Practical operation of ultra low carbon steel production in vacuum furnace

In the production of ultra-low carbon steel in vacuum furnace, the most difficult process to control is decarburization in the early stage and rapid aluminum deoxidization and alloying in the later stage. In view of Huaqiang vacuum furnace, on the premise of stable equipment performance, by optimizing the decarbonization process, summarizing and analyzing the utilization coefficient of carbon oxygen reaction and the recovery rate of aluminum deoxidization and alloying, some empirical formulas are found out, which greatly shortens the vacuum treatment time, speeds up the production rhythm and improves the product quality.

1. Production process of ultra-low carbon steel in vacuum furnace
Molten steel from converter → temperature measurement and oxygen determination sampling → light treatment for 7 minutes → temperature measurement and oxygen determination again → deep decarburization treatment for 8 minutes → temperature measurement and oxygen determination sampling → adding aluminum for deoxidization and alloying → after 3 minutes of circulation → temperature measurement and oxygen determination sampling → tapping after 5 minutes of clean circulation.
In view of this process, there are two aspects to control the difficulty: first, how to remove carbon to the lowest range within 15 minutes; second, how to add aluminum to the position once according to the residual oxygen content in the molten steel determined after decarburization within 15 minutes, which can not only ensure aluminum deoxidization, but also aluminum alloying. In this way, smelting time can be controlled between 25 and 30 minutes. Otherwise, the vacuum treatment time will be greatly prolonged. In the production process of ultra-low carbon steel, with the extension of smelting time, the contact between the refractory and the molten steel will also lead to the carbon recovery of the molten steel. In addition, due to the large amount of aluminum added to ultra-low carbon steel, there are many Al2O3 inclusions. Therefore, the earlier aluminum is added, the more conducive to the floating of Al2O3 inclusions, the more conducive to the operation of the next process and the improvement of product quality.

2. Carbon control
In the production of ultra-low carbon steel, the converter molten steel is directly decarburized in the vacuum furnace without deoxidation. Therefore, we must know the content of carbon and oxygen in the molten steel from the converter. According to the practical experience, about 150 ppm oxygen is needed for 100 ppm carbon. If we want to remove carbon to a very low range, we must also ensure that there is more than 200 ppm oxygen in the molten steel. For example, the carbon content of converter molten steel is 300ppm. In order to remove carbon to a very low range, the oxygen content needed is: 3 × 150 + 200 = 650 ppm. Of course, the higher the excess oxygen content is, the more conducive to decarburization. However, the higher the excess oxygen content is, the more aluminum will be needed to deoxidize, which will cause waste of raw materials and affect the purity of molten steel. Fig. (1) is the curve of the time of vacuum decarburization and the carbon content in the molten steel during the production of ultra-low carbon steel. From the figure, we can see that in the first six minutes of decarbonization, the carbon content decreased rapidly, and in the next nine minutes, the carbon amplitude decreased less! But it can be reduced to about 15 ppm. In order to facilitate the analysis, we divided the decarburization process of 15 minutes into two stages. The first six minutes were light treatment and the last nine minutes were deep decarburization.
3.1. Light treatment
At the beginning, due to the high content of carbon and oxygen, the reaction is more intense and easy to splash, so we need to control the reaction speed of carbon and oxygen to avoid some accidents. In the first treatment, set the lifting gas flow to 80nm3 / h, and control the vacuumizing speed. In general, it takes 2 minutes to pump 1000mbar to 400mbar, 2 minutes to pump 400mbar to 150mbar, and then keep 150mbar for about 2 minutes. In the case of high oxygen content, after the light treatment, the carbon content in molten steel can basically drop to about 30ppm.

3.2 deep decarburization treatment
With the decrease of carbon content in molten steel, the decarburization rate becomes slower, and the diffusion of carbon becomes a limiting link. In order to continue decarburization, other measures must be taken. At this time, it should be determined according to the oxygen content determined after the light treatment. First, when the excess oxygen content in the steel water is still very high (more than 300ppm), it is necessary to increase the lift gas flow to 150nm3 / h, and turn on the E3, E2, E1 booster pumps to reduce the vacuum degree to about 1mbar, ensure the cycle decarburization time of 9 minutes, and reduce the carbon to about 15ppm. Second, if the excess oxygen content of steel water is low, the first carbon content and oxygen content can be combined. According to the empirical formula, 100 ppm carbon needs about 150 ppm oxygen, plus 200 ppm oxygen, the additional oxygen content can be calculated. Finally, t-cob oxygen gun is used for deep decarburization. The production practice shows that when the lance position is 4220 mm, the oxygen pressure is 14 to 16 bar, and the oxygen flow rate is 1800 Nm3 / h, 550ppm oxygen can be added to the molten steel for every 100nm3 oxygen blowing. The volume of oxygen blowing can be calculated.