Analysis of liquid level fluctuations in peritectic steel caused by failure of continuous casting billet bulge compensation system

Based on the relevant data in PDO, the impact of the billet belly compensation system on liquid level fluctuations was analyzed. It was found that periodic fluctuations of the liquid level slightly outside the conventional frequency range would cause the compensation system to completely fail. Excessive liquid level fluctuations and pulling speed fluctuations can easily cause the bulge compensation system to work intermittently, and the intermittent work of the compensation system is an important reason why liquid level fluctuations cannot be effectively controlled. By optimizing the drum belly compensation system parameters and stabilizing the continuous casting process, the liquid level fluctuations of peritectic steel are effectively reduced.

Keywords: PDO; peritectic steel; crystallizer; liquid level fluctuation; bulge compensation system

The liquid level fluctuation in the continuous casting mold of peritectic steel slab affects the surface quality and production efficiency of the cast slab, which is the focus of attention at home and abroad. One of the causes is that the primary billet shell is thin and grows unevenly, which causes the billet shell to periodically shrink and press back in the crystallizer to a large extent. In addition, the unstable bulging of the billet shell in the secondary cooling zone is also an important reason. Unstable drum belly is mainly affected by factors such as changes in pulling speed, steel grade composition, mold slag performance, secondary cooling strength, and the distance between the lathe and the machine. Not only will it cause defects on the surface of the cast slab, but the liquid level fluctuation caused will also increase the risk of slag entrapment in the molten steel in the mold, resulting in excessive inclusion content in the cast slab. It seriously affects the quality of the cast slab and finished products, and even causes vicious production accidents such as vertical cracking of the slab and steel leakage, slag inclusions and steel leakage.

In view of the unsteady bulge of the slab, companies have developed bulge compensation systems to control the quality of the slab. This article uses the crystallizer liquid level, pulling speed, and wavelengths of the bulge compensation system in the PDO data to focus on analyzing the liquid level fluctuation problem caused by the failure of the bulge compensation system.

Introduction to production

The casting process of a steel plant uses a medium slab continuous caster, which has functions such as light reduction and bulge compensation system. Among them, the bulge compensation system is calculated based on parameters such as the casting machine roller spacing, and sets 7 compensation wavelengths. The compensation wavelengths at a pulling speed of 1.2 m/min are 45.5mm, 54.6 mm, 68.25mm, 91 mm, 136.5mm, 180mm, 267.7mm. When producing peritectic steel DP590, it was discovered that a large liquid level fluctuation problem occurred. This steel type is a carbon-manganese-silicon system, with a C content of 0.10% to 0.15%, a Mn content of 1.5% to 1.8%, and a Si content of 0.40% to 0.60%.

Analysis of causes of liquid level fluctuations

Unconventional liquid level fluctuation frequency

The periodic fluctuation of the peritectic steel liquid level is due to the periodic uneven extrusion of the billet shell when the billet shell bulge passes through the second cooling roller. The liquid level fluctuation information sampling points of the casting machine are discrete interval sampling, and the finite frequency energy is obtained through discrete Fourier transform for compensation, as shown in formula (1).

In the formula, X(k)-the intensity of the spectrum, mm;

During the casting process of DP590 steel, serious periodic fluctuations in the liquid level occurred at a pulling speed of 1.2 m/min, with a maximum of ±25mm. The liquid level fluctuation curve and the spectrum energy after Fourier transform are shown in Figure 1.

It can be seen from the curve shape of the liquid level fluctuation that the fluctuation of DP590 steel is a standard sinusoidal signal, which is a very typical periodic fluctuation of the liquid level of peritectic steel. Analyzing its fluctuation frequency through Fourier transform, it was found that there is a very obvious peak at the frequency of 0.52 Hz. After calculation, the corresponding wavelength s is 38.4 mm, which is not within the 7 compensation wavelengths set by the drum system. Therefore, the belly compensation system has no response to the liquid level fluctuation at this frequency, causing the liquid level fluctuation to continue to develop, up to ±25 mm.

The liquid level fluctuation frequency range caused by the peritectic steel drum in the compensation system is generally 0.015 ~ 0.5 Hz. If the detected fluctuation frequency is not within this range, compensation processing will not be performed. The seven wavelengths set by the existing bulge compensation system correspond to a frequency range of 0.075 to 0.44 Hz at a pulling speed of 1.2 m/min. Calculated according to the setting interval (0.075 Hz), the next compensation frequency point of the maximum compensation frequency of the existing drum belly compensation system is 0.44 Hz, plus the frequency interval, it is 0.52 Hz. Therefore, the periodic fluctuation of the liquid level slightly beyond the conventional frequency range leads to the complete failure of the compensation system, which is the main reason for the large fluctuation of the liquid level of DP590 steel.

Figure 1 DP590 steel liquid level fluctuation curve and spectrum energy

Belly bulge compensation system works intermittently

As the liquid level fluctuation intensifies, the uneven bulging phenomenon of the peritectic steel billet gradually becomes serious and changes. Therefore, during the casting process of DP590 steel, the frequency of liquid level fluctuation will also change as time goes by. In addition to the complete lack of compensation for the drum belly at the frequency of 0.52 Hz, the system also works intermittently during the compensation process for fluctuations at other frequencies, making it difficult to control the amplitude of fluctuations once they occur. There are two reasons for the intermittent work of the compensation system: liquid level fluctuations and pulling speed changes.

