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Products and solutions, 2011-11-07, 03:38 PM

GammaProtect®: Cathodic corrosion protection for hot stamping

For long-lasting corrosion protection, manufacturers of automotive components use galvanized steel: A simple truth, but one that only applies to cold formed parts. In hot stamping, above all in the highly cost-effective direct or one-stage hot stamping process, the corrosion protection question has always been far more difficult to solve. With GammaProtect®, ThyssenKrupp Steel Europe has now developed a new effective anti-rust finish that is also suitable for hot stamped components.

GammaProtect® is an electrolytic coating with a high melting point that withstands the extreme temperatures of hot stamping. As well as protecting against scaling it also provides active cathodic corrosion protection on a par with the conventional galvanizing of cold forming steels. The coating therefore expands the range of applications for hot stamping to include parts in the wet area of the car body particularly at risk from corrosion.

Hot stamping – or hot forming – is enjoying a boom in the auto industry because as well as meeting strict safety standards it also helps reduce vehicle weight. The strengths of GammaProtect® are demonstrated primarily in direct hot stamping, where steel sheet is first heated to around 900 degrees Celsius and then immediately stamped and simultaneously cooled in a special die. The rapid cooling produces components with strengths of up to 1,650 Megapascals, permitting the production of parts with thinner walls and lower weight.

With its relatively low melting point of 419.5 degrees, zinc liquefies in the heating phase of the hot stamping process. That is a risk in direct hot stamping: When hot blanks are stamped, liquid zinc can attack the microstructure of the material and cause cracks in the finished part. In indirect hot stamping this phenomenon is less pronounced because the part is first cold-formed before being heated and cooled to achieve the required strength, i.e. without being subject to any further forming stress. However, this method takes longer and is higher in cost than direct hot stamping. With GammaProtect® automakers can for the first time utilize the advantages of low-cost direct hot forming without having to forgo active corrosion protection.

Newly developed coating for higher melting point
The composition of GammaProtect® increases the melting point of the coating to over 870 degrees. And the zinc content is high enough to ensure it retains its cathodic corrosion protection properties. The coating is so resistant that it can tolerate timing variations in the production cycle, which ensures process stability. On the other hand, sheet coated with GammaProtect® can be heated at significantly faster rates, shortening holding time in the furnace by up to 20 percent compared with current production parts.

With the rapid growth in the range and quantity of parts being produced by hot stamping, shorter cycle times are a key development target. Using new heating technologies such as induction or infrared radiation, hot forming steels could be heated to the required temperature up to 100 times faster. Even under these conditions the new coating from ThyssenKrupp Steel Europe remains stable and provides long-lasting corrosion protection.

GammaProtect® has already been tested under near-production conditions and has demonstrated that it is suitable for all typical auto industry processing steps. Suitable joining techniques are adhesive bonding, resistance spot welding as well as MIG/MAG welding. The coating’s corrosion protection potential has been verified in extensive standardized tests.

The new coating is also suitable for the innovative tailored tempering technology, which is used to produce hot stamped parts with customized strength and elongation properties. In addition, the coating simplifies the production of tailored blanks for hot forming, known as Hotform Blanks.

With GammaProtect®, ThyssenKrupp Steel Europe AG has expanded its range of hot-stamping steels. To meet customer requirements these steels are also available in an uncoated variant and with an aluminum-silicon coating which prevents scaling and provides passive corrosion protection.

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