Q:How do the mechanical properties change after cold rolling?
A:1. Significantly improved strength and hardness (work hardening)
Principle: During cold rolling, the crystal structure inside the steel produces a large number of dislocation entanglements and grain breakage due to plastic deformation, resulting in obstruction of dislocation movement, which is manifested in the macroscopic increase of strength and hardness.
2. Decreased plasticity and toughness (increased brittleness)
Principle: Work hardening intensifies the lattice distortion inside the steel, and the grains are elongated along the rolling direction, resulting in reduced elongation and impact toughness.
Q:What changes occur in process performance after cold rolling?
A:1. Changes in formability
Advantages: Cold-rolled steel sheets have a smooth surface and uniform thickness, and are suitable for simple forming such as shallow drawing and bending (such as stamping of home appliance panels).
Limitations: Unannealed cold-rolled sheets have low plasticity and are difficult to be deep-drawn (such as automotive covers and cans), and work hardening must be eliminated by annealing.
2. Changes in welding performance
Unannealed cold-rolled sheets: High strength leads to stress concentration during welding, which is prone to cracking, and requires control of welding parameters (such as reducing heat input) or post-weld heat treatment.
Annealed cold-rolled sheets: After plasticity is restored, the welding performance is close to that of hot-rolled sheets, and are suitable for conventional welding processes (such as automotive body welding).
Q:How does the organizational structure change after cold rolling?
A:1. Grain morphology changes
Hot-rolled state: The grains are equiaxed and uniform in size (depending on the hot rolling final rolling temperature and cooling rate).
Cold-rolled state: The grains are significantly elongated or flattened along the rolling direction to form a fibrous structure.
The greater the deformation, the more obvious the fibrous structure, which may cause anisotropy in the steel.
2. Substructure refinement
During the cold rolling process, a large number of subgrain boundaries and dislocation cells are formed inside the grains, which refines the substructure size from micron to submicron, further strengthening the material (substructure strengthening).
Q:What changes have occurred in surface quality and dimensional accuracy?
A:1. Improved surface finish
The surface of hot-rolled plates is rough due to iron oxide scale. Cold rolling can reduce the surface roughness to Ra 0.5~3μm through rolling and degreasing processes, which is suitable for direct use as appearance parts (such as home appliance panels, decorative materials). 2. Improved dimensional accuracy
Cold rolling uses high-precision rolling mills and automatic control systems, and the thickness tolerance can be controlled within ±0.01~0.03mm, meeting the needs of precision processing (such as electronic component stamping).
Q:What are the selection principles for cold-rolled steel?
A:When high strength is required, unannealed cold-rolled plates are preferred (such as shelves and keels); when deep processing (such as stamping and welding) is required, annealed cold-rolled plates or further heat treatment (such as quenching and tempering) are selected.