The surface quality of hot-dip galvanized steel sheets is a core indicator of their quality, and zinc dross and streaks are the two most common and significant types of defects. These defects not only damage the product’s appearance but also significantly weaken the corrosion resistance and adhesion of the coating. The root cause of most surface problems is not the substrate itself, but rather directly related to the operation and control of the zinc pot area. This article will delve into the causes of these defects and provide systematic solutions.

In-depth Analysis and Control of Zinc Slag Defects

Zinc dross is essentially formed by solid inclusions in the molten zinc adhering to or being pressed into the surface of the steel strip.

The main types and origins of zinc slag

Top scum

◦ Cause: Formed by the accumulation of zinc liquid surface oxides, equipment corrosion products, and external contaminants (such as refractory material fragments).

◦ Features: Typically consists of dark, loose, flaky or lumpy material.

◦ Key control points:

·Regular cleaning: Each production team must perform system slag removal at least twice per shift, using specialized tools to thoroughly remove the slag to the non-working side of the zinc pot.

·Furnace nose area management: It is essential to regularly (weekly is recommended) and during any downtime, clean the accumulated debris from the surface of the molten zinc in the furnace nose. This is crucial for preventing dross from being carried into the steel strip.

Bottom iron-zinc alloy slag

◦ Cause: The molten zinc reacts with the iron walls of the pot, submerged rollers, and other components, generating high-density intermetallic compounds such as FeZn13, which then deposit at the bottom of the pot.

◦ Characteristics: The particles are hard, have a metallic luster, and form stubborn protrusions when pressed into the steel strip.

◦ Key control points:

·Strictly adhere to the dross removal cycle: For traditional iron-made zinc pots, systematic dross removal should be performed every 7-10 days. For induction zinc pots, due to the absence of reactions with the pot walls, the cycle can be extended to 3-6 months, but this requires monitoring through regular zinc sample analysis.

·Isolation and emergency procedures: Process control must ensure that the bottom slag does not come into contact with the submerged rolls and stabilizing rolls. If slag is found adhering to the roll surface, the machine must be stopped immediately, and a copper scraper must be used to thoroughly remove it to prevent the continued generation of defects.

Suspended particulate matter

◦ Cause: This is caused by the bottom dross being stirred up by convection currents in the molten zinc, or by improper control of the aluminum content in the molten zinc.

◦ Key control points: Precisely control the aluminum content in the zinc bath (typically between 0.18% and 0.22%), and ensure sufficient settling time for the zinc bath after slag removal.

The causes of air knife streaks and their fine-tuning

Streak defects directly reflect abnormal operating conditions of the air knife system.

The three main core reasons for the formation of streaks

Process parameter mismatch

◦ Problem: The air knife height and blowing pressure are not matched to the strip speed and target coating weight. When the speed increases, if the pressure is not increased accordingly or the height is not adjusted, uneven streaks are very likely to occur during the wiping process.

◦ Solution: Establish and strictly enforce a “Air Knife Process Parameter Specification Table,” enabling coordinated adjustment of speed, pressure, and height, and recording the results of each adjustment.

Nozzle is clogged or damaged

◦ Problem: Zinc splashes or environmental dust clog some of the nozzles, resulting in uneven, streaky airflow distribution.

◦ Key maintenance: Implement a pre-shift inspection system and use specialized tools to clean the nozzles. The air knife should be disassembled monthly for thorough maintenance, and the inside of the nozzles should be checked for corrosion or wear.

Interference from zinc liquid level fluctuations

◦ Mechanism of influence: The zinc liquid level serves as the reference point for the air knife height. Fluctuations in the liquid level directly alter the actual distance between the air knife and the strip, leading to uneven coating thickness and streaks at the edges.

◦ Core control: Relies on a high-precision automatic zinc liquid level control system, using a stable, low-speed zinc addition strategy to strictly control liquid level fluctuations within ±3mm.

Stable zinc liquid level: The cornerstone of safety and quality

Maintaining a stable zinc level is not only crucial for surface quality, but also a vital aspect of safe production.

·Safety Red Line: The zinc liquid level must always be below the lower opening of the furnace nozzle (typically at a depth of 150-300 mm). If the liquid level is too low, exposing the furnace nozzle, the protective gas barrier will be compromised, allowing air to enter the furnace. This not only causes oxidation and incomplete plating of the steel strip but also poses a serious risk of explosion.

·Automatic control key points: A high-quality control system should include real-time liquid level monitoring, automatic feedback adjustment, and over-limit alarms. Zinc addition should be performed at a fixed location using a dedicated lifting device for smooth operation.

Establish a systematic defect prevention and control system

·Standard Operating Procedures (SOP): Standardize and document all critical operations (such as slag removal, dross removal, air knife adjustment, and zinc addition) to ensure consistency in operations.

·Key parameter monitoring and logging: Daily recording and analysis of zinc bath temperature (recommended 455±5℃), aluminum/iron content, air knife parameters, defect types and frequency, etc., to achieve data-driven management and trend forecasting.

·Preventive Maintenance (PM) Plan: Develop a detailed schedule for periodic maintenance, including weekly maintenance of air knives, monthly cleaning of the furnace nose, regular endoscopic inspections of the zinc pot, and quarterly calibration of sensors.

·Personnel capacity building: Regularly provide theoretical and practical training to operation and maintenance personnel to ensure they understand the principles rather than simply performing tasks mechanically, and to equip them with basic problem diagnosis and troubleshooting skills.

A perfect hot-dip galvanized surface is the result of precise control. By thoroughly understanding the mechanisms of dross and streaking formation, and systematically implementing strict process discipline, preventive maintenance, and data-driven management, manufacturers can not only effectively eliminate surface defects but also achieve stable improvements in product quality and continuously enhance their competitiveness.