Problems and Solutions of LF-120 Ladle Refining Furnace

1. General Introduction

The LF-120 type ladle refining furnace of Tonggang Steel Mill is special refining equipment, which mostly adopts submerged arc refining operation. Its main features are: the molten steel melted in the primary smelting furnace is sent to the ladle, and then the electrode is inserted into the slag on the upper part of the molten steel of the ladle to generate an electric arc. The synthetic slag is added to form a high alkalinity white slag, which is stirred with argon to maintain a strong reducing atmosphere in the ladle for so-called submerged arc refining. As argon stirring accelerates the chemical reaction between slag and steel, arc heating for temperature compensation can ensure a long refining time, so as to reduce the content of oxygen and sulfur in steel (the minimum content of sulfur can be up to 10ppm and the total oxygen can be below 25ppm).

The LF-120 ladle refining furnace has less equipment investment, can significantly improve the steel output, cooperate with the converter for production, and can effectively evenly adjust the temperature and composition of molten steel before pouring (casting), so that the converter steelmaking plant can produce high-quality steel products at low cost. At present, the steel refined by LF refining furnace in the plant includes low carbon steel (SPHC / D), medium carbon steel (SS400, Q235B, s275jr), and weather-resistant steel (SPA-H), etc.

2. Problems Existing in the Use of LF Refining Furnace

Since the LF-120 ladle refining furnace was put into operation, it has been nearly 10 years. During this period, the furnace has had large and small fault problems. In recent years, the most frequent fault problem is the water leakage of the horizontal furnace cover and waste gas connector water pipe on the main equipment, which seriously restricts the smooth operation of production.

Horizontal furnace cover and exhaust gas connector are the key parts of LF ladle refining furnace design, because in many cases, the metallurgical effect of ladle refining furnace largely depends on the atmosphere control in the furnace. In order to prevent air from entering the furnace from the gap between the furnace cover and the ladle and the opening of the furnace cover, some necessary protective measures are taken, including the cleaning and maintenance of the ladle mouth that must be taken during use.

The furnace cover of 120t ladle refining furnace is a fully water-cooled tubular furnace cover. The whole furnace cover is welded with 20g seamless steel pipe and the special elbow to form a high-efficiency water-cooled forced circulation with the uniform flow and no dead point, which instantly takes away the heat generated in the ladle and protects the integrity of the furnace cover.

At present, it is found that there are three main reasons for the leakage of the full cooling water pipe of the furnace cover:

1. During production, molten steel splashes, steel slag splashes on the horizontal furnace cover and waste gas connector water pipe, and the water pipe is burned.

2. When refining molten steel, a large amount of dust is generated, that is, there are countless metal particles floating in the air. Due to the continuous movement of these metal particles, they constantly "wash" the cooling water pipes of the horizontal furnace cover and exhaust gas connector. Over time, under the "washing" of this metal particle, the water pipe wall becomes thinner and thinner until the weakest part of the whole water pipe bursts.

3. Steel slag splashes on the horizontal furnace roof and exhaust gas connector. When the electrode is energized and inserted into the steel water, the distance between the electrode and the furnace cover is very close. A large amount of dust containing metal particles floats in the air, which will connect the energized electrode with the furnace cover, resulting in a short circuit. In an instant, the cooling water pipe with a thickness of 8mm and a diameter of 89mm on the furnace cover will be broken down, resulting in water leakage.

The above three reasons constitute the main reason for the leakage of the cooling water pipe of the furnace cover, of which the third reason has a high frequency and the greatest impact. After the water leakage of the furnace cover and the water supply pipe of the waste gas connector, the production of the LF furnace was forced to be interrupted, and the repair welding of the primary water pipe takes 4 ~ 5 hours each time, which seriously affects the normal production rhythm.

3. Solutions to Existing Problems

If there is a problem with production equipment, we must always try our best to solve the problem in order to maintain the normal progress of production. Facing the problems of LF refining furnace horizontal cover and waste gas connector, equipment maintenance personnel successively consider three ways to solve the existing problems.

3.1 The first thought is to increase the thickness of the 20g steel pipe wall to prevent the water pipe from being burned through or broken down. The original 89mm outer diameter steel pipe with an 8mm thick wall is changed to the 108mm outer diameter steel pipe with the 12mm thick wall, which is added to the furnace cover through cold bending, welding and other technologies. After all the replaced thick wall cooling pipes are put into use, the problem remains. The pipes are still easy to be burned or broken down, and still can not meet the needs of production.

3.2 It is considered to modify the center distance of the three-phase electrodes, that is, to make the electrodes far away from the water pipe of the furnace cover to avoid power on and short circuits. However, due to the design reason of the electrodes, they can not rotate and can only be adjusted back and forth, and the fault still cannot be eliminated.

3.3 Remove the original high-efficiency water-cooled forced circulation full water-cooled pipe cooling mode, and make a refractory with the same shape according to the shape of horizontal furnace cover and waste gas connector. The thickness is 100mm. The refractory is insulator and non-conductive. The electric spark and splashed steel slag generated by the electrode during production will not damage the refractory and affect the use of the equipment. The third method is also the method being adopted at present. The implementation of this technology can avoid the influence of three factors: molten steel splashing, dust "scouring" and power on short circuit ignition at the same time, solve the problem of water pipe leakage, and effectively protect the horizontal furnace cover and waste gas connector from the influence of high temperature and heat. The equipment maintenance personnel can ensure the integrity of the horizontal furnace cover and waste gas connector by replacing the refractory regularly, The replacement process takes a short time and the operation is very simple.

4. Conclusion

Indeed, the full water-cooled tube cooling method has advantages that other cooling and heat protection methods do not have. However, the transformation of ladle refining furnace cover from water cooling to the refractory method in Tonggang refining and rolling plant shows that any technology has advantages and disadvantages. If applied under different environmental conditions, it will show different effects. It can give full play to its advantages. The technology to solve practical problems is the best technology application. The refractory form adopted by the LF-120 ladle refining furnace cover of the plant solves the problem of water leakage of the furnace cover pipe bothering the production. Up to now, this technology is still used in two ladle refining furnaces and has received good application effect. It not only runs the production smoothly, and ensures the stable operation state of the equipment, but also reduces the equipment maintenance cost, reduces the labor intensity of equipment maintenance personnel and improves the production efficiency.