Apr 15, 2021
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Dephosphorization in the steelmaking process

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The effect of phosphorus on the properties of steel

Phosphorus exists in the form of iron phosphide in steel, generally represented by [P]. Its specific effects on steel properties are:

1) It can deteriorate the welding performance of steel;

2) Reduce the plasticity and toughness of the steel, and make the steel cold brittle, that is, the impact toughness of the steel is significantly reduced under low temperature conditions;

3) It can improve the cutting performance of free-cutting steel;

4) It can improve the fluidity of molten steel and improve the casting performance of molten steel;

5) It can improve the corrosion resistance of alloy steel to the atmosphere and seawater;

6) It can improve the magnetic permeability of silicon steel for electrical engineering. For most steel grades, [P] is a harmful element.

Basic conditions and methods of dephosphorization reaction

Removal of phosphorus in steel to the required range is one of the tasks of steelmaking.

1.The basic reaction of dephosphorization

2[P]+5(FeO)+4(CaO)=(4CaO·P2O5)+5[Fe] Exothermic

It can be seen from the reaction formula that lower temperature, high (FeO), and high (CaO) are beneficial to the dephosphorization reaction.

2.Basic conditions and methods of dephosphorization

1) The basicity of the slag should be appropriately high and the fluidity should be good. The research results suggest that the slag basicity during dephosphorization should be controlled within 2.5-3.

2) Properly increase the oxidizing property of the slag, that is, the iron oxide in the slag should be higher.

3) Appropriate temperature. Although low temperature is conducive to the exothermic reaction, low temperature is not conducive to the melting of lime and the diffusion reaction, which will eventually affect the dephosphorization reaction rate. Therefore, in order to obtain the best dephosphorization effect, the molten pool should have an appropriate temperature, which should not be too high or too low.

4) Large amount of slag is one of the effective methods to improve the dephosphorization effect. For the electric furnace, the automatic slag flow method is adopted, and the discharge of old slag and the creation of new slag are another operation form of large slag volume.

Back to Phosphorus

Phosphorus recovery refers to the phenomenon that the phosphorus content in the molten steel in the late smelting period has risen compared with the mid-term, and the phosphorus content in the finished steel is higher than the phosphorus content in the molten steel at the end of smelting.

1.Reasons for Phosphorus

1) Too high furnace temperature will reverse the dephosphorization reaction.

2) At the end of smelting and at the time of tapping, adding ferroalloys to the furnace or ladle will greatly reduce the ∑ (FeO) in the slag. At the same time, the deoxidation products such as SiO2 will also greatly reduce the alkalinity of the slag and cause deoxidation. The phosphorus reaction proceeds in reverse.

3) The ferroalloy itself brings in a certain amount of phosphorus.

Among the above-mentioned reasons, the reduction of Σ(FeO) has the most significant effect on phosphorus recovery, while the effect of alkalinity and temperature is smaller.

2.Ways to prevent phosphorus reversion

For the converter, the main measure to prevent the ladle from returning to phosphorus is to prevent slag, that is, to prevent the slag from entering the ladle.

The commonly used methods in production are:

1) Before tapping, add lime to thicken the final slag into the furnace, and at the same time, carry out slag tapping.

2) During the tapping process, put a small amount of lime powder into the ladle to thicken the slag in the ladle and maintain the alkalinity.

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