1.What are the characteristics of continuous annealing furnace process and cooling method?
Process Characteristics: The strip steel continuously passes through heating, soaking, and cooling sections. Cooling is completed rapidly in a specific area within the furnace (cooling section).
Cooling Time: Extremely short, measured in seconds or minutes. The cooling rate (e.g., temperature drop per second) is the core control parameter.
Cooling Methods:
Gas Spray Cooling: Forced convection cooling using high-speed jets of cold hydrogen or a nitrogen-hydrogen mixture; the rate is controllable.
Water Quenching: For steels requiring extremely high strength (e.g., some high-strength steels), rapid cooling is achieved using a water quenching tank.
Over-Aging Treatment: For low-carbon aluminum-killed steel (deep-drawing steel), after rapid cooling to a certain temperature, it enters an "over-aging section" for heat preservation to control carbide precipitation and eliminate aging brittleness.
Key Points: The time is automatically matched by the production line speed and tension. The operator sets the target cooling rate and final cooling temperature.
2.What are the process characteristics and cooling methods of bell-type annealing furnaces?
Process Characteristics: After the steel coils are stacked, they undergo prolonged batch annealing and cooling within a sealed furnace.
Cooling Time: This is very long and a bottleneck in the production cycle, typically accounting for 40%-60% of the total cycle (heating + holding + cooling). For large coils (e.g., over 20 tons), cooling may take several days.
Cooling Stages:
Slow Cooling in the Furnace: After heating is stopped, the coil is naturally cooled in a protective atmosphere (H₂-N₂) or controlled to a safe temperature (e.g., ≤150℃). This is the most critical stage; excessively rapid cooling can lead to:
Uneven Performance: Large temperature differences between the inside and outside of the coil, resulting in uneven microstructure and hardness.
Insufficient Recrystallization: Affects deep-drawing performance.
Hydrogen Embrittlement Risk (for annealing in a high-hydrogen atmosphere).
Air Cooling After Tapping: After reaching the safe tapping temperature, the coil is lifted from the furnace and allowed to continue cooling to room temperature in air.
3.What are the specific variables that affect cooldown time/rate?
Steel Grades:
Low-carbon mild steel (e.g., SPCC, DC01): Relatively insensitive to cooling rate, but requires control of the "slow-cooling brittleness zone" to prevent cementite precipitation at grain boundaries from affecting stamping properties.
Deep-drawing steel/IF steel (e.g., DC04, DC06, SUS430): Requires strict control of the cooling rate, especially the rate at which it passes through the 400℃~300℃ temperature range, to optimize the {111} texture and ensure a high r-value.
High-strength steel (e.g., DP, TRIP): The cooling rate is crucial for obtaining martensitic, bainitic, and other phase transformation structures, requiring precise continuous annealing process control.
Coil Dimensions:
Coil weight/diameter: This is the most significant factor affecting the cooling time in a bell-type furnace. Larger coils and higher stacks result in slower heat dissipation from the core, leading to a geometric increase in cooling time.
Strip thickness: Thicker plates have a larger heat capacity than thinner plates, resulting in slower cooling.
4.How do I determine when "cooling is complete"?
For continuous annealing: Cooling is considered complete when the strip temperature exiting the cooling section reaches the process set value (e.g., below 40℃).
For bell-type annealing: The actual measured temperature prevails.
Thermocouples are inserted into the core of the steel coil to monitor the real-time temperature.
The process specification will stipulate the exit temperature (e.g., ≤120℃). The bell can only be removed after the core temperature has dropped below the exit temperature and stabilized for a period of time.
The total cooling time can be estimated based on historical data. For example, for a certain specification of coil, cooling from 650℃ to 100℃ may take 30 hours. However, the temperature standard, not the time standard, must be strictly followed.
5.How long does it need to cool down?
This depends on your annealing process and product specifications. For continuous annealing, cooling is controlled within seconds on the production line; for bell-type annealing, cooling to a safe tapping temperature typically takes tens of hours, the exact time of which must be determined based on the core temperature of the steel coil displayed on the furnace gauges, and strictly adhere to the process specifications for that steel grade.