Excessive liquid level fluctuations

In order to ensure that the system can be put into use, the maximum belly compensation is increased from 8 mm to 12 mm. However, when the liquid level fluctuation is too large (such as DP590 steel), the system will still not be put into use, as shown in Figure 2.

Figure 2 System protection without compensation when liquid level fluctuation is too large

Figure 3. Pulling speed adjustment and fluctuation lead to intermittent deactivation of the compensation system and repeated liquid level fluctuations.

The root cause of periodic bulging of peritectic steel lies in the characteristics of peritectic reaction, which cannot be eliminated. The correction range of the bulge compensation system is limited. When the abnormal fluctuation exceeds the maximum range of the bulge system, the system cannot correct and compensate. Therefore, it can only be improved from the process, such as increasing the intensity of secondary cooling water distribution, etc.

Changes in pulling speed

When the liquid level of DP590 steel fluctuates, the current of the drive roller changes drastically, mainly because the periodic bulging of the peritectic steel passes through the roller, causing the rolling resistance of the roller to change periodically. At the same time, the pulling speed also changes periodically. For example, when producing DP590 steel in summer, the drawing speed often fluctuates by about ±0.01 m/min.

The characteristic frequency of the bulge compensation system is related to the casting machine’s pulling speed. Different pulling speeds correspond to different frequencies. Therefore, the characteristic frequency of the liquid level fluctuation cannot be accurately calculated when the pulling speed changes. For the sake of compensation accuracy, the original belly compensation system will be protectively deactivated when the pulling speed changes. Under the original process conditions of DP590 steel, the belly bulge was quite serious, causing slight fluctuations in the drawing speed that often occurred, which could easily cause the compensation system to start and stop repeatedly.

When the pulling speed fluctuates by ±0.01 m/min, the bulge compensation system starts and stops intermittently, which aggravates the liquid level fluctuation caused by the billet bulge, as shown in Figure 3. The liquid level fluctuations are gradually improved each time the compensation system is activated, and the liquid level fluctuations rapidly expand each time it is deactivated, eventually resulting in repeated liquid level fluctuations that cannot be controlled to a reasonable level.

Improvement measures

Optimization of liquid level compensation system

Modify the bulge compensation system. First, add the 38.4 mm wavelength that appears in the production of DP590 steel to the existing 7 compensation wavelengths. Second, when the pulling speed fluctuates slightly (within ±0.015 m/min), the drum belly system compensation will no longer be automatically turned off to avoid repeated fluctuations in the liquid level caused by the protective deactivation of the system.

Optimization of process measures

The root cause of the periodic bulging of peritectic steel lies in the characteristics of the peritectic reaction, which cannot be eliminated. The severity of the bulging depends on the uniformity of the shell coming out of the crystallizer and the thickness of the shell. The uniformity of the billet shell is mainly determined by the first cooling. The relatively uniform casting of the billet out of the mold can hinder the periodic changes in the thickness of the billet shell in the drawing direction. The thickness of the billet shell is mainly determined by the secondary cooling. A thicker billet shell can resist the static pressure of molten steel and reduce the degree of bulging.

Under the existing casting machine equipment conditions, the cooling intensity of the secondary cooling zone is increased in summer, the specific water volume is increased from 1.45L/kg to 1.70 L/kg, and the overall secondary cooling water volume is increased by about 17%. The carbon content of steelmaking components is narrowly controlled, and the carbon content is controlled between 0.10% and 0.12%. By selecting mold powder with high crystallization performance, the heat flux density of the crystallizer is reduced from 1.85 MW/m² to about 1.75 MW/m². These measures can improve the uniformity of the primary green shell growth.

Optimization of casting heats

The inhomogeneity of the peritectic reaction is the root cause of the bulging, and phenomena such as bulging, liquid level fluctuations, and pulling speed fluctuations in turn promote the inhomogeneity of the peritectic reaction, influencing and amplifying each other. The development process takes time. One pour of DP590 steel is tracked. The results are shown in Figure 4.

Figure 4 Liquid level fluctuation of DP590 steel in one pouring step

It can be seen that during the entire pouring process of DP590 steel, abnormal liquid level fluctuations are prone to occur in the late casting period, so the pouring times cannot exceed 15 furnaces.

Improvement effect

After taking optimization measures, when the continuous casting speed fluctuates slightly, the drum compensation system is put into use. With the stabilization of the peritectic steel steelmaking process, abnormal large fluctuations in liquid level are completely eliminated. The liquid level fluctuation of DP590 steel has been significantly improved, and the liquid level fluctuation is controlled within the range of ±3 mm, as shown in Figure 5.

Figure 5 Liquid level fluctuation of DP590 steel in one pour after process improvement

Conclusion

(1) The inhomogeneity of the shell caused by the peritectic reaction promotes each other with liquid level fluctuations and drawing speed fluctuations, eventually leading to out-of-control liquid level fluctuations.

(2) Periodic fluctuations of the liquid level beyond the design frequency range of the compensation system will also cause complete failure of the compensation system.

(3) Liquid level fluctuations that exceed the conventional bulging compensation value and rapid changes in pulling speed will cause the bulging compensation system to work intermittently.

(4) The root cause of periodic bulging of peritectic steel lies in the characteristics of peritectic reaction, which cannot be eliminated. By optimizing the parameters of the bulging compensation system, the production process of peritectic steel is stable, and the liquid level fluctuation can be controlled within ±3 mm. within.

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