Analysis of the causes of liquid level fluctuations in medium-thin slab peritectic steel

Through mechanism analysis, we find out the causes of liquid level fluctuations when producing peritectic steel from medium-thin slabs. It is believed that the low accuracy of the mold liquid level control system, peritectic steel reaction, and improper control of inclusion types are the main reasons for liquid level fluctuations. factor. By taking corresponding measures such as improving the control accuracy of the liquid level control system, optimizing the deoxidation alloy system, and controlling the oxidation at the end of the converter, the technical problem of excessive liquid level fluctuations in the crystallizer of the medium-thin slab continuous casting machine was solved, and the quality of the continuous casting slabs was improved.

Keywords: medium thin slab; peritectic steel; liquid level fluctuation

With the advancement of related technologies and the improvement of product quality, the range of varieties that thin slab continuous casting can produce continues to expand, but the scope of peritectic steel is generally avoided, although some companies have introduced that their thin slab continuous casting technology has the ability to cast peritectic steel. However, in actual scale production, we still try to avoid the peritectic area. If peritectic steel must be produced, the drawing speed is required to be significantly reduced. There are currently no reports of normal mass production of peritectic steel. After Jinan Iron and Steel Co., Ltd.’s medium-thin slab continuous caster was put into operation, the composition design could not avoid the range of peritectic steel. Through a series of process optimization, the production capacity and quality of peritectic steel reached the level of other steel types.

There is a problem with production

After Jinan Iron and Steel Co., Ltd.’s medium-thin slab continuous casting machine was put into operation, the production of steel types was very complicated. It not only had to undertake the task of supplying slabs to hot rolling mills, but also the task of providing high-quality slabs to medium-thick plate mills, and also produced some parts according to the contract. Export and export products, so the range of peritectic steel cannot be avoided in actual production. Especially when producing a certain steel type for hot continuous rolling, the carbon content is required to be 0.12~0.14%, and a lot of problems are encountered in the early stage of production. The main phenomenon is that the crystallizer liquid level fluctuates too much. The fluctuation range is generally ±6-12mm, of which the majority is between ±8-10 mm, and the maximum is ±16mm, which is far beyond the normal range of ±3-5 mm. The specific fluctuation situation As shown in Figure 1a. Excessive fluctuations in the mold liquid level are the main reasons for defects such as slag inclusions, pits, and longitudinal cracks on the surface of the slab. The casting speed was forced to be reduced to 1.8m/min, and the pouring cycle reached about 65 minutes. However, defects such as slag inclusions and pits on the surface of the billet were still caused, and a large number of longitudinal cracked waste products appeared. In severe cases, the casting rate reached nearly 10%, and leakage was also caused. Steel accident.

(a) Before improvement (b) After improvement

Figure 1 Fluctuation trend of peritectic steel mold steel liquid level before and after taking improvement measures

Reason Analysis

The causes of crystallizer liquid level fluctuations are complex, involving factors including mechanical equipment, electrical control, continuous casting refractory materials, inclusion shapes and quantities, etc. Through a systematic analysis and summary of excessive mold liquid level fluctuations and a series of comparative tests, the real reason for the large mold liquid level fluctuations in the production of peritectic steel produced by the ASP continuous casting machine was finally analyzed.

Crystallizer liquid level control system

During the production process, liquid level fluctuations as shown in Figure 2 are found, and generally begin to appear about 30 minutes after the tundish is poured. It is believed to be caused by poor control accuracy of the crystallizer liquid level control system.

Under normal circumstances, after the crystallizer liquid level detection system detects a certain difference between the liquid level and the set value, it sends a command signal to the stopper rod flow control system. Objectively, the stopper rod moves up and down at a basically certain frequency, so that the crystallizer liquid level The surface is stable near the set value. However, through observation, it was found that the tundish plug rod flow control system “nods” in the direction of the ladle during its up and down movement. The reason is that some lifts of the flow control system have problems such as slight structural deformation or poor lubrication during use. It causes the elevator to vibrate at a certain frequency. As the steel is drawn, the temperature of the elevator rises. The vibration frequency and the frequency of the stopper rod moving up and down produce a phenomenon similar to “resonance”, which increases the movement range of the stopper rod, thereby causing the crystallizer liquid Surface fluctuations increased.

