1. What is residual emulsion? What specific harm does it cause to the quality of galvanized products?
Emulsion is a liquid used for lubrication and cooling during cold rolling. After rolling, some remains on the surface of the strip steel. After the moisture evaporates, it forms an oil film composed of rolling oil and metal debris.
These residues pose a fatal threat to subsequent galvanizing processes, mainly causing three major problems:
**Causing coating peeling and incomplete coating:** If the emulsion and iron powder on the strip steel surface are not thoroughly cleaned before entering the zinc bath, some areas will not be wetted by the zinc liquid, eventually forming "bald spots" or resulting in poor adhesion of the entire galvanized layer, easily peeling off during subsequent processing.
**Causing surface rust spots:** If the steel coil still carries residual emulsion during transportation or storage, the electrolytes and moisture in it will react with the steel plate to form rust spots that are difficult to wipe off, directly affecting the appearance and use.
Contamination of zinc pots and generation of waste residue: When impurities such as iron powder in the emulsion are introduced into the zinc pot, they react with zinc to generate zinc slag that is insoluble in the zinc liquid. This waste residue not only contaminates the plating solution and increases production costs, but also adheres to the surface of the strip steel, causing particulate defects.
Second: How to prevent residues from the source by controlling the emulsion system itself?
"Prevention is better than cure" is the first step in controlling residues. The key is to ensure that the emulsion used in the rolling mill is "clean."
Maintaining precise physicochemical indicators: Establish a strict testing system to implement process control. For example, too low a concentration will lead to insufficient lubrication and increased friction, resulting in a large amount of iron powder contaminating the strip; too high a concentration will hinder post-rolling volatilization. The key is to control the iron powder content below 300 ppm and maintain core performance parameters such as conductivity and saponification value within their optimal operating range.
Preventing contaminant oil from entering the system: Hydraulic oil and bearing oil leaking into the emulsion must be promptly removed using skimming devices, and oil leaks should be reduced through system maintenance. Once contaminated, these contaminant oils will damage the stability and volatility of the emulsion, resulting in graphite-like carbon black stains remaining after annealing.
3. How to maximize the removal of emulsion adhering to the strip using physical methods?
Physical removal at the mill exit is one of the most direct and effective means of reducing residue, primarily achieved through a high-pressure air purging system.
Optimizing the purging system: The core is to enhance the air purging capacity at the mill exit, including increasing the number of purging nozzles, increasing compressed air pressure, scientifically setting the purging angle to form a complete air curtain barrier, and adding edge suction fans to remove excess emulsion from the edges. Through the combination of "blowing" and "suction," the liquid emulsion adhering to the steel strip surface can be removed to the greatest extent possible.
Utilizing post-rolling high temperatures: The high temperatures generated during rolling cause the residual emulsion to evaporate rapidly. Studies have shown that higher post-rolling temperatures are more conducive to reducing residual oil. Therefore, in production, the rolling speed and cooling water volume are precisely adjusted to ensure that the post-rolling strip temperature is sufficient to promote emulsion evaporation.
4. How do the coiling and annealing processes affect residues, and how should the process be adjusted?
If residues remain on the strip surface during coiling, these substances will be trapped inside the coil and undergo complex physicochemical changes during subsequent high-temperature annealing.
Key optimization of the coiling process: Poor strip shape (such as the presence of corrugations) can lead to excessively tight bonding between coil layers, hindering emulsion evaporation. In this case, the coiling tension can be appropriately reduced to increase the interlayer gap, providing a channel for the volatiles to escape.
Fine adjustment of the annealing process: During the critical stage of heating the annealing furnace to approximately 300°C to 500°C, a high-flow-rate hydrogen and nitrogen gas should be used for powerful purging to quickly remove the evaporating emulsion and oil vapors from the furnace, preventing them from decomposing at high temperatures and forming stubborn carbon black residues on the strip surface.
5. When residues are unavoidable, what are the key cleaning and remedial measures for subsequent inspection?
For any sophisticated production process, the final "cleaning stage" is crucial; it is the last line of defense to ensure product quality.
Establish a comprehensive cleaning process: A dedicated cleaning section is set up on the hot-dip galvanizing production line. Through a triple mechanism of chemical dissolution of oil stains by alkaline solution, electrochemical removal of extremely fine iron powder by an electric field, and physical scrubbing by brush rollers, the strip steel is ensured to reach a "mirror-like" clean state before entering the zinc bath.
Strengthen surface quality inspection: The finished galvanized sheets require rigorous surface quality inspection to verify the effectiveness of control measures. Any unacceptable dezincification, incomplete galvanizing, or spots are judged as defective products, thereby driving further optimization of emulsion management in the preceding rolling process.