Analysis and preventive measures of mold overflow accident

The harm caused by mold overflow to ASP continuous casting machine production is introduced, and its causes are systematically analyzed. The results show that Al in the steel generates a large amount of Al2O3 flocculent inclusions due to secondary oxidation. During the pouring process, the flocculent Al2O3 inclusions continue to accumulate at the head of the plug rod and fall off after being washed away by the steel flow, resulting in the amount of Al2O3 flocculent inclusions entering the crystallizer per unit time. The amount of molten steel increases instantaneously, causing an overflow accident in the crystallizer. The steelmaking plant has formulated corresponding measures, significantly reducing steel spill accidents and achieving good results.

Keywords: crystallizer; nozzle blockage; steel overflow; prevention

The mold overflow accident reduced the production efficiency of the continuous casting machine and the molten steel yield, and seriously damaged the equipment. Since the ASP continuous casting machine in the steelmaking plant was put into operation, many mold overflow accidents have occurred, and most of the accidents occurred during the production of low-carbon, high-aluminum steel. This article focuses on analyzing the causes of steel spill accidents caused by molten steel flocculation, and formulating corresponding preventive measures.

 Main technical parameters of ASP continuous casting machine

The ASP continuous caster of the steelmaking plant was put into operation in June 2005. It is an important part of the continuous casting and rolling project. It is a low-head straight arc medium thin plate continuous caster. The main technical parameters are shown in Table 1.

Table 1 Main technical parameters of ASP continuous casting machine

projectParameters
Number of units/unit2
Metallurgical length/m24.2
Arc radius/m5
Steel package capacity/t150
Tundile capacity/t30
Crystallizer length/mm1200
Annual output/10,000 tons260
Section/mm135/150×900~1620
Pulling speed range/(m·min-‘)0.6~2.8

Analysis of causes of steel overflow

Judging from the several steel overflow accidents that occurred in the ASP continuous casting machine, the main reason for the steel overflow in the mold was the flocculation of the molten steel. After the Al2O3 inclusions gathered in a large amount, they suddenly fell off under the erosion of the molten steel and flooded the nozzle. The instantaneous steel flow increases, and the ASP pouring section is relatively small (high aluminum steel is generally 135 mm × 1070 mm), and the unshed Al2O3 is padded between the stopper rod and the upper nozzle of the tundish, so that the stopper rod cannot be completely closed. Die, the steel flow entering the mold is out of control. When the molten steel injected from the tundish into the mold is greater than the pouring speed of the mold, the liquid level in the mold will rise rapidly until the molten steel overflows the mold, causing mold overflow. Steel accident. The trend of relevant parameters when the steel spill accident occurred is shown in Figure 1. It continues to accumulate on the head of the plug rod, causing the stroke of the plug rod to continue to increase.

Figure 1 Trend chart of relevant parameters when the steel spill accident occurred

Causes and locations of flocculation at the nozzle

The main reason for the flocculation of molten steel is that Al in the molten steel is oxidized to form Al2O3 with a high melting point. Al2O3 is deposited and attached to the inner wall of the nozzle, so that the instantaneous steel flow through the immersed nozzle cannot meet the pulling speed requirements, causing the opening of the stopper rod to continuously increase. The itinerary keeps rising. In the absence of Al2O3 inclusions falling off, the inner diameter of the immersed nozzle continues to become smaller or even blocked. Practice has shown that the two most sensitive parts of the immersed nozzle blockage caused by the flow of molten steel are around the entrance of the stopper rod and the outlet of the immersed nozzle, as shown in Figure 2.

Figure 2 Sensitive area of water inlet blockage

Main chemical reactions

During the secondary oxidation of molten steel, Al is oxidized to form Al2O3 inclusions, which are deposited in sensitive areas where the nozzle is clogged.

