1.What effect does the base material composition have on the hardness of galvanized steel?
Carbon content
Core function: Carbon is the main element that affects the hardness of steel. As the carbon content increases, the amount of solid-dissolved carbon in the matrix ferrite increases, the lattice distortion intensifies, and the hardness increases significantly.
Galvanizing process adaptability: When high-carbon steel (C ≥ 0.6%) is galvanized, it is easy to cause the coating to become brittle due to excessive dissolution of the matrix, and the zinc liquid temperature and immersion time need to be controlled.
Alloying elements
Silicon
When the Si content is greater than 0.1%, it easily reacts with zinc to form a brittle phase, resulting in an increase in the hardness of the coating but a decrease in toughness.
Manganese
Strengthens the matrix ferrite and inhibits grain growth, indirectly improving the overall hardness of galvanized steel (for every 0.1% increase in Mn content, the hardness increases by about 5-10HB).
Aluminum
Adding 0.01-0.05% Al to the matrix can refine the grains and increase the matrix hardness; the Al content (0.1-0.2%) in the zinc liquid affects the coating structure and reduces the formation of brittle phases.
2.What effect does zinc liquid temperature and immersion time have on the hardness of galvanized steel?
Too high temperature
The dissolution rate of the matrix iron is accelerated, the thickness of the Fe-Zn alloy layer in the coating increases, the hardness increases significantly, but the brittleness increases.
Immersion plating time is too long: The alloy layer grows excessively, forming a thick and hard layered structure, resulting in a decrease in the toughness of the coating.
3.What effect do grain size and defects have on the hardness of galvanized steel?
Fine grain strengthening: The finer the coating grains (such as by adding Ni and Mg to refine the grains), the higher the hardness (the grain size decreases from 10μm to 5μm, and the hardness increases by about 15%).
Porosity and cracks: When the coating has high porosity or microcracks, the hardness test value may be low (due to the decrease in local bearing capacity), but in actual use, it is easy to cause brittle fracture due to stress concentration.
4.What effect does quenching and tempering have on the hardness of galvanized steel?
Quenching treatment: Rapid cooling (such as water cooling) after galvanizing can form a martensite-like structure in the alloy layer, and the hardness increases sharply (such as from 200HV to 400HV), but it is extremely brittle.
Tempering treatment: Tempering at 200-300℃ can eliminate the internal stress of the coating, reduce the hardness (such as from 300HV to 250HV), and improve toughness;
Tempering at too high a temperature (＞400℃) will cause the alloy layer to have coarse grains, and the hardness will increase again, but the plasticity will deteriorate.
5.What effect does surface passivation have on the hardness of galvanized steel?
The passivation film (thickness 0.5-1μm) formed by chromate passivation (such as rainbow passivation) has a hardness of about 150-200HV, which can slightly increase the surface hardness and enhance the corrosion resistance;
Chromium-free passivation (such as silane treatment) has little effect on hardness (hardness change <5%) and mainly plays a protective role.