The introduction of vacuum degassing

crafting process of the vacuum degassing 

First, the molten steel is sucked into the vacuum chamber, and then argon is blown into the molten steel on the side walls of the two risers. The argon gas rapidly expands under the action of the high temperature of molten steel and the low pressure of the upper part of the vacuum chamber, causing the density of the mixture of molten steel and gas to continuously decrease along the height of the immersion tube. Under the action of the pressure difference caused by the density difference, The molten steel enters the vacuum chamber. The mixture of molten steel and gas entering the vacuum chamber releases gas under the action of high vacuum. At the same time, the molten steel becomes molten steel beads, and the gas to be removed from the molten steel beads is released into the vacuum under the action of high vacuum. In the process, the molten steel beads are turned into smaller molten steel beads, thereby achieving a very good degassing effect. The molten steel that has released the gas returns to the ladle along the downcomer. After several cycles in this way, the gas in the molten steel can be reduced to a relatively low level.

At the beginning of the cycle treatment, sampling and temperature measurement are required every 10 minutes; when the treatment is about to end, sampling and temperature measurement are performed every 5 minutes. According to the results of sampling and analysis, if it is necessary to add alloy materials (including deoxidizers or other additives), an automatically controlled feeding hopper can be operated, and the alloy materials can be fed into the vacuum chamber at a constant feeding rate without breaking the vacuum. .

Advantages

(1) Better degassing effect. Due to the input of driving gas, a large number of bubble nuclei are generated in the riser, and the molten steel entering the vacuum chamber is sprayed into fine droplets, which greatly increases the degassing surface area of the molten steel, which is beneficial to the progress of degassing.

(2) The temperature drop is small. Generally, the processing temperature drop is only 30-50℃, and electric heating can also be carried out during the degassing process, so the molten steel only needs a little overheating in the furnace.

(3) Large processing range. The same equipment can process molten steel of different capacities, and it can also be processed in electric arc furnaces and induction furnaces.

Because the vacuum cycle degassing method has the above advantages, it develops rapidly and has a wide range of applications. According to statistics, more than 100 RH devices have been put into production, and the processing capacity of the largest equipment is 350t. Some factories in my country have also selected RH devices. For example, after Daye Steel Plant first installed a 60-100tRH device (commissioned in 1967), then Shanghai Heavy Machinery Plant (commissioned in 1972), Wuhan Iron and Steel Company's second steelmaking plant (1#RH1979, 2#RH1990 put into production), and Baoshan Iron and Steel General Plant's steelmaking plant (put into operation in December 1985) also installed this equipment. Practice has proved that this kind of equipment is flexible in operation, reliable in operation and good in treatment effect.

Process parameter 

When designing or using a cyclic degassing device, reasonable selection of process parameters is of great significance for achieving good degassing effect, longer lining life and reducing operating costs.

Parameters such as processing capacity, circulation factor, processing time, circulation flow rate, vacuum degree and the pumping capacity of the vacuum pump are the main process parameters that should be considered in the operation and design of the cyclic degassing method.

(1) The processing capacity refers to the amount of molten steel to be processed. Too small amount of molten steel will cause the temperature to drop too fast and fail to achieve the purpose of treatment. The general capacity of ladle is ≥30t. When the molten steel capacity is large, the thermal stability is better, and the treatment can be carried out calmly to achieve good treatment results.

(2) The cycle factor is also called the number of cycles, which refers to the ratio of the amount of molten steel passing through the vacuum chamber to the amount of molten steel in the ladle during processing. The general cycle factor is 3, that is, the degassing process is completed after 3 cycles.

(3) The pressure of the vacuum chamber of the vacuum cycle degassing equipment is about 50-100 Pa. After 3 cycles, the degassing of molten steel is completed. Each cycle requires 3 minutes, and 3 times requires 10 minutes in total. Add 5 minutes for alloying and For other operations, a total of 20 minutes is required. Therefore, the vacuum cycle degassing has the characteristics of a fast processing rhythm, which can be matched with the rhythm of the converter refining. The rhythm of the vacuum cycle degassing equipment used in the electric arc furnace workshop can be slower.

(4) The circulating flow rate of the vacuum cycle refers to the amount of molten steel passing through the riser per unit time (t/min). The factors that affect the circulation flow rate are the diameter of the riser or the downcomer; increase the gas flow rate; and increase the distance between the gas and the steel surface in the vacuum chamber. Increasing the values of these parameters can increase the circulating flow of molten steel, but an excessive increase will make the density of the molten steel and gas mixture too small, and the gas will block the passage of molten steel and reduce the circulating flow.

(5) The vacuum pump must have sufficient capacity to ensure the vacuum level during the degassing process and ensure the processing quality of molten steel.

Processing effects

(1) Dehydrogenation After cyclic treatment, about 65% can be dehydrogenated by deoxidation, and 70% can be dehydrogenated without deoxidation, so that the hydrogen content in the steel is reduced to less than 2ppm.

(2) Deoxidation. Carbon has a certain deoxidation effect during cyclic treatment, especially when the original oxygen content is high, such as the treatment of undeoxidized steel, this effect is more obvious. The actual measurement found that the removal of dissolved oxygen in steel mainly relies on the deoxidation of carbon under vacuum.

(3) Nitrogen removal The RH method has a small amount of nitrogen removal. When the original nitrogen content is less than 50 ppm, there is almost no change in the nitrogen content before and after the treatment. When the nitrogen content is greater than 100 ppm, the nitrogen removal rate is generally only 10-20%.

(4) The quality of degassed steel The range of steel grades processed by vacuum cycle degassing is very wide, including forging steel, high-strength steel, various carbon and alloy structural steels, bearing steel, tool steel, stainless steel, electrical steel, deep Red steel and so on. After the molten steel is treated, the purity can be improved, the longitudinal and transverse mechanical properties are uniform, and the elongation, reduction of area and impact toughness can be improved. For some steel grades that require heat treatment, the heat treatment time can generally be shortened after degassing.

(5) Economic effect Practice has proved that vacuum degassing will not increase the production cost per ton of steel. For some steel grades, the vacuum degassing method can significantly reduce costs and has better economic benefits.