VOD Furnace Decarburization Process

VOD Furnace Decarburization Process

The VOD (Vacuum Oxygen Decarburization) furnace decarburization process is a refining method for reducing carbon content in molten steel. It is primarily controlled through the oxygen supply rate (intensity), the position of the top‑blown oxygen lance, the vacuum level (determined by the number of operating vacuum pumps), and the flow rate of bottom‑injected argon.

Process Stages

Decarburization in the VOD furnace occurs in two distinct stages:

1. Vacuum Oxygen Blowing Stage  

   During the initial oxygen injection, desiliconization takes place for approximately two minutes, followed by active decarburization. To minimize splashing and prevent blockage of the feed system, the lance is maintained at a high position (typically ≤900 mm) while argon flow is reduced. The rate of decarburization depends on factors such as oxygen supply intensity, argon stirring intensity, initial carbon content, and steel temperature. For example:

   - With an initial carbon of 0.27%, the average decarburization rate is about 0.0125%/min.

   - At 0.41% initial carbon, the rate increases to approximately 0.0188%/min.

   Strong bottom stirring with an argon intensity of 0.01–0.02 Nm³/(t·min) not only enhances decarburization but also reduces chromium oxidation.

2. Vacuum Decarburization Stage  

   When the carbon content in the steel reaches about 0.08%, oxygen blowing is stopped and decarburization continues under vacuum. The lower the target final carbon, the higher the required vacuum level and the longer the decarburization time, generally ranging from 15 to 30 minutes.

Process Control and End‑Point Determination

Since VOD refining operates under vacuum, precise automated control is essential. Accurate determination of the end‑point carbon content is critical for process stability and final product quality. The two main methods used are:

- Exhaust Gas Analysis  

  An infrared analyzer continuously monitors the concentrations of CO, CO₂, and O₂ in the off‑gas. Combined with vacuum level data, this method allows real‑time control of the refining process and precise determination of when to stop oxygen blowing. It is widely applicable and not limited by specific equipment configurations.

- Gas‑Phase Carbon Measurement  

  This approach calculates carbon removal by measuring the total volume of exhaust gas and its CO and CO₂ content, generating a real‑time carbon‑decrease curve. Oxygen blowing is terminated once the target end‑point carbon is reached.

Through careful regulation of operating parameters and reliable end‑point detection, the VOD process enables efficient, controlled decarburization while minimizing loss of valuable alloying elements such as chromium.

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