The tundish is also a place where molten steel is polluted. The molten steel in the refined ladle meets the cleanliness requirements, but when it passes through a refractory container, various pollution problems will occur, reducing the cleanliness of the molten steel. No matter what measures the tundish takes, it can only reduce this pollution, and it is impossible to further purify the ladle of molten steel.
The continuous casting process not only requires stable drawing speed and uniform cooling to improve product quality, but also the protection of casting and the reduction of refractory contamination of molten steel are also crucial aspects, which are precisely related to tundish operations.
Several key points of the continuous casting process polluting molten steel are:
- Ladle Drainage Sand:
It is difficult to purify the molten steel during refining, but the drainage sand of dozens of pounds enters the tundish, just like the entry of a pile of garbage, the molten steel is seriously polluted.
- Secondary oxidation:
It is inevitable that part of the molten steel comes into contact with the atmosphere, causing secondary oxidation, which is also a headache. All the protective casting measures in continuous casting are to isolate the air.
- Ladle covering agent:
The last molten steel of the ladle enters the impact zone of the tundish with the ladle covering agent, and it is also some oxides. If it cannot be removed in time, it will also pollute the molten steel.
- Tundish covering agent and mold slag:
These two slags are used to achieve protective casting, forming a layer of liquid slag on the molten steel surface of the tundish and the liquid surface of the mold, which can isolate the air and prevent the secondary oxidation of the molten steel. effect. However, the tundish covering agent and mold flux are also some oxides, if they are entrained into the steel, they will bring fatal inclusions of large particles to the steel. At the same time, some elements in the molten steel can also be exchanged with the liquid slag in the tundish, changing the composition of the molten steel and inclusions, and adversely affecting the cleanliness and performance of the molten steel.
- Tundish refractory:
Especially at the beginning of casting, the scouring of the tundish refractory by the molten steel flow and the erosion of the tundish slag line.
- Long nozzle and submerged nozzle:
If the molten steel stream cannot be centered and descended, the molten steel flow will flow down along the inner wall of the nozzle, which will inevitably cause erosion of the refractory channel and erosion of the refractory material, resulting in molten steel pollution.
During the pouring of the first furnace of the tundish, the protection has not yet been formed. During the casting, the molten steel is exposed to the atmospheric environment, resulting in secondary oxidation, thereby absorbing nitrogen and oxygen, polluting the molten steel. This process is inevitable. What can be done under the current technical conditions is to reduce the exposure time and exposure area of molten steel, and enter the stable casting stage as soon as possible.
After the tundish is in place, the first furnace of molten steel is received. At this time, the tundish space is in the atmospheric environment. Even if it is filled with argon gas, it cannot completely guarantee that the inner cavity of the tundish is occupied by argon gas. I have not seen this article. . Exactly how to fill with argon to keep the atmosphere free of oxidation and nitriding in the bag, and there is no report on the relationship between the atmosphere in the tundish and the nitrogen content of the slab. The Indians have done some real work in this regard. According to the tundish receiving the first batch of molten steel, the molten steel in the tundish is affected by the atmosphere to generate oxygen and nitrogen absorption until the molten steel floods the outlet of the long nozzle. At this time, it enters a stable stage. Subsequent ladle molten steel continued to leak and began to gradually dilute the molten steel that had been inhaled to increase the nitrogen content, until the fifth billet became normal, indicating that the nitrogen content of the first four billets, especially the first three billets increased significantly, It also shows that the secondary oxidation has a non-negligible effect on these slabs.
The steel plant introduced in the article is a two-strand continuous casting machine, which produces a 200mm billet. It is estimated that the plant is smelted by an electric furnace, which has a small capacity and a small production capacity. To produce SCr420HB steel, according to the pulling speed of 0.9m/min, four 4.5-meter-long cast billets are produced. After 20 minutes of production, the nitrogen content is stabilized, and the flow rate of four steels is more than 11 tons. This small electric furnace data is too low for most domestic steel mills, generally more than 5 streams in China, and the billet pulling speed for producing 200 square billets of this type of steel must be above 1.4m/min. Calculated according to the corresponding configuration of the 6-strand continuous casting machine, the height of the molten steel level in the tundish during normal casting is 900mm, and the depth of the nozzle inserting the molten steel is 250mm. At the end face, the height of the molten steel is 650mm. It is estimated that the weight of the molten steel at this time is about 25 tons. According to the pulling speed of 1.4m/min special steel, it takes 9.6 minutes to digest 25 tons of molten steel. Generally speaking, the pouring starts when the molten steel reaches the height of about 300~400mm in the tundish, and the molten steel in the ladle continues to descend during this period, so the time for 20 minutes to stabilize the quality of the slab is credible. The quality requirements of steel grades are particularly strict and can be implemented accordingly.
