The VOD (Vacuum Oxygen Decarburization) furnace decarburization process is a well - established method aimed at eliminating carbon from materials. The refining operations within a VOD furnace are primarily regulated by four key parameters: the oxygen supply rate (or intensity), the height of the top - blowing oxygen gun, the degree of vacuum (which is determined by the number of vacuum pumps in operation), and the flow rate of argon at the bottom.
The decarburization refining process in a VOD furnace can be divided into two distinct stages: the vacuum - blown oxygen decarburization stage and the vacuum decarburization stage.
In the initial phase of oxygen - based decarburization, desiliconization takes place for approximately 2 minutes, after which the actual decarburization process commences. To prevent excessive splashing and blockage of the feeding tube during this stage, it is advisable to adopt high gun positions (with a maximum height of ≤900mm) and reduce the argon flow rate.
The speed of oxygen - based decarburization is closely related to several factors. These include the oxygen supply intensity, the argon blowing intensity, the initial carbon content of the molten steel, and its temperature. For instance, when the initial carbon content is 0.27%, the average decarburization rate is 0.0125% per minute, and when the initial carbon content is 0.41%, the average rate increases to 0.0188% per minute.
Intense stirring, achieved by maintaining a bottom - blown argon intensity in the range of 0.01 to 0.02 Nm³/(t·min), offers multiple benefits. It not only helps in reducing the oxidation of chromium but also significantly accelerates the decarburization rate.
Once the carbon content in the steel drops to 0.08%, it is time to transition to the vacuum decarburization stage. The lower the target end - carbon content, the lower the required vacuum degree and the longer the decarburization time. Typically, this stage lasts between 15 and 30 minutes.
Given that the decarburization process in a VOD furnace occurs under vacuum conditions, automatic control of the furnace operation is essential. Among all the control aspects, the management of the end - carbon content holds great significance for the smooth progress of the entire smelting process and the precise control of the carbon content in the final product.
There are two main methods for determining the end - carbon content: the exhaust gas analysis method and the gas - phase carbon determination method.
This method employs an infrared analyzer to examine the components of the exhaust gas, specifically CO, CO₂, and O₂. By referring to the changes in vacuum, it can effectively control the refining process of the VOD furnace. One of the major advantages of this method is its ability to accurately determine the optimal time to stop oxygen blowing. Moreover, it is not constrained by the specific conditions of the equipment.
In this approach, the total volume of exhaust gas and its CO and CO₂ content are measured. Based on these measurements, the decrease curve of the carbon content over time is calculated. Once the target end - carbon content is reached, oxygen blowing is stopped. This method provides a reliable way to control the decarburization process and ensure the desired carbon content in the final product.
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