Differences between AOD and VOD furnaces

What is AOD?

AOD refining method is the abbreviation of argon oxygen decarburization method. When refining stainless steel, it blows oxygen into molten steel under standard atmospheric pressure, and blows inert gas (Ar, N2) to achieve the effect of false vacuum by reducing CO partial pressure, so that the carbon content is reduced to a very low level. level, and inhibits the oxidation of chromium in steel.

AOD furnace is suitable for producing low carbon and ultra low carbon stainless steel, and it is easy to control the S content in special steel below 0.005%. The AOD refining furnace can carry out secondary steelmaking, and generally can flexibly add or reduce related elements to achieve the purpose of refining. In the smelting process, scrap iron and iron sand are generally used to make steel. The quality of the product is relatively high and the ductility is good. The deep-drawn products are generally smelted in a refining furnace.

What is vacuum oxygen decarburization?

The VOD method is an out-of-furnace refining technology for producing high-chromium stainless steel by blowing oxygen and decarburizing under vacuum conditions and stirring with argon. The decarburization refining of the VOD furnace is mainly controlled by the supply speed (intensity) of oxygen, the height of the top blowing oxygen lance, the degree of vacuum (the number of starting vacuum pumps) and the flow rate of the bottom blowing argon.

The differences between AOD and VOD

1. The working principle is different:

(1) The AOD furnace body is placed on a support ring that can be tilted back and forth and fixed by pins. The bottom side of the furnace is equipped with a lance for blowing gas into the horizontal direction of the molten pool. The number of lances varies according to the tonnage of the furnace, generally 2~3. The meeting point of the lance extension lines coincides with the vertical axis of the furnace. The spray gun is composed of two layers, an inner tube (copper) and an outer tube (stainless steel). The inner tube introduces the main blowing gas (composed of ArO2. or Ar-N2), and the annular gap between the inner tube and the outer tube introduces Ar and N2 or air for cooling to protect the gun. The lance is consumable, buried in the refractory layer when the furnace is built, and burnt out synchronously with the furnace lining during the smelting process.

The oxygen blown in is mainly used to heat up carbon oxide, the argon gas blown in is mainly for strong convection stirring, and nitrogen blowing can be used for alloying when producing high-nitrogen alloys. -4N steel grades save a lot of metal nitrides, such as the addition of nitride complexes, and save a lot of production costs.

(2) The decarburization process of the VOD furnace is divided into two stages, namely the vacuum oxygen blowing decarburization stage and the vacuum decarburization stage. In the initial stage of oxygen blowing decarburization, desiliconization occurs for about 2 minutes first, and then decarburization begins. At this time, in order to prevent problems such as large splash and block the feeding pipe, measures can be taken to reduce the argon flow rate and the high gun position (≤900mm). .

The oxygen blowing decarburization rate is related to the oxygen supply intensity, the argon blowing intensity, the initial carbon content and the molten steel temperature. The initial carbon was 0.27%, the average decarburization rate was 0.0125%/min, and at 0.41%, it was 0.0188%/min. The strong stirring with the intensity of bottom blowing argon reaching 0.01~0.02Nm3∕(t·min) can not only reduce the oxidation of chromium, but also significantly increase the decarburization rate. When the carbon content in the steel reaches 0.08%, vacuum decarburization should be carried out. The lower the required end-point carbon is, the lower the vacuum degree is, and the longer the decarburization time is, generally 15~30min.

2, the effect is different:

(1) The AOD furnace body is built with magnesia-calcium bricks or chrome-magnesium bricks or magnesia-rich dolomite bricks; the fused chrome-magnesium bricks of higher quality are used around the spray gun to withstand high temperature and molten steel erosion. The removable furnace cap is built with chrome-magnesium bricks or aluminum-magnesium bricks, and can also be poured with heat-resistant concrete. The size ratio of the furnace is roughly: the depth of the molten pool: the diameter of the furnace: the total height of the furnace is 1:2:3. 

When the furnace is tilted forward at a certain angle, the initial molten steel is poured from the transfer ladle, and the gas spray gun is above the molten steel surface. When the furnace returns to the upright position, the spray gun is submerged into the depth of the molten steel, and air-blown refining begins. After the refining is completed, the steel is still tapped through the furnace cap.

(2) Since the decarburization of the VOD furnace is carried out in a vacuum can, the operation of the VOD furnace must be controlled automatically. Among them, the control of end-point carbon is extremely important to the smooth progress of the entire smelting process and the control of the carbon content of the finished product. The determination of end-point carbon mainly includes exhaust gas analysis method and gas-phase carbon determination method. The exhaust gas analysis method is to analyze the CO, CO2 and O2 components in the exhaust gas with an infrared analyzer, and refer to the change of the vacuum degree to control the refining process of VOD. This method can accurately determine the oxygen stoppage time, and is not limited by the specific conditions of the equipment. The gas-phase carbon determination method is to measure the total amount of exhaust gas and the content of CO and CO2 to calculate the decrease curve of alkali content with time, and stop blowing oxygen after reaching the end carbon.

3. Features are different

AOD furnace method is a more advanced technology for refining stainless steel. It has the advantages of simple equipment, convenient operation, strong adaptability, low investment and low production cost, and is widely used.

VOD has strong alloying, degassing, decarburizing, and powder spraying capabilities; in addition, it has strong stirring, heating, slag washing, and inclusion processing capabilities. The tank cover has a large area and is easy to arrange devices for different purposes; the tank can accommodate ladles of different sizes; it is easy to connect with a vacuum pump; the upper part of the ladle does not need to be flanged, the structure is simple, and a small free space can be used; it is easy to set up a splash-proof cover; It is easier to ensure the advantages of sealing and so on.