1.How to quickly determine the type of color difference?
Longitudinal stripe color difference (along the rolling direction):
Appearance: One bright and one dark stripe, parallel to the rolling direction.
Main suspects: Uneven air knife process parameters, strip vibration, zinc liquid fluctuation.
Transverse color difference (perpendicular to the rolling direction):
Appearance: Alternating light and dark banded stripes, resembling "wood grain" or "water ripples".
Main suspects: Unstable annealing process (uneven furnace temperature, speed fluctuation), poor condition of submerged rolls/stabilizing rolls.
Flake/cloud-like color difference:
Appearance: Large areas of light and dark without a fixed shape.
Main suspects: Poor substrate surface cleanliness, uneven zinc liquid composition (especially aluminum content), uneven passivation solution spraying or drying.
2.What causes color differences?
Poor surface cleanliness: Residual iron powder, grease, oxides, etc., on the substrate surface cannot be completely reduced or removed after annealing, affecting wettability during zinc plating, leading to uneven zinc layer adhesion and color differences.
Uneven substrate surface roughness: Areas with different roughness exhibit varying diffuse reflection of light, resulting in inconsistent brightness.
Uneven substrate composition and mechanical properties: These affect recrystallization behavior and the degree of surface oxide reduction in the annealing furnace.
Uneven furnace temperature: Uneven transverse temperature within the annealing furnace leads to varying degrees of recrystallization and surface oxide reduction states in different parts of the strip, thus affecting its reactivity with the molten zinc.
Uneven furnace atmosphere: Improper control of dew point and hydrogen-nitrogen ratio can affect the reduction of surface oxides.
Production line speed fluctuations: Changes in speed cause variations in the heating and reduction time of the strip within the furnace.
3.How to adjust it?
Strengthen incoming material inspection: Ensure the surface cleanliness and roughness of the substrate meet requirements.
Optimize the cleaning process: Ensure the efficiency and effectiveness of alkaline washing, brushing, electrolytic cleaning, and rinsing to ensure the strip steel is absolutely clean before entering the annealing furnace.
Adjustment methods:
Optimize annealing profile: Set appropriate furnace temperatures for each section based on steel grade and specifications.
Check and calibrate burners: Ensure uniform transverse furnace temperature.
Stabilize furnace atmosphere: Strictly control the dew point and composition of the protective gas.
Maintain stable production line speed: Avoid unnecessary acceleration and deceleration.
4.What factors can cause color differences during the galvanizing process?
Uneven blade lip gap: This leads to uneven transverse air blowing pressure, uneven zinc layer thickness, and longitudinal stripe color difference. Adjustment: Stop the machine to check and adjust the blade lip gap, ensuring its parallelism and consistency.
Improper or unstable distance between the air knife and the strip: Changes in distance significantly alter the gas impact force. Adjustment: Optimize and stabilize this distance.
Air knife pressure fluctuations: Unstable pressure causes longitudinal variations in zinc layer thickness. Adjustment: Check the air knife pressure control system and air supply stability.
Air knife nozzle blockage: Partial blockage causes localized airflow changes, resulting in linear color differences. Adjustment: Regularly clean and maintain the air knife nozzle.
5.When color differences occur, how do you find the cause?
Observation and Recording: Record in detail the morphology (longitudinal/transverse/flaky), location, color, and batch and specification of the color difference.
Identify the Problem Area: Based on the color difference morphology, preliminarily determine the problematic环节 (link/step) (e.g., for longitudinal stripes, prioritize checking the air knife; for transverse stripes, prioritize checking the annealing process).
Process Parameter Review: Retrieve all key process parameter records from the production of this coil (annealing temperature, speed, air knife parameters, zinc bath temperature, etc.) and compare them with the parameters from normal product production to identify abnormal fluctuations.
Equipment Status Inspection:
Online Inspection: Check air knife distance, strip vibration, passivation coating, etc.
Offline Inspection: Schedule a shutdown to inspect the air knife lip, submerged roll, stabilizing roll, nozzles, etc.
Sample Analysis: If possible, perform scanning electron microscopy (SEM/EDS) analysis on the color difference sample to observe the zinc layer morphology and composition, providing a scientific basis for determining the cause.