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In the production of low nitrogen steel in the electric furnace process, some high-grade steel grades have strict requirements on nitrogen, such as high-grade hot-rolled steel plate with nitrogen requirement less than 40 × 10-4%, high-strength pipeline steel with nitrogen requirement less than 30 × 10-4%, if cold-rolled steel plate with nitrogen requirement less than 25 × 10-4%, 82B wire rod is drawn and combined into high-strength steel wire, prestressed steel wire, steel strand, steel wire rope, etc. if the nitrogen content is high, it will appear. Therefore, nitrogen control in steel is an important topic in the research project of microalloy and high carbon steel.

 

In the production of low nitrogen steel, the electric furnace process is obviously inferior to the converter process. The lowest nitrogen content in the tapping liquid of the converter process is 10 × 10-4%, and the final nitrogen content in the steel is 20 × 10-4% ~ 40 × 10-4%. Due to the high nitrogen content in scrap and the easy nitrogen absorption of molten steel in the arc zone, the nitrogen content in the product is generally 80 × 10-4% ~ 120 × 10-4%. With the development of modern EAF steelmaking technology, the increase of carbon distribution and oxygen consumption, some enterprises have been able to produce liquid steel less than 50 × 10-4% or even less than 40 × 10-4% through EAF process, but systematic research is needed to achieve this effect stably.

 

Reasonable control and optimization of oxygen supply system is the basis

 

Firstly, in the process of EAF smelting, decarburization can effectively remove nitrogen at the same time. In the process of oxygen blowing and decarburization, the denitrification is due to the fact that the carbon oxygen reaction product co bubble is equivalent to a small vacuum chamber, by which the [n] in the molten steel can be brought out. In the process of decarburization, the rate of denitrification is directly proportional to that of decarburization, and the rate of denitrification increases with the increase of temperature. Oxygen and sulfur are surface active elements, which hinder the denitrification of molten steel.

 

Secondly, in the process of EAF smelting, the process of denitrification and denitrification of molten steel exist simultaneously. Denitrification mainly depends on the C-O reaction to form co bubble carrying method. When the carbon content in the molten steel is high, the reaction of carbon and oxygen in the molten steel is strong, and a large number of CO bubbles are produced in the molten pool to remove nitrogen from the molten steel. The higher the carbon content in the molten steel, the lower the tapping amount. In order to reduce the nitrogen content of tapping, the denitrification amount in the smelting process of electric furnace must be increased, which requires that the furnace charge should have high carbon distribution and more C-O reaction. The oxygen consumption per ton of steel is increased due to the increase of the charge carbon content, which is almost close to the oxygen consumption level of converter.

 

When the amount of hot metal added is basically the same, the nitrogen content in tapping varies greatly, which is caused by the different oxygen supply intensity during the smelting of electric furnace. Under high oxygen blowing intensity, decarburization is rapid in the early stage of melting, which can rapidly reduce the nitrogen content in the molten pool. However, in this fashion, some scrap is not completely melted, and the nitrogen brought by the melting of this scrap is difficult to be removed due to the lack of carbon oxidation in the molten pool, so the nitrogen content in the molten pool is relatively high; on the contrary, under low oxygen blowing intensity, the nitrogen content in the molten pool initially decreases slowly, but at the In the later stage of smelting, the recovery of nitrogen in molten steel is also slow due to scrap melting, and the nitrogen content in tapping is low.

 

After the first batch of scrap is added to the EAF, the molten pool metal is heated by the high temperature of the arc. When the electrode is heated, a pit can be formed at the impact point, and the exposed liquid steel will appear. If the temperature of the exposed liquid steel is higher than that of other parts, nitrogen will be absorbed. In addition, the strong jet of the arc will draw a lot of gas into the arc column, which provides conditions for the dissolution of [n] in the molten steel. As long as a molten metal pool is formed, the liquid steel has the opportunity to absorb nitrogen from the atmosphere. Therefore, in order to reduce nitrogen absorption from the atmosphere, it is an effective measure to make foam slag to prevent molten steel from being exposed. Especially after melting, the thickness of slag layer of foam slag should be higher than the depth of impact pit.

 

Taking into account the above factors, determining the reasonable amount of hot metal, optimizing the oxygen supply system and foaming slag as soon as possible can reduce the nitrogen content in tapping liquid steel.

 

The key is to improve the process and relevant measures

 

In this paper, cm490 and 20CrMo are used as two kinds of test steel. Aluminum is used to deoxidize cm490 and aluminum is not used to deoxidize 20CrMo. With the increase of the aluminum content in the molten steel, the amount of nitrogen increased. Under the same condition of adding aluminum, the higher the aluminum content in the molten steel is, the less aluminum is consumed in deoxidization. Before the nitrogen in the molten steel reaches saturation, the aluminum in the molten steel promotes the nitrogen absorption of the molten steel. At the same time, aluminum deoxidation can reduce the dissolved oxygen in the molten steel greatly, and create good conditions for nitrogen absorption.

 

During LF refining process, the amount of nitrogen added can reach a good level of less than 5 × 10-4%. The main reasons for nitrogen increase in LF are the contact between molten steel and atmosphere, arc ionization, nitrogen in raw materials, and the power supply system will also affect it. For the refining process of steel without aluminum deoxidation, even if the steel liquid contacts with the atmosphere, the amount of nitrogen absorption is very small, or even no nitrogen absorption. Once the steel liquid contacts with the atmosphere, it will absorb nitrogen, but the chance of nitrogen absorption is less.

 

The effect of electric arc heating on nitrogen content of liquid steel is the same as that of electric furnace heating. No matter whether aluminum is deoxidized or not, nitrogen will be absorbed in the bare area exposed to arc. In the process of LF refining, foam slag must be made to prevent the liquid steel from being exposed in the arc area. In addition, when the surface temperature of refining slag is between 600 ℃ and 1500 ℃, the liquid steel hardly absorbs nitrogen. Therefore, it is very important to keep a reasonable amount of slag and keep the slag covered well in the refining process to control the nitrogen increase of molten steel. In addition, reducing the high temperature time of slag and the existence time of high temperature slag as much as possible is also conducive to preventing nitrogen absorption of molten steel.

 

For the cm490 Liquid Steel Deoxidized by aluminum, the tundish is empty and the liquid steel is exposed at the same time, so the nitrogen absorption of the liquid steel is serious, and the nitrogen content almost doubled. With the pouring process