Time-frequency characteristics of instantaneous abnormal fluctuations of liquid level in slab continuous casting mold

Research background

The crystallizer is called the “heart” of continuous casting and is a key link to ensure the safety and smoothness of continuous casting and the quality of cast slabs. The stability of molten steel level is one of the important indicators for mold process control. Abnormal liquid level fluctuations not only easily cause slag entrainment, but also increase the inclusion content near the primary solidified shell, affecting the uniform growth of the primary shell. At the same time, it will also affect the lubrication performance of the mold slag. In severe cases, it will cause surface cracks on the continuous casting billet or even steel leakage accidents. The development of high-efficiency continuous casting with high casting speed and no defects as its core content is the goal that steel companies strive to achieve. However, as the pulling speed increases, the frequency and amplitude of abnormal copper mould tube liquid level fluctuations will become more serious, which brings unprecedented challenges to the stability control of abnormal copper mould tube liquid level fluctuations. In order to explore the special phenomenon of abnormal instantaneous liquid level fluctuations in the mold, Northeastern University’s Zhu Miaoyong team collected real-time data on mold liquid level fluctuations and stopper rod position data under different process conditions of slab continuous casting in a steel plant. A spectrum analysis of the liquid level fluctuation data was carried out to determine whether the drum belly had a significant impact on the instantaneous abnormal fluctuations of the copper mould tube liquid level. Through time-frequency characteristic analysis of process big data, the correlation between the stopper rod position and the instantaneous abnormal fluctuation of the copper mould tube liquid level was determined. This excavates the time-frequency characteristics of the high-frequency area of the stopper rod position and achieves accurate prediction of abnormal instantaneous liquid level fluctuations in the copper mould tube.

02Research methods

An electromagnetic liquid level sensor is used to collect the liquid level fluctuation data of the on-site slab continuous casting mold in real time. The electromagnetic liquid level sensor is produced by the Czech company VUHZ, and the sensor model is SH-D. The measurement principle is based on the interaction between the high-frequency magnetic field generated by the coil in the detection and tracking detector and the copper mould tube level fluctuations to determine the copper mould tube level. The size of the copper mould tube is approximately (910–1300) mm × 210 mm × 900 mm, and the size of the electromagnetic sensor is 540 mm × 150 mm × 63 mm. The sensor is fixed on the back plate of the crystallizer, located directly behind the immersion water inlet and close to the meniscus. The measurement area is approximately 600 mm wide, 80 mm high, and has a response time of 0.1 s. In addition, stopper rod position data of different steel types and different pulling speeds were collected in real time, with a sampling time of 0.1 s. A total of 13 consecutive furnace data were collected for continuous wavelet transform (CWT) analysis.

03Research results

Continuous wavelet transform was used to study the time-frequency characteristics of instantaneous abnormal copper mould tube liquid level fluctuations and stopper rod position under different conditions, as shown in Figure 1. Three obvious red areas are marked in the figure, corresponding to the rapid occurrence and disappearance of instantaneous abnormal fluctuations in the copper mould tube liquid level. At the 45th, 228th and 515th sampling points, the CWT coefficients are 3.88, 3.40 and 6.61 respectively. As can be seen from Figure 1(a), obvious instantaneous abnormal fluctuations in the copper mould tube liquid level occurred at the 45th, 228th and 515th sampling points, which shows that the CWT method can clearly reflect the characteristics of instantaneous abnormal fluctuations in the copper mould tube liquid level.

In addition, the light blue area (whose CWT coefficient range is 1.5-2.0) is distributed throughout the area, which means that the copper mould tube liquid level fluctuations during this period are disordered, and indicate that the fluctuations are relatively stable, and no obvious abnormal fluctuations occur. CWT analysis is more sensitive to the state and trend of instantaneous abnormal fluctuations in the copper mould tube liquid level. Figure 1(c) shows the position change curve of the stopper rod of medium carbon steel. As can be seen from Figure 1(c), the change in the stopper rod position consists of two parts: a stable Z-shaped rise and a sudden drop. The CWT results for the stopper rod position in Figure 1(c) are shown in Figure 1(d). It can be seen from Figure 1(d) that at the sampling points at the same position, the CWT coefficients of the parts marked with dotted lines in Figures 1(b) and (d) have the same upward trend. In addition, before the maximum value appears, the CWT coefficient of the stopper rod position shows an increasing trend in the high-frequency region, with a growth rate in the range of 0.1-0.2/s.

Time domain diagram of copper mould tube liquid level fluctuation

copper mould tube liquid level fluctuation CWT diagram

 Time domain diagram of stopper rod position

Stopper rod position CWT diagram

Figure 1 Characteristics of instantaneous abnormal fluctuations in medium carbon steel mold liquid level

04Research conclusion

WT provides an effective method for analyzing subtle changes in instantaneous abnormal fluctuations in copper mould tube liquid level. The CWT results of the stopper rod position can reflect the time-frequency domain characteristics of the instantaneous abnormal fluctuation signal of the copper mould tube liquid level. The growth rate of the CWT coefficient in the high-frequency area of the stopper rod position can be used to infer in advance the occurrence of abnormal instantaneous fluctuations in the copper mould tube liquid level. When the growth rate is in the range of 0.1-0.2/s, instantaneous abnormal fluctuations in the copper mould tube liquid level will occur after about 10 s. It has strong application potential in judging and predicting instantaneous abnormal fluctuations in copper mould tube liquid level, which can greatly improve the stability of copper mould tube liquid level fluctuations. Improve the quality of continuous casting billets, ensure the efficient and smooth flow of the continuous casting process, and generate higher benefits for steel companies.

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