In the production of ultra-low carbon steel by vacuum furnace, the most difficult process is decarburization in the early stage and rapid aluminum deoxidization and alloying in the later stage. For Huaqiang vacuum furnace, under the premise of fundamental safety and stability of equipment functions, by optimizing the operation process of decarbonization technology,
This paper summarizes and analyzes the service coefficient of carbon oxygen reaction and the yield of aluminum deoxidization and alloying, explores some experience formulas, greatly shortens the time of vacuum treatment, speeds up the production rhythm and improves the product quality.
1. Technical process of ultra-low carbon steel produced by 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.
For this process, there are two main aspects of difficulty control, the first one is to set the carbon removal to the lowest scale within 15 minutes; the second one is 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 ensure aluminum alloying. In this way, the exercise time can be controlled between 25 and 30 minutes. Otherwise, the vacuum processing time will be greatly extended. In the production process of ultra-low carbon steel, along with the extension of the exercise time, the touch between the refractory and the molten steel will also cause the molten steel to return to carbon. In addition, the ultra-low carbon steel because of the large amount of aluminum participation, the occurrence of many Al2O3 impurities, so the earlier aluminum participation, the more conducive to the floating of Al2O3 impurities, 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, converter molten steel is directly decarburized in vacuum furnace without deoxidation. Therefore, it is necessary for us to know the carbon and oxygen content in the molten steel from converter. According to the practice experience, 100 ppm carbon needs about 150 ppm oxygen. To remove carbon to a very low scale, it is necessary to ensure that there is sufficient oxygen content of more than 200 ppm in the molten steel. For example, the carbon content of converter molten steel is 300ppm. In order to remove carbon to a very low scale, the oxygen content required is: 3 × 150 + 200 = 650 ppm. Of course, the higher the oxygen content is, the more conducive to decarburization. However, the higher the oxygen content is, the more aluminum will be required to deoxidize, which will result in material waste and affect the purity of molten steel. Fig. (1) is the curve of the change between the time of vacuum decarburization and the carbon content of molten steel in the production process 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 fluctuation of carbon decreased less! But it can be reduced to about 15 ppm. For the convenience of analysis, we divide the decarbonization process of 15 minutes into two periods to comment, the first six minutes are light treatment, and the last nine minutes are deep decarbonization.
3.1. Light disposal
At the beginning, because the carbon and oxygen content are relatively high, the response is sharp, and it is easy to splash, so we need to control the response speed of carbon and oxygen to prevent some incidents. At the beginning of disposal, set the promoted gas flow rate to 80nm3 / h, and control the vacuumizing speed. Generally, it takes 2 minutes for 1000mbar to 400mbar, 2 minutes for 400mbar to 150mbar, and then it takes about 2 minutes for 150mbar. When the oxygen content is high, the carbon content in the molten steel can be reduced to about 30ppm after the light disposal.
3.2 deep decarbonization treatment
With the decrease of carbon content in molten steel, the decarburization rate becomes slower, and the carbon dispersion becomes a restrictive link. In order to continue decarburization, it is necessary to adopt other methods. At the moment, the decision should be based on the oxygen content determined after light treatment. At the top of the list, when the abundant oxygen content in steel water is still very high (more than 300ppm), we need to improve the gas flow rate to 150nm3 / h, turn on E3, E2, E1 booster pumps, reduce the vacuum degree to about 1mbar, and ensure that the carbon can be reduced to about 15ppm in 9-minute cycle decarburization time. Second, if the abundant oxygen content of the molten steel is low, the first carbon content and oxygen content can be contacted. According to the experience formula of 100 ppm carbon, 150 ppm oxygen is needed, plus the abundant 200 ppm oxygen, the oxygen content that needs to be added 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 when blowing 100nm3 oxygen. Thus, the volume of oxygen blowing can be calculated.