2Al+3[O]=Al2O3

The chemical reaction between the refractory material on the surface of the immersed nozzle (made of Al-C) and the molten steel continuously occurs, resulting in Al2O3 precipitation, which adheres to the inner wall of the immersed nozzle.

Reactions within refractory materials under high temperature:

Si02(s)+C(s)=Si0(g)+CO(g)

Na2O(s)+C(s)=2Na(g)+CO(g)

K2O(s)+C(s)=2K(g)+CO(g)

Reaction between refractory material and liquid steel interface:

3Si0(g)+2[Al]=Al2O3↓+3[Si]

3CO(g)+2[Al]=Al2O3↓+3[C]

The overall response is;

3SiO2(s)+3C(s)+4[Al]=2Al2O3↓+3[Si]+3[C]

Analysis of Al2O3 clogged water outlet

According to microscope observation, the nozzle blockage is composed of three layers, as shown in Figure 3. From left to right, they are the decarburization layer, the first deposition layer and the second deposition layer. A decarburized layer of about 500 μm formed by dissolved carbon; next to the decarburized layer is the first deposition layer with a thickness of 100 to 300 μm, also known as a networked Al2O32 dense layer, mainly composed of Al2O3 particles and (25% to 30%) Al2O3+(50%~60%)SiO2+(5%~12%)Na2O+(1%~4%)

It is composed of the glass phase of K2O; in contact with the molten steel, there is a second deposit composed of Al2O3 and nodular metal that is several millimeters to several centimeters thick, also known as accumulated Al2O3. Al2O3 is flat and has a size ≤ 20μm. At the same time, MgO-Al2O3 and MgO-Fe2O3-like spinels were observed from the ladle and tundish refractory materials.

Figure 3 Three-layer structure of the nozzle blockage

The reticular Al2O3 dense layer and decarburization layer are the products of the reaction between the tundish nozzle refractory material and the molten steel. Their thickness is far from reaching the level of blocking the nozzle, and such structures are easy to form when pouring other steel types; while the accumulated Al2O3 is mainly It is formed by adsorbing Al2O3 contained in molten steel and is the main cause of nozzle blockage.

Measures taken

(1) When the roughness of the working surface of the nozzle refractory material is >0.3 mm, the role of the protective layer under the viscous layer disappears. Therefore, the working surface of the nozzle refractory material must be as smooth as possible to ensure that the molten steel surface is intact during the casting process.

(2) Before pouring, add an appropriate amount of Ca-Fe powder into the tundish to convert the high melting point Al2O3 generated after pouring into low melting point 12CaO2·Al2O3.

(3) Strictly carry out protective pouring work of molten steel. The entire process of molten steel from the ladle to the crystallizer is protected and poured, and the relevant connection parts are sealed with argon gas. The liquid steel level in the tundish cannot be exposed or turned over.

(4) After the flocculation phenomenon occurs, especially when the liquid level suddenly rises, try not to manually close the stopper rod, but use the system’s “stopper rod emergency stop” button to avoid the problem that manual operation cannot be completed.

(5) If the plug rod cannot be completely closed, the pulling speed should be continuously increased to increase the amount of steel poured per unit time to alleviate the rise in liquid level. At this time, try not to reduce the drawing speed, let alone suddenly turn the drawing speed into a “crawling” state, which will directly lead to the occurrence of steel overflow accidents in the mold.

(6) Try to avoid low-temperature pouring. Under low-temperature conditions, inclusions are easy to gather and difficult to float.

(7) Improve the cleanliness of molten steel and reduce the number of non-metallic inclusions in the molten steel.

Conclusion

In view of the situation that the ASP continuous casting machine has small cross-section, fast casting speed and high frequency of pouring high-aluminum steel, several mold overflow accidents caused by floc flow were carefully analyzed and summarized, and relevant process and operation improvement measures were proposed, which greatly It reduces the frequency of mold overflow accidents and lays a good foundation for improving the productivity of the ASP continuous casting machine and reducing unplanned shutdowns.

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