Bake hardening steel

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Bake hardening steel (short: BH steel) is a material which has a by a controlled aging process after heating to temperatures of about 170 ° C increase in strength experienced. The name refers to the process bake (Engl. Bake ) and solidifying ( hardening ). This steel is often used for car body parts, as in he good press is formable and the burning of paint undergoes an increase in strength.

The accumulation of carbon atoms on dislocations creates so-called Cottrell clouds or very fine precipitates , which hinder the movement of the dislocations . This increases the tension required to initiate plastic deformation → the strength increases.

Cottrell clouds are formed by diffusion of the carbon and nitrogen atoms dissolved in the iron lattice. These are embedded in the interstitial spaces, distort the lattice and thereby tighten it. The stored atoms take up a little more space than is available in the interstitial spaces. The lattice distortion is reduced by diffusion of the interstitial atoms onto the dislocations. The interstitial atoms therefore prefer to diffuse there, thereby blocking the dislocation movement and thereby increasing the strength. The diffusion rate increases with the temperature, which means that even with a slight increase in temperature, as is the case with e.g. B. is required for baking varnish, which form Cottrell clouds.

In order to obtain good formability, the Cottrel clouds must not have formed before the sheet metal forming. Since aging (diffusion of the interstitial atoms to the dislocations) only takes place more slowly at room temperature, the material has to be processed a few weeks after production.

The BH effect is usually described by the BH2 value. This indicates the increase in the yield strength after baking at 170 ° C after 2% pre-stretching and subsequent heat treatment. The BH0 value (increase in strengthening without pre-stretching) is less common.

Mainly visible inner and outer parts of the body are made from BH steel . This can be made from thinner sheet metal. One example is the bonnet of a motor vehicle, which is now only 0.43 millimeters thick instead of the usual 0.7 millimeters.

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  1. ↑ Wolfgang Bleck (Ed.): Material Science Steel. Verlag Mainz, Aachen 2004, ISBN 978-3-89653-820-8 .

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