1. Forms of Thermal Forming for Coated Strip
Two families dominate industrial practice:
Hot stamping or press hardening: the blank is heated to the austenitising range, around 900 C and above, then transferred quickly to a die where it is formed and quenched by the die cooling system, producing an ultra-high-strength martensitic structure.
Medium and low temperature hot forming: the blank is heated to a lower range, roughly 200 C to 600 C, to raise plasticity and reduce springback so that more complex shapes can be formed.
2. Challenges of Hot Pressing Galvanized Coil
Zinc evaporation. Zinc boils at about 907 C, which is below or at the level of most hot stamping cycles. The coating evaporates violently, destroying the anti-corrosion layer, and the vapour forms a gas cushion between blank and die that leaves surface defects such as orange peel and pitting.
Alloying and embrittlement of the coating. At high temperature, zinc and iron diffuse into each other and build an iron-zinc alloy layer composed mainly of FeZn13. This layer is hard but brittle, so it microcracks during subsequent deformation and can flake away.
Liquid metal embrittlement. If molten zinc reaches the grain boundaries of the steel, it can penetrate and embrittle them, cracking the base sheet and cutting the strength and safety margin of the component. This is the most serious failure mode.
Because of these three effects, ordinary galvanized coil cannot be press hardened in the same way as uncoated or Al-Si coated material.
3. How the Industry Solves the Problem
| Solution | Working principle | Status |
|---|---|---|
| Al-Si coating | An aluminium-silicon layer with a melting range around 600 C stays stable through heating and forming, blocking oxidation and decarburisation of the steel | Mainstream and proven; used for A-pillars, B-pillars, bumpers and other safety structures |
| Modified zinc-based coatings | Magnesium and aluminium additions raise the melting point and stability of the coating, reducing evaporation and liquid metal embrittlement risk | Under development; less mature and less widely applied than Al-Si |
For parts that must reach a martensitic structure through press hardening, the practical rule is to select Al-Si coated sheet rather than a standard galvanized coil.
4. Controlling Medium and Low Temperature Forming
Below the austenitising range the process is far easier to manage. The key is to keep the peak temperature under the point at which the zinc layer softens badly, melts or over-alloys, and a ceiling of about 300 C is a practical guideline. Within that window the steel gains sufficient ductility for complex shapes while the zinc coating stays largely intact and continues to provide corrosion protection.
5. Choosing Material for Ultra-High-Strength Structural Parts
Press-hardened safety parts with a martensitic structure: specify Al-Si coated sheet rather than galvanized coil.
Complex shapes that do not need full hardening: consider low-temperature hot forming of galvanized coil, keeping the temperature below roughly 300 C.
Room-temperature forming with standard corrosion protection: conventional roll forming, bending or drawing of galvanized coil remains the most economical route.
6. FAQ
Q: Can galvanized coils be hot-pressed directly?
Not for press hardening above 900 C. Zinc evaporates at around 907 C, and the molten coating can embrittle the steel at the grain boundaries.
Q: What is liquid metal embrittlement?
It is cracking of the base steel caused by molten zinc penetrating the grain boundaries during high-temperature forming, and it reduces both strength and safety of the part.
Q: Why is Al-Si coating used instead of zinc?
Its melting range around 600 C keeps the coating stable during hot stamping, so it protects against oxidation without evaporating or embrittling the steel.
Q: What temperature is safe for forming galvanized coil?
A practical limit is about 300 C, where the steel becomes more formable while the zinc layer remains essentially intact.
Q: Are zinc-magnesium-aluminium coatings a proven alternative?
They show promise for higher temperature stability, but the technology is less mature and far less widely used than Al-Si coatings.