The recarburizer addition time isn't a fixed point in time; it's determined dynamically based on multiple factors, including the smelting process, furnace type, and the state of the molten steel/iron. The core principle is to create optimal dissolution and diffusion conditions for the recarburizer while avoiding its negative effects.
The following are detailed control methods for different smelting equipment:
1. Induction Furnace (Medium Frequency Furnace) - Most Commonly Used, Most Flexible
This is the furnace type most commonly used by small and medium-sized foundries, and its addition timing is the most precisely controlled.
Principle: Add with the charge, supplemented by subsequent adjustments.
Optimal Timing:
Preferred: Add the majority of the recarburizer to the furnace bottom during charging or mix it with the metal charge (pig iron, scrap, etc.).
Reason: During the charge melting process, the recarburizer has ample time to dissolve and diffuse in the molten steel/iron. With the formation of the molten pool and the effects of electromagnetic stirring, the carbon is evenly distributed, maximizing absorption.
Second Choice: Add the recarburizer after the charge is mostly melted (approximately 60%-80%) and the temperature reaches approximately 1350°C-1400°C, after slagging.
Reason: At this point, the molten pool has already formed and is highly fluid. The added recarburizer is quickly drawn below the melt surface and rapidly dissolved by the high temperature and electromagnetic stirring. This method is suitable for situations where the charge carbon content is uncertain or when additional recarburization is required.
2. Electric Arc Furnace
The electric arc furnace smelting cycle is long and is generally divided into an oxidation phase and a reduction phase. The timing of recarburizer addition is closely related to the smelting objectives.
Add after the oxidation phase and at the beginning of the reduction phase:
Reason: The oxidation phase results in significant decarburization, resulting in a very low carbon content in the molten steel. Once the reduction phase begins, deoxidation is well established and the furnace atmosphere becomes reducing. Adding the recarburizer at this time will prevent oxidation and ensure a stable absorption rate. There is still ample time for stirring and homogenization after addition.
Add during tapping (in-ladle recarburization):
Reason: The high impact of the steel stream during tapping facilitates rapid dispersion and dissolution of the recarburizer in the ladle. This method is often used to precisely control the final composition. However, it requires a fine particle size recarburizer, and ideally, the ladle should have a stirring mechanism such as bottom argon blowing.
3. Cupola
Cupola furnaces operate continuously, making the addition of recarburizer relatively simple.
Optimal timing: Add it with the batch (metal charge and coke) through the charging port.
Reason: As the recarburizer descends, it comes into contact with the high-temperature furnace gases and coke, providing ample time and temperature for absorption during the iron droplet formation process.
Most practical advice:
For the most commonly used induction furnaces, adhere to the principle of "bottom-of-the-furnace primary, supplemented by process adjustments." Add the majority of the recarburizer (e.g., 80% of the planned total) to the bottom of the furnace during charging, leaving the remainder for purging and subsequent fine-tuning based on chemical composition analysis. This ensures high absorption while precisely controlling the final composition.
By strictly controlling the addition timing, combined with high-quality recarburizer and proper operation, you can significantly improve product quality and production efficiency.
