304 stainless steel strip is one of the most widely used metal materials, popular in automotive parts, home appliances, and hardware—thanks to its corrosion resistance and versatility. But after cold rolling, 304 strip becomes hard and brittle, with low elongation (usually 20-30%), making it hard to stamp into complex shapes. Continuous annealing is the key to fixing this: by heating the strip to 800-900℃ in a controlled process, we can soften the metal, refine its structure, and boost elongation to ≥40%—meeting the strict stamping requirements of most industries. This article breaks down the continuous annealing process improvement for 304 strip, using simple language, real factory cases, and actionable tips—no overly technical jargon.
Why 304 Stainless Steel Strip Needs Continuous Annealing
Cold rolling makes 304 strip stronger but less workable—here’s why annealing is non-negotiable for stamping:
Cold rolling side effects: After cold rolling, 304 strip’s internal grain structure becomes distorted, creating internal stress. This makes it hard, brittle, and prone to cracking during stamping.
Stamping performance requirements: Most stamped parts (e.g., automotive trim, washing machine drums) need elongation ≥40%. This means the strip can stretch and bend without breaking during complex forming.
Continuous annealing advantage: Unlike batch annealing, continuous annealing processes the strip in a single pass—fast, efficient, and consistent, perfect for mass production of 304 strip.
Key Focus: 800-900℃ Annealing Temperature
The annealing temperature is the most critical factor—800-900℃ is the “sweet spot” for 304 strip. Here’s why this range works, and what happens if you go outside it:
1. Why 800-900℃ Is Optimal
At 800-900℃, the distorted grain structure from cold rolling is restored to a uniform, fine state.
Impact on elongation: This grain recovery and recrystallization boost elongation from 20-30% to ≥40%, making the strip flexible enough for stamping.
Test data: A 0.5mm thick cold-rolled 304 strip has 25% elongation. After annealing at 850℃, elongation reaches 43%—well above the 40% requirement.
2. Risks of Temperature Below 800℃
If annealing temperature is too low (＜800℃), grain recovery is incomplete—internal stress remains.
Problems: Elongation stays below 35%, and the strip cracks or tears during stamping. We’ve seen factories waste 20% of strips by annealing at 750℃.
3. Risks of Temperature Above 900℃
If temperature is too high (＞900℃), grains grow too large, making the strip soft but weak.
Problems: Elongation may reach 45%, but the strip bends or deforms easily during stamping—parts lose dimensional accuracy. It also increases energy costs and causes surface oxidation.
Continuous Annealing Process Improvement Tips
To hit 800-900℃ perfectly and ensure elongation ≥40%, follow these practical tips (used by real stainless steel factories):
1. Temperature Control
Use a digital temperature controller (accuracy ±5℃) to monitor the annealing furnace in real time.
Tip: Keep the temperature stable at 820-880℃—this range avoids the risks of too low/too high and ensures consistency.
2. Annealing Time & Speed
Continuous annealing speed: 5-8m/min (adjust based on strip thickness). Thicker strips (1.0mm+) need slower speed (5m/min) for full grain recovery.
Hold time: The strip should stay in the 800-900℃ zone for 3-5 minutes—enough for recrystallization but not too long to cause grain growth.
3. Cooling Process
After annealing, cool the strip slowly (air cooling or water cooling at 50℃/min) to prevent new internal stress.
Tip: Avoid rapid cooling—this can reduce elongation by 5-8% and make the strip brittle again.
4. Surface Protection
Use a protective atmosphere (nitrogen or argon) in the furnace to prevent surface oxidation.
Why it matters: Oxidized surfaces need extra polishing, which increases cost and can reduce stamping performance.
Real Industrial Cases
Case 1: Home appliance factory improvement. A Chinese factory made 304 strip for washing machine drums, but stamping scrap rate was 18% (elongation only 32%). They adjusted annealing temperature to 850℃, slowed speed to 6m/min, and added nitrogen protection. Elongation jumped to 42%, scrap rate dropped to 3%.
Case 2: Automotive parts upgrade. An American automaker needed 304 strip with ≥40% elongation for car trim. Their old process (annealing at 780℃) only hit 34% elongation. After raising temperature to 860℃ and optimizing cooling, elongation reached 44%—no more stamping cracks, and parts passed quality inspections.
Case 3: Cost-saving success. A European factory was annealing 304 strip at 920℃, wasting energy and getting inconsistent elongation. They lowered temperature to 840℃, kept speed at 7m/min, and cut energy costs by 15%. Elongation stayed at 41%, meeting all stamping requirements.
How to Verify Stamping Performance (Elongation ≥40%)
After annealing, test the strip to make sure it meets the 40% elongation requirement—simple and straightforward:
Test method: Use a tensile testing machine to pull a small strip sample until it breaks. Measure the length before and after breaking to calculate elongation.
Frequency: Test every 10 rolls of strip—catch any issues early before mass stamping.
Tip: If elongation is 38-39%, adjust the furnace temperature up by 10-15℃—this usually boosts elongation by 2-3%.
Common Mistakes to Avoid
Mistake 1: Ignoring temperature fluctuations. Even a 10℃ drop below 800℃ can reduce elongation by 5%—always monitor the furnace.
Mistake 2: Speeding up the annealing line. Trying to increase production by running the strip faster (＞8m/min) leads to incomplete annealing—elongation drops below
