1.What is a suitable range for the drying temperature of cold-rolled coils?
The drying section of a cold-rolled steel cleaning line typically uses hot air drying or electromagnetic induction heating. The temperature setting depends on the strip specifications and production line speed.
Generally, the drying temperature (hot air temperature) is recommended to be controlled between 80℃ and 120℃, and the strip temperature (actual strip temperature) when exiting the drying oven should reach 60℃ to 90℃.
2.What quality problems can arise from improper drying temperature settings?
Low Temperature (Insufficient Drying):
Residual Moisture: Incomplete drying of the strip surface leaves moisture residue after coiling, leading to water stains or yellow rust.
Annealing Defects: Moisture vaporizes upon entering the annealing furnace, potentially causing atmosphere fluctuations and even the risk of oxidation discoloration or hydrogen embrittlement.
Decreased Coating Adhesion: For products requiring subsequent oiling or plating, residual moisture weakens coating adhesion.
High Temperature (Over-drying):
Oxidation Discoloration: High temperatures cause the strip surface to come into contact with oxygen, forming a bluish-brown oxide film, affecting appearance and subsequent processing performance.
Changes in Mechanical Properties: For thin-gauge or high-strength steels, excessively high temperatures may cause age hardening or abnormal fluctuations in yield strength.
Oil Coating Failure: If rust-preventive oil is applied immediately after drying, the high strip temperature can cause the oil film to evaporate or oxidize, reducing its rust-preventive effect.
Energy Waste: Overheating increases energy consumption and accelerates equipment aging.
3.What are the main factors that affect the setting of drying temperature?
Strip thickness and heat capacity: Thicker strips have higher heat capacity and slower heating, requiring higher hot air temperatures or longer drying times; thinner strips heat up quickly, but excessively high temperatures can cause overheating.
Production line speed: Higher speeds mean shorter strip time in the drying section, necessitating higher temperatures or longer drying sections to ensure sufficient heat input.
Inlet strip moisture content: The residual water content on the strip surface after rinsing is affected by the rinsing method and the condition of the squeeze rollers. If the squeeze rollers are worn or under insufficient pressure, the strip will have a high water content, requiring a higher drying temperature.
Subsequent process requirements:
If the subsequent process is annealing, drying only needs to ensure no water is present, and the temperature should be relatively low.
If the subsequent process is oiling, the strip exiting the drying oven temperature must be controlled ≤50℃ to prevent oil evaporation.
If the subsequent process is galvanizing, the drying temperature must be matched with the pre-galvanizing heating section.
Drying equipment type: Hot air drying has relatively low efficiency and high temperature requirements; electromagnetic induction drying has high efficiency and rapid heating, allowing for the setting of lower hot air auxiliary temperatures.
4.Are there different requirements for drying temperature for different product types?
Ordinary cold-rolled coil (CQ): The primary goal is to ensure complete drying, with some fluctuation allowed, and cost is the priority.
Automotive outer panels (O5): Strictly control the upper limit to prevent oxidation color and surface quality damage. Some production lines adopt a low temperature + extended drying time strategy.
Silicon steel (electrical steel): The insulating coating on the surface of silicon steel is sensitive to moisture and needs to be thoroughly dried. At the same time, high temperature should be avoided to prevent damage to the coating performance. Electromagnetic induction drying is often used to precisely control the temperature.
Galvanized substrate: It needs to be connected with the temperature of the pre-plating heating furnace to avoid sudden temperature changes that cause fluctuations in the shape of the strip and to prevent a decrease in the adhesion of the zinc layer.
5.How can drying temperature be effectively monitored and managed during production?
Key Monitoring Points:
**Strip Temperature Meter:** An infrared thermometer is installed at the dryer oven outlet to monitor the actual strip temperature in real time. This is the most direct feedback parameter.
**Hot Air Temperature:** Monitor the hot air temperature in each zone of the dryer oven to ensure the heating system is functioning properly.
**Speed Linkage:** PLC-linked control of drying temperature and production line speed-automatically adjusting hot air temperature or fan frequency when speed changes to maintain stable heat input.
**Daily Inspections:**
Check the condition of the squeeze rollers every shift to ensure minimal water content in the incoming strip.
Check for blockages in the dryer oven nozzles to ensure even hot air coverage across the strip width.
Observe the strip surface after exiting the dryer for residual steam or oxidation and adjust accordingly.
**Abnormal Handling:**
**Low Strip Temperature:** Check the hot air temperature setting, fan speed, production line speed for excessive speed, and whether the squeeze rollers are malfunctioning.
**High Strip Temperature:** Check for temperature sensor drift, heating system malfunction, and abnormal speed reduction.