Control Measures for Reducing Oxygen Content in Ladle Refining Furnaces

Control Measures for Reducing Oxygen Content in Ladle Refining Furnaces

1. Selecting a Suitable Refining Slag System

Refining slag not only needs to possess functions such as deoxidation, desulfurization, and inclusion adsorption, but also needs to effectively promote slag formation. To reduce the oxygen content in steel, a high-basicity slag system should be prioritized. Increasing the basicity of the slag helps reduce the equilibrium oxygen content in the steel and simultaneously increases the sulfur distribution ratio in the slag-steel mixture, thus benefiting deoxidation and desulfurization. However, the basicity of the slag should not be too high. If the basicity exceeds 5.0, the melting point of the slag will increase, leading to a slower slag formation process, thereby affecting the deoxidation and desulfurization effects. Therefore, in practice, it is recommended to control the basicity of the refining slag between 2.5 and 3.0, which maintains good deoxidation and desulfurization capabilities while effectively adsorbing inclusions and ensuring slag formation.

2. Control of Argon Blowing in the Ladle

Argon agitation is an important means of reducing the oxygen content in steel, mainly achieved by removing dissolved oxygen and inclusions in the molten steel.

Argon's Role in Removing Dissolved Oxygen

During the smelting process, argon gas is blown in through the permeable bricks at the bottom of the ladle. The bubbles act as small vacuum chambers in the molten steel. As the bubbles rise, they agitate the molten steel, causing dissolved oxygen to diffuse into the argon bubbles, thus removing oxygen.

Argon's Role in Removing Inclusions

During argon blowing, solid inclusions in the molten steel are primarily removed through the flotation effect of the bubbles. Inclusions collide with and adhere to the bubble surface; as the bubbles rise, the inclusions are carried away. Studies show that when the contact angle between solid particles and bubbles is greater than 90°, almost all particles reaching the bubble surface adhere to the bubble. When the contact angle is less than 90°, the adhesion rate decreases sharply. Because Al₂O₃ and SiO₂ have relatively large contact angles with the molten steel, they easily adhere to the bubble surface, achieving the effect of removing inclusions. Therefore, the removal efficiency of inclusions mainly depends on their collision probability with bubbles, and larger inclusion particles have a significantly higher collision probability with bubbles than smaller particles.

The Number and Size of Argon Bubbles

The number and size of bubbles during argon blowing directly affect the inclusion removal effect. The more numerous and smaller the bubbles, the better the inclusion removal effect. A higher argon flow rate results in larger bubbles leaving the blowing element; therefore, using low-intensity argon blowing (i.e., soft argon blowing) and appropriately extending the blowing time is beneficial for inclusion removal. Furthermore, using multiple blowing elements simultaneously can inject more small bubbles into the molten steel within a limited purification time, thereby improving removal efficiency.

The Protective Effect of an Inert Atmosphere

During ladle smelting, argon gas covers the surface of the molten steel, forming a protective atmosphere. This inert atmosphere helps prevent secondary oxidation of the molten steel, thereby reducing the oxygen content in the molten steel.

3. The Influence of Refractory Material Selection on Oxygen Content

The oxygen content in steel comes not only from the molten steel itself but also from the corrosion of refractory materials. Therefore, when selecting refractory materials, it is essential to ensure their excellent resistance to rapid heating and cooling. Since the ladle refining process is intermittent and involves significant temperature fluctuations, the furnace lining is subjected to strong erosion during the entire argon blowing process. Therefore, the high-temperature strength and erosion resistance of the refractory materials are crucial, effectively reducing oxygen introduction and preventing an increase in oxygen content.

4. Control of AlS Content in Steel

The AlS content in steel has a significant impact on the oxygen content. Studies show that when the AlS content is controlled at 0.015%, the change in oxygen content in the steel is small; while when the AlS content increases to 0.030%, the oxygen content essentially stops changing. However, excessively high aluminum content (above 0.030%) causes aluminum to combine with oxygen in the slag, thereby reducing compounds such as SiO₂ and MnO in the slag and producing more Al₂O₃ inclusions. Furthermore, excessive aluminum can cause secondary oxidation during the steel pouring process, forming Al₂O₃ inclusions that remain in the finished product. Therefore, it is recommended to control the residual aluminum content in the steel between 0.015% and 0.030% to ensure the quality of the molten steel.

Summary

Reducing the oxygen content in the ladle refining furnace is a multifaceted process involving the selection of a suitable refining slag system, optimization of argon blowing operations, the influence of refractory material selection, and precise control of the AlS content in the steel. By comprehensively considering these factors, the oxygen content in steel can be effectively controlled, the quality of molten steel can be improved, and the stability and efficiency of the smelting process can be ensured.