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Internal Friction Study of a 340MPa Bake-hardenable ULC Steel

Internal Friction Study of a 340MPa Bake-hardenable ULC Steel
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Bake hardenable (BH) steels are widely used for automotive applications, in particular for exposed panels, due to their superior formability, their low yield strength (YS) in the as-produced condition and their increased strength in the pre-strained and paint-baked condition. Static strain aging causes the mechanical properties of BH steel to change slightly over time. The increase of YS and the decrease of elongation can result stamping surface defects. Steelmakers must therefore produce BH steel grades with minimized static strain aging properties. The static strain aging is reduced by means of a precisely controlled interstitial carbon content and a low dislocation density.The internal friction (IF) technique is a powerful method to measure the interstitial atom (C, N) content in Fe, ferrous alloys and steels. The interstitial content is analysed by the measurement of the amplitude and position of the Snoek peak. In addition to the analysis of the interstitials, the IF technique also provides microstructural information by means of the analysis of the dislocation-related IF peaks. These peaks are due to elementary dislocation displacements or interstitial-dislocation interactions. The Snoek-Köster (SK) peak is due to the process of double kink formation on screw dislocation segments dragging interstitial atoms. The Bordoni peak or g-peak is caused by the process of double kink formation on screw dislocation segments which do not interact with interstitials. The dislocation-enhanced Snoek peak (DESP) is also due to dislocation-interstitial interactions. It results from the formation of double kinks on non-screw dislocation segments dragging interstitial atoms.The evolution of the amplitude of the Snoek peak, the γ-peak and the SK peaks during the static strain aging of a 340MPa ultra low carbon (ULC) BH steel was analysed by the impulse IF technique, and the kinetics of static strain aging was determined.It was found that an increasing amount of deformation resulted in an increase of the SK peak amplitude. The Snoek peak also appeared to be enhanced by the deformation (DESP). The Tmax of the Bordoni or γ peak was obtained by curve fitting. The Tmax temperature of the BH steel was found to be the similar as the Tmax temperature for pure ferritic Fe. The activation energy for Cottrell atmosphere formation and carbide formation were determined by IF method. The measured activation energies were slightly lower than the reported values. It was also shown that the IF method could in principle be used to measure the BH-kinetics in-situ.
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