Let's take a look at the specific annealing process.
Annealing is a key process in metalworking that uses heating, holding, and cooling to improve material structure and properties (e.g., stress relief, softening, and grain refinement). Annealing processes vary depending on the material (e.g., steel, aluminum alloy) and application (e.g., softening and stress relief after cold rolling). The following uses recrystallization annealing of cold-rolled steel sheets (most notably, SGCC substrate processing) as an example to provide a detailed description of the specific process:
1. Pre-annealing Preparation
Raw Material Inspection: Confirm the material (e.g., low-carbon steel), thickness, and surface condition (free of oil, rust, etc.) of the cold-rolled steel sheets to avoid defects during heating. The materials are then processed in batches according to the production schedule.
Furnace Loading: Load the cold-rolled steel sheets (usually coils or stacked flat sheets) into a continuous annealing furnace or a box annealing furnace. Continuous annealing furnaces are suitable for large-scale production, with the steel sheets continuously moving on a roller conveyor. Box annealing furnaces are suitable for small batches, with workpieces positioned statically, and require temperature uniformity within the furnace. Furnace Atmosphere Control:
For easily oxidized materials such as mild steel, a protective atmosphere (such as a nitrogen + hydrogen mixture, with a hydrogen content of 5%-20%) is required to prevent surface oxidation of the steel plate (preventing scale formation that could affect subsequent galvanizing or processing) and to reduce any trace oxide film (FeO to Fe) that may exist on the surface.
The dew point of the protective atmosphere must be strictly controlled (usually ≤ -40°C) to prevent moisture from entering and causing oxidation.
2. Heating Phase
Heating Rate Control: The heating rate should be set based on the material thickness and furnace type (generally 5-20°C/min). Avoid excessive heating, which could lead to a large temperature difference between the inside and outside of the workpiece and cause thermal stress or deformation (especially for thick plates).
Heating Temperature: Reach the recrystallization temperature (approximately 650-720°C for mild steel, adjusted based on carbon content). The recrystallization temperature is crucial for softening cold-rolled steel sheets. During cold rolling, plastic deformation creates numerous dislocations (work hardening). When heated above the recrystallization temperature, these dislocations disappear, new equiaxed grains form, and the material regains its plasticity, reducing its hardness.
Temperature Uniformity: Multiple heating devices within the furnace (such as radiant tubes and resistance wires) ensure consistent temperatures across the steel sheet, with deviations controlled within ±5°C to avoid localized performance variations.
III. Holding Phase
The holding time is set based on the steel sheet thickness and target performance, generally ranging from 1 to 4 hours (the thicker the sheet, the longer the holding time). The goal is to allow for a complete transformation of the internal microstructure of the steel sheet:
The "fibrous grains" after cold rolling are completely transformed into "equiaxed recrystallized grains," eliminating work hardening;
Allowing for uniform diffusion of elements such as carbon and nitrogen across grain boundaries, reducing internal stress.
Atmosphere Maintenance: A protective atmosphere is continuously introduced during the holding phase to maintain positive furnace pressure (preventing air infiltration) and ensure a surface free of oxidation. IV. Cooling Stage
Cooling Method: The cooling rate is selected based on the annealing purpose. Recrystallization annealing typically uses slow cooling (furnace cooling or controlled cooling) to avoid rapid cooling that could cause new stresses or structural defects (such as martensitic transformation).
In continuous annealing furnaces, the steel plate is gradually cooled through water or air cooling sections at a cooling rate of approximately 5-10°C/min.
Box-type annealing furnaces can be heated with the heating device turned off, allowing the workpiece to cool naturally in the furnace to below 200°C before removal.
Final Cooling Temperature: The workpiece must be cooled to below 200°C (usually ≤150°C) before removal from the furnace to prevent oxidation caused by high temperatures upon exposure to air.
V. Post-Annealing Treatment
Performance Testing: Samples of steel plates are tested for hardness (e.g., Vickers hardness HV) and elongation (e.g., δ5) to confirm whether softening requirements are met (e.g., the hardness of cold-rolled low-carbon steel is approximately 150 HV, which is reduced to 80-100 HV after annealing, while the elongation increases from 20% to over 30%). Surface Inspection: Observe the surface for defects such as oxidation discoloration and pitting to ensure the effectiveness of the protective atmosphere.
Subsequent Processing: Annealed steel sheets can proceed directly to the next process (such as galvanizing or stamping). At this stage, the material exhibits excellent plasticity and low stress, making it less susceptible to cracking during processing.
Key Points: Matching Temperature and Time: Insufficient temperature or too short a holding time prevents recrystallization, resulting in a hard and brittle material. Excessive temperature or prolonged holding time results in coarse grains and reduced performance.
Atmosphere Control: The purity and dew point of the protective atmosphere are crucial for preventing oxidation, especially for steel sheets that will be subsequently galvanized (such as SGCC substrates). Surface oxidation can lead to poor zinc adhesion.
The processes for different annealing types (such as stress relief annealing and spheroidizing annealing) vary slightly, but the core principle revolves around controlling parameters in the three stages of "heating - holding - cooling" to achieve target performance.