Changes in microstructure during cold rolling process
After cold rolling, the steel strip is flattened, elongated, and its lattice is distorted and broken, resulting in reduced plasticity and increased strength and hardness. This phenomenon is called work hardening.
After cold rolling, the grains of steel coils are crushed and elongated, accompanied by severe lattice distortion. This leads to an increase in hardness and a decrease in plasticity and toughness of the steel, namely the work hardening phenomenon.
When the steel is heated to a temperature above its recrystallization temperature but below the austenitization temperature, followed by a period of heat preservation, the unstable atoms at grain boundaries rearrange via thermal motion. Through the whole process of nucleation and grain growth, new equiaxed grains are generated, which is defined as the recrystallization phenomenon.
As a result, the strength and hardness of the steel decrease, while its plasticity and toughness improve, endowing the steel strip with excellent processing and service properties.
Recrystallization requires specific temperature and holding time conditions. Nevertheless, excessively high heating temperature or unduly prolonged holding time will cause boundary fusion between the newly formed equiaxed grains, triggering continuous abnormal grain growth. This process will in turn deteriorate all mechanical properties of the steel.
Accordingly, recrystallization annealing can be summarized as follows: via controlled heating and heat preservation, the process shall fully induce recrystallization in cold-rolled steel, and meanwhile strictly restrain excessive grain growth.
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The main factors affecting the grain size after recrystallization during the annealing process are the annealing temperature and the holding time. Under certain cold deformation conditions, the grain size after recrystallization varies with the annealing temperature and the holding time. The higher the temperature and the longer the holding time, the coarser the grains.
Reasons for Annealing
After cold rolling deformation, the internal microstructure of steel features elongated and fractured grains, along with a massive accumulation of crystal defects. This raises the internal free energy of the metal, placing it in an unstable thermodynamic state. The material inherently tends to spontaneously revert to a complete, orderly microstructure with low free energy and high stability.
At room temperature, atoms possess low kinetic energy, weak diffusion capability, and extremely slow diffusion rates. As a result, this spontaneous structural recovery cannot occur on its own. External activation energy is therefore required. This activation is achieved by heating the steel to a specified temperature, which provides atoms with sufficient diffusion kinetic energy, eliminates lattice distortion, and induces favorable transformations in both microstructure and mechanical properties.
For the above reasons, all cold-rolled steel products must undergo annealing heat treatment.
Purposes and Functions of Annealing
Annealing refers to the technological process in which steel strip is heated to a specified temperature, held at that temperature, and then cooled down.
Annealing of cold-rolled sheets is one of the most critical heat treatment procedures in cold-rolled steel strip production. Based on different steel grades, annealing processes for cold-rolled steel strip are classified into primary annealing, intermediate annealing and finished product annealing. Finished product annealing is the most widely adopted process.Its core objectives are to eliminate internal stress and work hardening induced by cold rolling, and to enable steel sheets to meet the specified requirements of mechanical properties, technological properties and microstructures. This type of heat treatment is generally defined as recrystallization annealing.
Comparative Differences between Batch Annealing and Continuous Annealing Lines
•Batch Annealing: Features an extremely long production cycle (tens of hours). It is arranged separately from the pickling line and leveler line, yet integrated into a complete cold strip production process. Degreasing and pickling can be performed optionally, and the impact of steel grades and specifications on overall production is minimal. It supports flexible small-batch and organized production with single-furnace operation. The number of furnaces can be adjusted at any time according to production output and product variety changes. Inventory buffers exist between intermediate processes, endowing it with strong adaptability to output and product variety. It is ideal for multi-variety, small-batch production and experimental production.
•Continuous Annealing: Boasts a short production cycle (several minutes). Degreasing/pickling, annealing, and leveling are integrated into a single unified production line, ensuring high production efficiency. However, it incurs high costs due to shutdowns caused by production planning and the switching of specifications and varieties. It is not suitable for an overly wide coverage of product specifications or excessively low output. It is optimized for high-volume, single-variety production.
•Batch AnnealingThe heating and cooling rates of bell-type furnace annealing are slow, which provides sufficient time for carbide precipitation and grain growth. This recrystallization annealing method is conducive to the formation of pancake grains that benefit deep drawing performance, and can achieve excellent n-value and r-value. It enables convenient property control in the production of deep-drawing and ultra-deep-drawing steel grades, and possesses distinct superiorities in this field.
•Continuous AnnealingIt features rapid heating and cooling rates, as well as short holding time. To achieve sufficient recrystallization and favorable formability, it imposes strict requirements on chemical composition and annealing schedule. Compared with batch-annealed products, low-carbon steel products produced via continuous annealing present higher hardness, higher strength and lower plasticity. This process is more suitable for manufacturing high-strength automotive steel.
•Batch AnnealingThis process performs annealing on whole steel coils. Uneven temperature distribution between the inner and outer layers of the coil easily leads to coil sticking. Residual oil on the strip surface is volatilized via heating; inadequate volatilization will result in poor surface cleanliness (optional degreasing and cleaning processes are available). It is unsuitable for the production of ultra-thin product specifications, yet has no production restrictions on medium and thick gauge products.
•Continuous AnnealingThe strip undergoes pre-process degreasing and cleaning to achieve a clean surface. During uncoiling and heating, the steel strip maintains uniform temperature distribution, delivering excellent flatness and strip shape. This process is suitable for manufacturing products with strict surface quality requirements and thin-gauge steel products.
