1. What fundamental changes occur in the toughness of galvanized coils during low-temperature processing?
Low temperature has a two-fold cumulative effect on the overall toughness of galvanized coils. Firstly, the steel matrix follows a general low-temperature softening principle-as the temperature decreases, the strength indicators (yield strength and tensile strength) of the steel increase, but its plastic deformation capacity decreases and its brittleness increases. This phenomenon is called low-temperature brittleness. When the temperature drops to a certain range, the fracture mode of the steel suddenly changes from ductile fracture to brittle fracture; this temperature point is called the ductile-brittle transition temperature. Secondly, the galvanized layer itself is extremely sensitive to low temperatures: zinc is a temperature-sensitive metal. Under low-temperature conditions, its crystal structure becomes more stable, and the interatomic bonding force increases, significantly reducing the ductility and increasing the brittleness of the zinc layer. This makes it more prone to peeling or cracking during bending and processing. In other words, during low-temperature processing, the steel matrix becomes "hard and brittle," and the zinc layer becomes "fragile." When subjected to forming operations such as cold bending and stamping, the cumulative effect significantly increases the risk of overall cracking.
2. Are the low-temperature embrittlement mechanisms of the steel substrate and the galvanized layer the same? What kind of mutual influence exists between them?
The mechanisms are different, but they can exacerbate each other's damage during processing. The root cause of low-temperature embrittlement in the steel substrate (body-centered cubic structure metal) lies in the increased resistance to dislocation movement at low temperatures, and the increased interaction intensity between interstitial impurity atoms and dislocations and grain boundaries, which drastically weakens the material's adaptability to plastic deformation. The low-temperature embrittlement of the zinc layer (close-packed hexagonal structure) stems from its crystal structure becoming more rigid at low temperatures, naturally reducing its ductility. The mutual influence between the two during processing is mainly manifested in the following ways: When the steel substrate undergoes cold bending deformation at low temperatures, its outer surface bears a large tensile strain. The zinc layer, due to its high brittleness and insufficient ductility, cannot deform synchronously with the substrate, leading to microcracks or even blocky peeling of the coating. The greater the deformation of the steel substrate, the more severe the strain borne by the zinc layer, and the earlier the initiation of low-temperature embrittlement cracks. Conversely, once the coating develops cracks, stress will concentrate there, which may further induce brittle propagation of the steel substrate, leading to through-thickness failure.
3. Cracking of galvanized coils during low-temperature processing: Is the metallurgical quality of the sheet metal itself the most critical contributing factor?
Undoubtedly, the internal quality of the base material is the decisive prerequisite for successful low-temperature processing. Even at room temperature, metallurgical defects in the steel coil can easily lead to cracking during processing; at low temperatures, the negative impact of these defects is magnified many times over.
Specifically, the causes of failure vary depending on the steel grade. Case study data shows that even at room temperature, poor-quality base materials may crack due to internal defects during a 180° cold bending test. Metallographic analysis indicates the presence of numerous composite inclusions of silicates, sulfides, and mold flux at the crack sites. These inclusions have insufficient local elongation, becoming stress concentration points during bending, leading to crack initiation. Furthermore, excessively high levels of free cementite are also a significant cause of bending cracks in Q195C steel, while the severe banded structure of Q355B steel can lead to delamination during shearing, and the incomplete annealing of the microstructure in Q420XG steel also results in insufficient overall plasticity of the base material. It is clear that under low-temperature conditions, the material selection criteria must be significantly raised-the minor effects of impurities will be dramatically amplified, and the cleanliness and uniformity of the substrate will change from a "bonus" to a "survival baseline."
4. How significant are the differences in low-temperature processing toughness between galvanized coils of different strength grades and compositions?
The differences are very significant, primarily stemming from the design and development of the original steel. Conventional ordinary steel substrates exhibit a drastic decrease in toughness at low temperatures, often falling below the acceptable lower limit of impact energy at -20°C and exhibiting a high probability of brittle fracture at -40°C. In contrast, specially designed low-temperature resistant galvanized sheets demonstrate entirely different performance: employing a low-carbon niobium microalloying composition design, and achieving a uniform, fine-grained structure through controlled rolling and cooling, with a grain size reaching grade 11, they maintain reliable toughness even under extreme cold conditions of -40°C. Their impact energy at -40°C is no less than 34J, and their Z-direction elongation is no less than 35%. Suitable for welded structures and extremely cold regions, they can be safely used in applications with extremely high safety requirements, such as bus structural frames. Studies have also shown that the ultimate load-bearing capacity of galvanized components at low temperatures may even be about 8-9% higher than at room temperature. This demonstrates that the low-temperature toughness design of the substrate is the fundamental factor determining success or failure, rather than the coating itself.
5. What effective measures can be taken to prevent cracking during galvanized coil processing in winter?
When processing galvanized coils at low temperatures in winter, the following key preventative measures can be taken:
First, select steel grades suitable for low-temperature conditions. If the ambient temperature is below -20℃, priority should be given to using low-temperature specialized steel substrates (such as Q355ND, which has undergone micro-alloying treatment), rather than ordinary Q235 or Q195 substrates.
Second, manage temperature before processing. Perform forming operations such as shearing, punching, and bending indoors or in a relatively warm environment whenever possible, avoiding direct, significant cold deformation of the galvanized coil below -10℃. If processing must be carried out in a low-temperature environment, consider moderate preheating of the steel plate, but be careful not to overheat to avoid damaging the galvanized layer.
Third, control the deformation speed and amount. Rapid, significant deformation at low temperatures is most likely to cause cracking. Therefore, it is recommended to reduce the bending angle per pass and use a "multi-pass progressive forming" method instead of "one-time forming"; at the same time, reduce the processing speed to allow the material enough time for micro-stress relaxation and avoid brittle fracture.