Steel material hardens after cold rolling due to the dislocation tangling generated by plastic deformation. Annealing is therefore carried out to soften the material. The annealing process comprises heating, holding of the material at an elevated temperature (soaking), and cooling of the material. Heating facilitates the movement of iron atoms, resulting in the disappearance of tangled dislocations and the formation and growth of new grains of various sizes, which depend on the heating and soaking conditions. These phenomena make hardened steel crystals recover and recrystallize to be softened.

Furthermore, precipitates decompose to solute atoms which subsequently dissolve into the steel matrix on heating and holding, then reprecipitate in various sizes and distributions, depending on the rate of cooling. These changes in the size and distribution of the grains and precipitates also affect the hardness of the material.

The annealing of cold rolled coils has conventionally been conducted by grouping and annealing the coils in batches stacked in a bell-type furnace. This process is called batch annealing. However, continuous annealing is now more commonly used. This type of annealing involves uncoiling, and welding strips together, passing the welded strips continuously through a heating furnace, and then dividing and recoiling the strips. The figure shows a continuous annealing line, which is composed of the entry-side equipment, furnace section, and delivery-side equipment.

The main entry-side equipment comprises payoff reels, a welder, an electrolytic cleaning tank, and an entry looper.

The furnace section comprises a heating zone, soaking zone, and cooling zone. The cooling zone is divided into three sub-zones so that complex cooling patterns such as cooling-heating-holding-cooling can be performed. The delivery equipment comprises a delivery looper, shears, and coilers, and may be linked to a temper rolling mill and plating equipment as part of a larger continuous line.

The heating cycle applied to strips by continuous annealing differs from product to product, but the three patterns shown in the figure are typical. For cold-rolled strips for general use, it is normal practice to adopt a heating pattern in which the strip is heated to 973K (700) for about 1 minute, rapidly cooled, held at about 673K (400) for 1 to 3 minutes to precipitate the solute carbon, and then cooled to room temperature.

Although the total equipment length is 150 to 300m, the total length of the strip in the line is as much as 2,000m. The travel speed of the strip is 200 to 700 m/min. However, a recently developed line for can material passes strip 0.15mm in thickness at a maximum speed of 1,000 m/min. To operate such lines, speed control, tension control, and tracking control of the strip are necessary, in addition to a high level of automatic temperature and atmosphere control.