1.What effect does the chemical composition of steel have on the annealing temperature?
Low-carbon steel (e.g., SPCC, DC01, SPCD): The main purpose is to achieve recrystallization and soften the microstructure. Temperatures are typically between 680°C and 750°C.
Ultra-low carbon/interstitial atom-free steel (e.g., DC04, DC05, SPCE, SPCEN): To obtain excellent deep-drawing properties, higher temperatures are required to allow sufficient grain growth and promote favorable texture formation. Temperatures are typically between 750°C and 850°C.
Medium-carbon steel or high-strength low-alloy steel: Temperatures are even higher, potentially reaching 750°C to 900°C, to ensure the dissolution and homogenization of carbides.
2.What are the final performance requirements for the product?
Standard hardness (e.g., SPCC-SD): Use the lower-middle temperature limit to ensure recrystallization.
High deep-drawing performance (e.g., DC04): Use the upper-middle temperature limit to promote grain growth and {111} texture development.
Balance between high strength and high elongation (e.g., duplex steel): Annealing temperature will approach or exceed the Ac1 point, performing two-phase annealing, making the process more complex.
3.How does strip thickness affect annealing temperature?
Thinner strips heat up quickly, requiring shorter heat treatment times and more precise temperature control.
Thicker strips may require slightly higher temperatures or longer holding times to ensure sufficient recrystallization in the core.
4.How do the type of annealing furnace and the process affect the annealing temperature?
Continuous annealing furnace: Strip steel passes through rapidly (from tens of seconds to several minutes), and the temperature is usually set in the upper limit range (e.g., 780°C ~ 850°C), which is a short-time high-temperature process.
Bell-type annealing furnace: Steel coils are stacked and annealed for a long time (tens of hours), and the temperature is usually set in the lower limit range (e.g., 680°C ~ 720°C), which is a long-time low-temperature process. The final properties achieved are slightly different from those of continuous annealing.
5.What is the meaning of "precision" control?
Heating rate: Controls the initial grain state.
Soaking (holding) temperature: The core temperature range as described above, ensuring uniform temperature across the strip's transverse and longitudinal directions.
Holding time: Ensures sufficient recrystallization and grain growth.
Cooling rate: Another key factor in controlling final properties. For example, by controlling the slow and rapid cooling sections, the solution carbon and nitrogen content can be adjusted, affecting aging time and yield strength.
Furnace atmosphere: Typically, a nitrogen-hydrogen mixed protective gas (e.g., 5%~25% H₂, the remainder N₂) is used to prevent strip oxidation and promote the reduction of surface oxides, resulting in a bright surface.