Q:What is work hardening?
A:Cold rolling is a process of reducing the thickness and increasing the strength of steel plates by rolling at room temperature. However, this process can cause the internal crystal structure of the steel plate to be distorted, the dislocation density to increase, the hardness and brittleness to increase significantly, and the plasticity and toughness to decrease significantly (i.e. "work hardening" or "cold work hardening").
Q:How does annealing eliminate work hardening?
A:Principle: Heat the cold-rolled steel plate to above the recrystallization temperature, keep it warm for a certain period of time, and then slowly cool it.
Function: The atoms inside the steel plate gain energy and rearrange to form new uniform grains (recrystallization), eliminating crystal distortion and dislocation accumulation;
The hardness is reduced and the plasticity is restored, so that the steel plate has good processing performance again.
Q:What improvements are achieved after annealing?
A:1. Optimize surface quality
During the cold rolling process, rolling oil, oxides or tiny defects may remain on the surface of the steel plate. Annealing reduces the iron oxide scale and purifies the surface, making the substrate surface cleaner and smoother, improving the adhesion and uniformity of the galvanized layer.
Comparison: If annealing is not performed, residual grease or oxide scale on the surface will cause the galvanized layer to "leak" or "peel".
2. Control the mechanical properties of the substrate
Galvanized steel for different purposes has specific requirements for the strength and elongation of the substrate.
Annealing can accurately control the strength, elongation and hardness of the substrate by adjusting the heating temperature and cooling rate.
Purpose: To match the performance of the substrate with the galvanized layer to meet the needs of subsequent processing or use scenarios.
Q:What are the common annealing process types?
A:1. Continuous annealing
Process: The cold-rolled steel plate passes through the heating section, insulation section, and cooling section in a continuous furnace, and is in a protective gas throughout the process to avoid oxidation.
Features: High production efficiency, uniform substrate performance, suitable for mass production of high-precision galvanized products such as automotive plates and home appliance plates.
2. Hood annealing
Process: The steel plate is rolled into an annealing furnace, sealed with a hood, and then a protective gas is introduced to slowly heat and cool.
Features: Low cost, but low production efficiency, suitable for small and medium-sized batch production or galvanized steel with low performance requirements.
Q:What are the negative effects of improper annealing?
A:1. Substrate performance is unqualified
Annealing temperature is insufficient or time is too short: work hardening is not completely eliminated, the steel plate is still hard and brittle, and it is easy to crack after galvanizing;
Annealing temperature is too high or time is too long: the grains grow excessively ("overheating"), the substrate strength decreases, and the product's bearing capacity is affected.
2. Galvanized layer quality defects
Surface oxide scale or grease remains: the galvanized layer has poor adhesion and is easy to fall off (such as the zinc layer peeling off in pieces when bending);
Abnormal surface roughness of the substrate: resulting in uneven thickness of the galvanized layer (such as local overthickness or thinness).