1.Why is substrate strength the decisive factor for the hardness of galvanized coil?
The higher the substrate strength (yield strength/tensile strength), the harder the galvanized coil generally is.
The strength (resistance to plastic deformation) and hardness (resistance to local indentation) of metallic materials are essentially derived from their internal crystal structure (such as grain size and dislocation density).
When substrate strength is high (e.g., through cold rolling and alloying), the internal grains are refined, the dislocation density is high, and the atomic arrangement is more compact, making it less susceptible to both overall plastic deformation (higher strength) and localized indentation (higher hardness).
For example, among low-carbon steel substrates, deep-drawing grade substrates (such as DC06) have a yield strength of approximately 130-180 MPa, corresponding to a galvanized coil hardness (HV) of approximately 50-70. High-strength steel substrates (such as HSLA 350) have a yield strength ≥350 MPa, and the galvanized coil hardness can reach HV 90-120, a significantly higher value.
2.How does the galvanizing process regulate the relationship between "substrate strength - galvanized coil hardness"?
Hot-dip galvanizing:
During the hot-dip galvanizing process, the substrate is heated in an annealing furnace (800-900°C) to achieve recrystallization (eliminating cold-rolling stress and refining the grains), which reduces the substrate's strength and hardness.
If the substrate is cold-rolled hard steel (high strength and hardness), the substrate's strength decreases after hot-dip galvanizing (for example, the yield strength of a cold-rolled hard coil is 300 MPa, which drops to 200 MPa after annealing). Consequently, the hardness of the galvanized coil will also decrease (HV from 80 to 60).
Therefore, the hardness of hot-dip galvanized coil depends not only on the original substrate strength but also on the annealing process (temperature and time). The more thorough the annealing, the lower the substrate strength and the lower the hardness of the galvanized coil.
Electrogalvanizing (such as EG):
Electrogalvanizing only deposits a zinc layer on the substrate surface, eliminating the need for high-temperature annealing and leaving the substrate's original strength and hardness largely unchanged. At this time, the hardness of the galvanized coil is directly positively correlated with the strength of the substrate: the higher the original substrate strength, the higher the hardness of the galvanized coil (for example, after the cold-rolled hard substrate is electrogalvanized, the hardness is consistent with the substrate).
3.What effect does the zinc coating have on the overall hardness?
The hardness of the zinc coating itself (pure zinc or zinc alloy) is relatively low (pure zinc HV approximately 30-40), much lower than that of the substrate (mild steel substrate HV approximately 50-150). Since the zinc coating thickness is typically only 5-20μm (substrate thickness 0.3-3mm), its contribution to the overall hardness of the galvanized coil is negligible.
Only when the substrate is extremely thin (e.g., less than 0.1mm) and the zinc coating is thick (e.g., ≥10μm) may the zinc coating slightly reduce the overall hardness (due to the softer zinc layer), but this situation rarely occurs in industry.
4.What is the core relationship between galvanized coil hardness and substrate strength?
Substrate strength is the dominant factor determining the hardness of galvanized coils, and the two are generally positively correlated. However, the annealing process during hot-dip galvanizing can reduce substrate strength, weakening this correlation, while electrogalvanizing retains the original correlation. In practical applications, the relationship between the substrate grade (e.g., DC01, S355) and the galvanizing process must be comprehensively assessed.
5.What are the hardness test methods for galvanized sheets?
Vickers hardness test, Brinell hardness test, micro Vickers hardness test, Knoop hardness test