Vacuum Furnace ultra low Carbon Steel Produced
Ultra-low carbon steel is produced in vacuum furnace. The most difficult process to control is decarbonization in the early stage and rapid aluminium deoxidation and alloying in the later stage. With regard to Huaqiang vacuum furnace, under the premise of stable equipment function, the operation process of decarbonization skills is optimized.
The utilization coefficient of carbon-oxygen reaction and the yield of deoxidation and alloying of aluminium are summarized and analyzed. Some experiential formulas are explored, which greatly shorten the vacuum disposal time, speed up the production rhythm and trudge the product quality.
1. Skills Process of Ultra-Low Carbon Steel Production in Vacuum Furnace
The molten steel coming from converter temperature and oxygen sampling light disposal for 7 minutes temperature and oxygen determination deep decarbonization for 8 minutes temperature and oxygen sampling aluminum deoxidation and alloying circulation for 3 minutes temperature and oxygen sampling net circulation for 5 minutes.
There are two main difficulties in controlling this process. Firstly, it is necessary to set the minimum carbon removal plan in 15 minutes. Secondly, how to add aluminium in place at one time according to the residual oxygen content in molten steel after 15 minutes decarbonization can ensure both aluminium deoxidation and aluminium alloying. In this way, the training time can be controlled between 25 and 30 minutes. Otherwise, the vacuum disposal time will be greatly extended. In the production process of ultra-low carbon steel, along with the extension of drilling time, the contact between refractory and molten steel will also cause the carbon return of molten steel. Otherwise, because the participation of aluminium in ultra-low carbon steel is larger than that in the control, there are many Al2O3 mixtures. Therefore, the earlier the aluminium participation, the more advantageous it is to the floatation of Al2O3 mixing, the more advantageous it is to the operation of the subsequent process and the journey of product quality.
2. Carbon control
When producing ultra-low carbon steel, the molten steel of converter is decarbonized directly in vacuum furnace without deoxidation. Therefore, it is necessary for us to know the content of carbon and oxygen in molten steel. According to the practice experience, 100 ppm carbon needs about 150 ppm of oxygen. In order to remove carbon to a very low level, it is necessary to ensure excess oxygen content in molten steel above 200 ppm. For example, the carbon content of molten steel in converter is 300 ppm. In order to remove carbon to a very low level, the oxygen content required is 3 *150+200=650 ppm. Of course, the higher the excess oxygen content, the better the decarbonization. However, excessive excess oxygen content will necessarily require more aluminium to deoxidize, which will constitute material waste and affect the purity of molten steel. Fig. 1 is the change curve of vacuum decarburization treatment time and carbon content in molten steel during the production of ultra-low carbon steel. From the figure, we can see that in the first six minutes of decarbonization process, the carbon content drops rapidly, and in the next nine minutes, the carbon content decreases less concavely and convex. But it can be reduced to about 15 ppm. In order to facilitate analysis, we divide the 15-minute decarbonization process into two periods. The first six minutes are for light treatment, and the second nine minutes are for deep decarbonization.
3.1. Light Disposal
In the initial period, because the carbon and oxygen content are higher than the illumination, the reaction is more intense than the illumination and easy to splash, so we should control the reaction speed of carbon and oxygen and prevent some incidents from happening. When the gas flow rate is set to 80Nm3/h, and the vacuum speed should be controlled. Usually 1000 mbar is pumped to 400 mbar for 2 minutes, 400 mbar to 150 mbar for 2 minutes, and then 150 mbar for 2 minutes. When the oxygen content is high, the carbon content in molten steel can be reduced to about 30 ppm at the end of light treatment.
3.2 Deep decarbonization disposal
With the decrease of carbon content in molten steel, the decarbonization rate slows down, and the loosening of carbon becomes a restrictive link. In order to continue decarbonization, it is necessary to adopt other methods. At this time, the oxygen content determined after light disposal should be selected. Firstly, when the excess oxygen content in molten steel is still high (more than 300 ppm), it is necessary to travel to 150 Nm3/h and turn on E3, E2 and E1 booster pumps to reduce the vacuum to about 1 mbar, so as to ensure a 9-minute cycle decarbonization time and reduce the carbon to about 15 ppm. Secondly, if the excess oxygen content in molten steel is low, the first carbon content and oxygen content can be contacted. According to the experience formula, 100 ppm carbon needs about 150 ppm oxygen, and the excess 200 ppm oxygen, the oxygen content of the additional demand can be calculated. Ultimately, T-COB oxygen lance is used for deep decarbonization by blowing oxygen. Production practice shows that when the gun position is 4220 mm, oxygen pressure is 14 to 16 bars and oxygen flow rate is 1800 Nm3/h, oxygen can be increased by 550 ppm per 100 Nm3 oxygen blowing. Thus the volume of oxygen blowing can be calculated.