Figure 2. Crystallizer liquid level fluctuation trend chart 2

Figure 3. Comparison chart of stopper rod stroke amplitude increase

Peritectic reaction

Analyzing the growth pattern of the billet shell after steel breakage, it can be found that the billet shell does not gradually thicken, but has a certain degree of wavy nature. Peritectic steel often causes fluctuations in the crystallizer liquid level. A large number of theories and practices have proven that peritectic steel with a carbon content of 0.09 to 0.18% (especially 0.12 to 0.15%) undergoes peritectic transformation during the solidification process, accompanied by a volume of about 0.38%. Shrinkage causes uneven flow of mold slag, uneven heat transfer in the crystallizer, relatively weak shell, and uneven growth. In the secondary cooling zone, the static pressure of the molten steel causes the billet to bulge between the two rollers, and the weak areas of the billet shell are more obvious. When the billet bulges, a pumping effect occurs, causing the liquid level in the mold to drop rapidly. The mold liquid level control system will raise the stopper rod position to fill the mold with molten steel. At the same time, as the billet drawing progresses, the bulging area is compressed to the middle of the two rollers, and the molten steel in the liquid phase cavity is also filled with molten steel into the crystallizer, causing the liquid level to rise rapidly and making the shell growth in the crystallizer more uneven. Repeatedly, the crystallizer liquid level fluctuation rapidly intensifies.

Influence of inclusions

Production statistics found that furnaces with large crystallizer liquid level fluctuations have high [O] content at the converter end point and low Si recovery rate, that is, the amount of SiO2 in the deoxidation product is relatively high.

The peritectic steel produced by Jigang No. 3 Steelmaking in medium and thin slabs uses Si+Mn+a small amount of aluminum for deoxidation. The resulting deoxidation products may include: ① rhodonite (2MnO·2Al2O3 ·5SiO2); ② spessartine garnet (3MnO·Al2O3 ·3SiO2); ③ pure Al2O3 (Al2O3>25%). Among them, spessartine garnet (3MnO·Al2O3·3SiO2) has the lowest melting point, which is about 1400°C. The melting points of the other two inclusions are higher than spessartine garnet. In steelmaking production, the deoxidation products should be controlled within the shadow area of spessartite in the phase diagram. Inclusions are easy to float and the fluidity of molten steel is good. When Mn/Si is greater than 2.5, spessartine garnet is generated. However, the target composition of peritectic steel currently produced is Mn/Si=1.75. During the deoxidation alloying process, the deoxidation product contains more SiO2, which leads to an increase in the production of rhodonite and adhesion to the plug rod. However, this adhesion is different from the adhesion caused by Al2O3 or CaS on the change of stopper rod stroke. Al2O3 or CaS adheres to the stopper rod or nozzle relatively firmly, causing the stopper rod stroke to continue to rise. However, the melting point of inclusions such as rhodonite is near the casting temperature of molten steel and is in a semi-molten state. The adhesion is not strong and changes with the molten steel. Erosion and vibration of the stopper rod, semi-molten inclusions frequently adhere and are washed away, exacerbating the “vibration” of the stopper rod stroke within a certain range, and the amplitude increases, see Figure 3. The washed-out inclusions are absorbed by the mold slag, which denatures the mold slag, exacerbating the uneven growth of the shell, and superimposes with the liquid level fluctuations caused by peritectic steel.

Key research measures and effects

Through systematic analysis and summary of excessive fluctuations in the crystallizer liquid level, corresponding improvement measures were taken. The crystallizer liquid level control system was comprehensively maintained and a long-term system was formed to greatly improve and stabilize the control accuracy. The phenomenon shown in Figure 2 did not reoccur. Optimizing the deoxidation alloy system, controlling the end-point oxidation of the converter, improving the ability of the mold slag to absorb inclusions and improving performance stability, etc., finally solved the technical problem of excessive fluctuations in the crystallizer liquid level of the ASP continuous casting machine.

After the research, the fluctuation range of the crystallizer liquid level of the ASP continuous casting machine has been greatly reduced. The liquid level fluctuation range of most heats is within 3mm. The occasional large fluctuations in the heats generally do not exceed 5mm. Figure 1b shows the crystallization after taking the key research measures. Liquid level fluctuations in the instrument.

This research has greatly improved the continuous casting machine speed, and the steel throughput has increased from 2.5t/min to 3.6t/min, and there is room for further improvement. From April 2006 to February 2007, more than 1 million tons have been produced. . The occurrence probability of steel breakouts and longitudinal cracks is reduced, the quality of the slab is rapidly improved, the crack rate of the slab is reduced from 12% to less than 1%, and there are no more steel breakouts caused by large fluctuations in the mold liquid level and longitudinal cracks.

Conclusion

Through system analysis, we found out the reasons for the serious fluctuation of liquid level when producing peritectic steel in Jigang’s medium-thin slab continuous casting machine:

⑴ The low control accuracy of the execution structure of the stopper rod control system is the mechanical cause of liquid level fluctuations;

⑵ The volume shrinkage of peritectic steel and the uneven inflow of mold slag are the fundamental reasons;

⑶ The deoxygenation system is unreasonable and improper control of inclusion types aggravates liquid level fluctuations;

⑷ By taking corresponding measures, the technical problem of excessive fluctuations in the mold liquid level of the medium-thin slab continuous casting machine was solved, and the casting speed and slab quality were improved.

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