Introduction to the Annealing Process Principle of Tin-Plated Coil Material

The tinplate annealing process is what turns a cold-rolled hard coil into soft, formable can stock. Without annealing, the steel remains work-hardened and cracks on the first draw. The process itself is not complicated, but every parameter—temperature, soak time, cooling rate—changes the final temper grade. Here is how it works.

Stage 1: Heating to Recrystallization

After cold rolling, the steel has flattened grains and high internal stress. Annealing heats the coil to around 600–700°C in a protective atmosphere (hydrogen-nitrogen mix to prevent oxidation). At this temperature, new strain-free grains nucleate and grow. Consequently, the hard, brittle work-hardened structure disappears, and the steel softens.

The heating rate matters. Heat too fast and the coil develops thermal stress that distorts the strip. Typical box annealing ramps up over 10–20 hours.

Stage 2: Soak Time at Temperature

Hold the coil at peak temperature for several hours. This soak lets recrystallization complete uniformly across the coil cross-section. A short soak leaves the inner wraps unrecrystallized; the result is inconsistent hardness from coil ID to OD. For box annealing, soak time runs 15–30 hours depending on coil weight.

Stage 3: Controlled Cooling

Cool slowly and uniformly. Rapid cooling introduces thermal stress and can re-harden the strip. Standard box annealing cools over 15–20 hours to below 200°C before the coil exits the furnace.

Stage 4: Temper Rolling (Skin Pass)

After annealing, the steel is too soft and wavy for can-making. A light cold reduction of 0.5–3% sets the final temper. This is where T1 through T6 are defined:

• T1: minimum temper reduction, softest, deep draw.
• T3: medium, general cans.
• T5: heavier reduction, stiffer.
• DR (double reduced): additional cold reduction without full anneal, for thin-gauge rigid cans.

Why Tin Temperature Window Is Narrow

Tin melts at 232°C. Consequently, the annealing atmosphere and temperature must not melt the tin layer prematurely. In practice, tinplate annealing runs at temperatures where tin is molten briefly, but the protective atmosphere and short cycle prevent excessive alloying. If you over-anneal, the tin diffuses into the steel, forming a brittle Fe-Sn intermetallic layer that hurts solderability and lacquer adhesion.

Annealing on Other Products We Supply

Annealing logic applies beyond tinplate:

• C11000 / T2 Copper Plate ships in 1/2-hard or full-hard temper, set by the annealing and cold-rolling cycle.
• C12200 / TP2 Copper Tube anneals bright in a nitrogen atmosphere for bendable refrigeration tubing.
• M19 Non-Oriented Electrical Steel final-anneals to set its magnetic properties—same principle, different temperature target.

Specify the Temper, Not Just “Annealed”

A complete tinplate order reads: “T3 temper, box annealed, 0.20 mm, 2.8/2.8 g/m², Standard finish.” Leave out the temper and you receive whatever the mill annealed last run.

Get the Right Annealed Tinplate

Tell us your can type, forming process, and required temper. We will confirm available stock and quote coil with a mill certificate.

• Email: vera.liang@ztwysteel.com
• Location: Xishan District, Wuxi, Jiangsu, China
• Supply range: electrical steel, copper & brass plate/pipe, nickel alloys, wear plate, mold steel
• Documentation: MTC with temper, hardness, coating weight, and dimensions

We reply within one working day.