1.What are the types of thermoforming?
Hot stamping (press hardening): This is the most common term. The steel sheet is heated to austenitizing temperature (approximately 900°C or above), then quickly transferred to a die for stamping. Simultaneously, the die is quenched by the die's cooling system, ultimately producing an ultra-high-strength martensitic structure.
Medium-low temperature hot forming: The sheet is heated to a relatively low temperature (e.g., 200°C to 600°C) to increase its plasticity, reduce springback, and facilitate the forming of more complex shapes.
2.What are the main challenges in hot press forming of galvanized coils?
Zinc evaporation: Zinc's boiling point is approximately 907°C, and hot stamping temperatures exceed 900°C, causing zinc to evaporate violently and burn. This not only damages the anti-corrosion layer, but the evaporated gas also forms an air cushion between the steel plate and the die, causing surface quality problems (such as orange peel and pitting).
Alloying and brittleness: At high temperatures, zinc and iron diffuse into each other, forming an iron-zinc alloy layer (primarily FeZn<sub>13</sub>). This alloy layer is hard but brittle, and can easily cause microcracks during subsequent stamping and deformation, and may even cause the coating to peel off.
Liquid metal embrittlement: Under the high temperatures of hot stamping, if zinc comes into contact with the grain boundaries of steel, it can cause liquid metal embrittlement, leading to cracks in the base steel sheet and seriously affecting the strength and safety of the part. This is one of the most serious risks.
3.How to overcome challenges in sheet metal hot stamping?
Aluminum-silicon coating: This is currently the most mainstream and successful solution. A layer of Al-Si alloy is applied to the base steel sheet. The melting point of the Al-Si coating is approximately 600°C, far above the boiling point of zinc in the hot stamping process. Therefore, it remains stable during heating and forming, preventing evaporation and effectively preventing oxidation and decarburization of the steel sheet. In the hot stamping of high-strength automotive structural parts (such as A-pillars, B-pillars, and bumpers), Al-Si coating has largely replaced pure zinc coating.
Special zinc-based coatings: Some steel mills are also developing zinc-based coatings that are more resistant to high temperatures. For example, by adding magnesium (Mg) and aluminum (Al) to increase the coating's melting point and stability, reducing the risk of evaporation and LME, but the technology and application are not as mature or widespread as Al-Si coatings.
4.How to control medium and low temperature heating molding?
This process is relatively easy to control. The key is to keep the temperature below the critical point where the zinc layer severely softens, melts, or over-alloys (usually below 300°C is recommended). At this temperature, the steel's ductility improves, enhancing formability while the zinc layer remains largely intact and still provides good corrosion protection.
5.How should we choose ultra-high strength structural parts with extremely high requirements?
It is the current standard practice in the industry to prefer Al-Si coated steel sheets for hot stamping rather than ordinary galvanized coils.