is a diagram showing a method for predicting breakout and FIG. 4 is a graph showing temperature changes over time at each measurement position when prevention is achieved, and FIG. 4 is a graph showing a similar relationship in a comparative example. Code 1, 3... Short side of the mold, 2, 4... Long side of the mold, a ₁ to a ₁₀ ... Measurement position, b ₁ ... Meniscus, b ₂ ...
breakout position.

Claims (1)

[Scope of Claims] 1. When measuring the temperature in the width direction of a continuous casting mold to predict signs of breakout and prevent breakout, the temperature in the width direction is measured at a small number of measurement positions and at only a small number of measurement positions. The distance (w) between measurement positions is
Detect at each temperature measurement position that is 100 mm or more and is determined such that the distance w and the distance l in the casting direction satisfy the following formula, 0.6 {(L-l)-k ₂・V _R・tr}×1/k ₁ <w<{(
L-l)-k ₂・V _R・tr}1/k ₁ However, w: Distance between measurement positions in the horizontal direction of the mold (mm), l: Distance from the meniscus of molten metal in the mold to the measurement position Distance in the casting direction (mm), L: Distance from the meniscus of molten metal in the mold to the mold outlet (mm), V _R : Casting speed (mm/min) k ₁ , k ₂ : Cast metal, cooling of the mold A constant determined by casting conditions such as casting conditions and casting speed.
k ₁ = 0.29 to 0.5, k ₂ = 0.008 to 0.017, tr: time (sec) from when the temperature of the mold suddenly changes until breakout occurs, and calculate the temperature of the mold by comparing the detected temperatures at each measurement position. Predict signs of breakout due to sudden changes,
A method for preventing breakout of slabs, which is characterized by adjusting the casting speed within the time (tr) from the sudden change in mold temperature until breakout occurs.