1.What is stress-relief annealing? What is the typical stress-relief annealing temperature for cold-rolled coils?
Stress-relief annealing is a heat treatment process that involves heating cold-rolled coils to a temperature below their recrystallization temperature, holding them at that temperature, and then slowly cooling them to eliminate the internal stresses generated during cold rolling without altering the primary grain morphology (i.e., preventing recrystallization).
The temperature range varies depending on the steel grade:
Ordinary low-carbon steel cold-rolled coils: Stress-relief annealing temperatures are typically between 550 and 650°C. If the purpose is softening (recrystallization annealing), the temperature will be higher, at 600–700°C (bell furnace) or 700–850°C (continuous annealing line).
Precision alloys/specific stainless steels: For certain materials requiring the retention of high strength and only stress relief (such as 4J42 alloy and 301 stainless steel semi-hard products), lower temperatures are typically between 250 and 400°C.
2.Are stress-relief annealing and recrystallization annealing the same thing? What's the difference in temperature?
Stress-relief annealing (low temperature): The goal is simply to release lattice distortion energy and reduce internal stress. After annealing, the grains remain fibrous (elongated) as in cold rolling, and the decrease in hardness is not significant. Suitable for products that need to maintain the cold-rolled hardening effect but also require a certain degree of toughness.
Recrystallization annealing (high temperature): The goal is to generate entirely new equiaxed grains through nucleation and growth, completely eliminating work hardening. After annealing, the material softens and its plasticity is significantly improved.
Temperature boundary: Usually, the recrystallization temperature of the material is used as the boundary. For example, for low-carbon steel, approximately 650℃ or higher is considered recrystallization annealing; while for 304 austenitic stainless steel, the solution annealing temperature is as high as 1000℃ or higher, and low-temperature stress relief (~400℃) hardly changes the hardness.
3.What factors influence the choice of stress-relief annealing temperature?
Material Composition (Steel Grade): This is the primary factor.
Low-carbon aluminum killed steel: Recrystallization temperature is relatively low; annealing at 600-700℃ is usually sufficient for complete softening.
Austenitic stainless steel (e.g., 304): For complete softening (solution treatment), heating to 1000-1050℃ is required. For stress relief (eliminating machining stress), temperatures are typically below 400℃ to avoid carbide precipitation or martensitic transformation.
Duplex steel (DP steel): Annealing temperature directly affects the martensite ratio, and is usually precisely controlled within the 750-820℃ range.
Cold Rolling Deformation: A larger deformation results in higher stored energy, and the recrystallization temperature will decrease slightly.
Final Performance Requirements: Whether a hard state (stress relief) or a soft state (recrystallization) is required determines the process curve.
4.What are the consequences of improper annealing temperature control?
Underheating (under-heating): Incomplete stress removal and excessive residual stress can lead to dimensional instability or deformation during subsequent stamping, and may also result in insufficient hardness.
Overheating (over-heating/over-aging):
For low-carbon steel: Excessive grain growth leads to low strength, causing "orange peel" defects on the surface during stamping, and may even result in adhesion failure.
For certain stainless steels (e.g., 301): Annealing at specific temperature ranges (e.g., 400℃) may actually cause martensite precipitation or decomposition, resulting in increased hardness and brittleness instead of decreased hardness.
For microalloyed steels containing Nb and Ti: Excessive temperature can cause coarsening of carbonitrides, resulting in loss of strengthening effect.
5.In actual production, how can we confirm that the set annealing temperature is correct?
Mechanical Property Testing: This is the most direct indicator. Test the hardness (HRB/HV), yield strength, and elongation after annealing. If the hardness is too high, the temperature is too low or the annealing time is insufficient; if the strength is too low, the temperature is too high.
Metallographic Observation: Observe under a microscope to confirm whether the expected microstructure has been achieved.
If it is only stress relief annealing, the elongated grains should still maintain the rolling direction.
If it is recrystallization annealing, complete newly formed equiaxed grains should be observed.
Sheet Shape and Residual Stress Testing: Cut the coil to measure the warpage, or use X-ray diffraction to measure the residual stress. A qualified stress-relief annealing should prevent the coil from deforming after slitting.