MAGNETIC EVALUATION OF THE EMBRITLLEMENT ON COLD-ROLLED STEELS
Y. Kamada, T. Nakano, S. Takahashi, K. Ara, H. Kikuchi, J. Echigoya and K. Yaegashi ;
Iwate University, Morioka, Japan.
The objective of this study was to demonstrate the correlation between magnetic properties and the degree of
embrittlement on cold-rolled steels, and to clarify its underlying mechanism. Transmission electron
microscopy observation, Charpy impact test and magnetic hysteresis measurement of ring-shaped specimens
were preformed on Fe-0.15wt%C steels with different cold-rolling ratios. As the cold-rolling ratio
increased, dislocation density, ductile-brittle transition temperature (DBTT) and magnetic coercive field
(Hc) increased simultaneously. The increase of dislocation density leads to the increase of yield stress, and
raises DBTT. Since the internal stress produced by dislocation becomes the obstacle of magnetic domain
wall movement, the increase of dislocation also raises the value of Hc. Hence, the observed correlation
between DBTT and Hc originates from the increase of dislocation density induced by plastic deformation.
We also developed special equipment, which enabled us to measure directly the magnetic hysteresis of
Charpy test pieces. Similar correlation was confirmed by using this equipment. These results show us a
possibility for the application of magnetic non-destructive methods to the evaluation of some embrittlements
of steels as an alternative technique of Charpy impact test.