When I was working in a steel mill more than 20 years ago, I produced high-quality special steel and developed new customers. At first, the order volume was small, less than one heat of steel. In order to meet the requirements of high-quality delivery, the first 1/3 of the steel was removed, only the 40~50 tons of billet after delivery, which was basically considered at that time, users felt good after using it, and then gradually opened the market, so there was no first furnace of this steel at that time, and there was no follow-up 10 furnace, 100 furnaces. We can only impress users with high quality and gain market recognition.
It is a good way to use nitrogen content to identify secondary oxidation, so that samples can be taken from the casting billet and the change of nitrogen content can be seen on the oxygen and nitrogen analyzer. If the oxygen content is used for identification, it cannot accurately reflect the on-site situation, because in chemical analysis, a large particle oxide inclusion can cause the oxygen content to rise sharply, and the process of preparing the sample will inevitably cause the surface of the sample to oxidize, even if Oxidation that is not visible to the naked eye can often cause an increase in the oxygen content in the analysis process. Therefore, the deviation fluctuates greatly during the analysis of oxygen content, but the nitrogen content is more accurate. The numerical fluctuation of the content of the same batch of samples is small and the reliability is high. gas condition.
Minimize the contact time of molten steel with air as much as possible. Generally speaking, the diameter of the slide plate of the ladle opening mechanism is φ60~65mm. After the ladle is poured, the slide plate should be fully opened, and the upper and lower slide plates should be aligned with the diameter of the molten steel. The tundish reaches the required liquid level. In the past, some measurements have shown that it is basically 6~10 tons of molten steel per minute. Now, the general continuous casting machine tundish car is equipped with a weighing mechanism, which can quickly know the speed of the molten steel.
To prevent secondary oxidation, the tundish covering agent should be added in time. When the tundish molten steel submerges the lower end of the long nozzle, the covering agent should be added in the impact area to form a liquid slag layer to isolate the atmosphere. The excellent and special steel continuous casting machine is divided into an impact area and a working area. The molten steel in the impact area enters the working area through the hole under the retaining wall. Once the molten steel in the working area submerges the hole in the retaining wall, the stirring of molten steel is greatly reduced. The addition of the covering agent is completed to form a liquid slag layer to isolate the atmosphere in the working area. Generally, the molten steel from the tundish enters the mold and the pouring can also start at this time.
The ladle drainage sand and ladle slag that pollute the molten steel in the tundish must be removed to minimize the impact of these two major pollution sources on the molten steel. Iron and steel enterprises have paid attention to this problem. A long-term must-do.
The continuous casting process is a process of reducing the cleanliness of molten steel. It is not only necessary to use long nozzles, submerged nozzles, tundish covering agents and mold slag to achieve protective casting, but also pay attention to the drainage sand entering the molten steel during the ladle opening process. Pollution occurs. After the ladle is poured, the ladle slag enters the tundish impact area. Necessary measures must also be taken to deal with it and guide it out in time.
After the masonry of the continuous casting tundish is completed, before placing the tundish cover, the sharp corners of the tundish refractory materials and the protruding rough areas should be cleaned up. , and use a broom to clean the bottom of the bag to remove as much garbage as possible.
If the tundish cover is made of refractory material, pay attention to the condition of the refractory material when hoisting and place it, and dispose of the hanging objects and the refractory material that will fall off in time. At the same time, use a broom to clean the tundish cover, and every grain of dust on the tundish cover If it enters the steel, it is a large particle inclusion.
Before the tundish is baked online, try to use compressed air to purge the inner cavity of the tundish to remove the dust falling in the workshop, so that a clean tundish can meet the clean molten steel.
