1.How is the purpose and importance of cleaning demonstrated?
Ensuring annealing quality: If residual grease remains on the surface, the oil film will carbonize and decompose in the high-temperature environment of the annealing furnace (~700°C), forming difficult-to-remove carbon spots or oil stains, damaging the surface smoothness of the strip steel, and in severe cases, causing the steel coils to stick together.
Ensuring coating/plating adhesion: Whether hot-dip galvanizing, electro-galvanizing, or color coating, a clean surface is a prerequisite for obtaining good coating adhesion, uniformity, and corrosion resistance. Oil stains can cause coating peeling and defects such as pinholes.
Enhancing product quality: For automotive and appliance steel sheets with high-level surface requirements, surface cleanliness is a core indicator.
2.What are the mainstream cleaning processes?
Pre-cleaning Section:
Physical Brushing: The strip steel first passes through one or more sets of nylon or steel wire brush rollers, mechanically removing most of the loose rolling oil and iron powder with the aid of a spray solution.
Hot Alkaline Spray: A powerful spray is performed using a high-temperature (60-80°C) and moderately concentrated alkaline cleaning solution to initially remove oil stains.
Main Cleaning Section (Core):
Electrolytic Cleaning: This is the key step in deep cleaning. The strip steel, acting as the cathode (or alternating anode/cathode), is immersed in a strongly alkaline electrolyte. When direct current is applied, a vigorous electrolytic reaction occurs on the strip steel surface, generating a large number of hydrogen (or oxygen) microbubbles.
Mechanical Peeling: These microbubbles, like "miniature bombs," peel the oil film off the metal surface.
Saponification and Emulsification Enhancement: Under the combined action of current and high temperature (70-90°C), the saponification reaction and emulsification of the alkaline solution and oil are greatly enhanced.
Continuous spraying and soaking: In conjunction with the electrolytic cell, there is continuous alkaline spraying and soaking.
Post-cleaning and rinsing section:
Hot water rinsing: After alkaline washing, alkaline silicates and other substances adhere to the surface of the strip. It requires multi-stage (2-4 stages) countercurrent rinsing with high-temperature hot water (usually deionized water) to ensure complete removal of residual alkaline solution and impurities.
3.What are the key components of a cleaning system?
Cleaning Solution:
Composition: Primarily sodium hydroxide, compounded with surfactants, complexing agents, defoamers, etc.
Management: Requires online monitoring and automatic control of concentration, temperature, and conductivity, and regular filtration and replacement.
Brushing System:
Brush Rollers: Material and bristle hardness are selected based on the surface condition of the strip steel.
Squeeze Rollers: Rubber squeeze rollers are used after each cleaning/rinsing stage to remove excess liquid and prevent cross-contamination.
Electrolysis System:
Electrodes: Typically made of stainless steel.
Power Supply: Provides stable and adjustable DC power.
Filtration and Circulation System:
Magnetic filtration + bag/cartridge filtration: Used for continuous removal of iron powder and suspended impurities from the cleaning solution, and is the core component for maintaining stable cleaning results.
Wastewater Treatment System: Modern production lines are equipped with wastewater treatment systems to achieve alkali recovery and compliant wastewater discharge.
4.What are the key indicators for evaluating cleaning effectiveness?
Surface residual oil content: Unit is mg/m² (common methods: hydrophilic method, ultraviolet spectrophotometry). High-grade plates require less than 5 mg/m², or even lower.
Surface residual iron powder content.
Water film continuity test: Observe the uniformity and continuity of the water film on the strip surface after rinsing to determine cleanliness.
Surface quality after actual annealing/coating: This is the final inspection.
5.What are the trends in technological development?
High Efficiency and Environmental Protection: Developing more efficient, environmentally friendly cleaning agents that can operate at lower temperatures, reducing energy consumption and wastewater treatment burden.
Nitric Acid-Free Pickling: A mild acid pickling process (acid-cleaning combined unit) is sometimes used after the cleaning stage. Traditionally, nitric acid was used, but the use of environmentally friendly acids (such as sulfuric acid and organic acids) is now being promoted as alternatives.
Intelligent Control: Intelligent dynamic optimization of cleaning process parameters based on sensors and models.
Dry/Semi-Dry Cleaning Technology Exploration: The use of laser or dry ice cleaning, for example, is currently under research or applied in specialized fields, aiming to reduce water and chemical usage.