The T.O. content in steel reflects the overall level of inclusions. Therefore, lower T.O. will extend the fatigue life of gears. High-quality gears generally require an oxygen content below 20 ppm. Controlling and reducing high-melting-point, high-hardness oxides like alumina in steel to improve molten steel purity is a key task in production.
End point carbon control of molten steel
Deoxidation of gear steel requires precise control of the carbon content at the time of tapping from the electric furnace to stabilize the initial oxygen content.
Based on the carbon-oxygen product, the carbon content of the steel at tapping determines the dissolved oxygen content at tapping. Appropriately increasing the carbon content of the steel at tapping from the electric furnace can reduce the oxygen content at tapping, thereby alleviating the burden of subsequent refining. The low carbon content required for gear steel increases the difficulty of deoxidation, often leading to overoxidation. This increases the burden of refining and deoxidation, and also increases iron loss and deoxidizer consumption.
Therefore, appropriately increasing the final carbon content can reduce overoxidation of the molten steel, facilitate degassing and inclusion removal, and effectively reduce the gas content in the steel.
Control of aluminum content in steel
Al is a strong deoxidizing element. Increasing the amount of acid-soluble aluminum in steel can effectively reduce the presence of dissolved oxygen.
During tapping from an electric furnace, aluminum should be added as much as possible at once, depending on the oxygen content of the steel. This allows the dissolved oxygen in the molten steel to react with the aluminum to form large Al2O3 clusters. These clusters grow, float, and are removed from the molten steel before reaching the refining stage.
In the refining stage, deep deoxidation of the molten steel can be performed using aluminum wire feeding to further reduce the oxygen content. After aluminum deoxidation, the Al2O3 generated by deoxidation must be removed from the molten steel. This can be achieved through intelligent argon purging during the refining process. The argon flow rate must be properly controlled to avoid exposing the molten steel. Literature suggests that the oxygen content after refining is largely affected by the oxygen content of the tapped steel. Adding alloys or aluminum blocks to reduce oxygen content during tapping is a widely accepted method. Some products of aluminum deoxidation will take on a needle-like shape and gradually grow, but 75%-85% of these inclusions will float and be removed within 4 minutes.
High basicity refined slag series
Refining slag systems not only need to meet deoxidation requirements but also perform desulfurization, submerged arc slag formation, and inclusion adsorption.
When oxygen content is low, appropriately increasing slag basicity not only reduces the equilibrium oxygen content in steel but also increases the sulfur distribution ratio between the slag and steel. Research suggests that refining slag basicity has a significant impact on T[O]. When the CaO/SiO2 ratio in the slag is controlled within the range of 3.0 to 8.5, the T[O] content in the molten steel gradually decreases as the slag basicity increases. However, if the basicity exceeds 8.5, its effect on T[O] in the molten steel becomes less significant.
Literature research suggests that for a slag system with good deoxidation capabilities, the CaO/SiO2 ratio in the slag should be greater than 1. The higher the ratio, the faster the deoxidation rate. However, the refining slag basicity should not be too high. If the basicity is 5.0, the melting point of the refining slag increases, slagging slows, and the slag fluidity decreases, which also affects the deoxidation and desulfurization effects. Therefore, it is not recommended to use refined slag with too high basicity.
Inclusion modification treatment
In gear steel production, Al2O3 modification is often used after Al deoxidation. This converts the high-melting-point Al2O3 into low-melting-point complex inclusions. A rational mixing process is then used to promote the floating and removal of these inclusions.
After deep deoxidation in the refining furnace, the number of fine aluminate inclusions increases, reducing the castability of the molten steel and making nozzle nodules more likely to form during the pouring process. Therefore, before tapping, an appropriate amount of calcium silicon wire should be added based on the acid-soluble aluminum content of the steel for modification. This treatment forms low-melting-point calcium aluminates, improves the flowability of the molten steel, and prevents nozzle nodules.
VD vacuum refining
The oxygen content in steel decreases with the decrease of vacuum degree. Prolonging the vacuum time is beneficial to the discharge of molten steel and total gas, and promotes the aggregation, growth, floating and removal of inclusions.
Fully enclosed non-oxidation protection casting.
Oxygen added during the continuous casting process can come from air, refractory materials, or mold slag. Using a non-oxidizing protective casting method throughout the casting process effectively prevents secondary oxidation of the molten steel. Adding a special tundish covering agent isolates the molten steel from air, reduces secondary oxidation, and prevents the formation of Al2O3 inclusions.
