On the plasticity mechanisms of lath martensitic steel

The plasticity mechanisms of press hardening steel with a fully lath martensite microstructure were examined experimentally by strain rate sensitivity measurements, repeated relaxation tests and internal friction measurements. The analysis of relaxation tests suggests that the micro-plasticity could be due to the motion of mobile non-screw dislocations, based on mobile dislocation exhaustion observed in the micro-plastic range. In the macro-plastic range, the plasticity is thought to be due to the generation of mobile screw dislocations. The solute carbon-dislocation interaction results in a negative strain rate sensitivity and a Snoek-Köster-Kê peak in the internal friction spectrum of the lath martensitic press hardening steel. The magnitude of the effective activation volume and its stress dependence indicate that plastic deformation is most likely controlled by screw dislocation motion by formation and lateral movement of kink pairs dragging solute carbon atom atmospheres. Both isotropic and kinematical hardening seem to play a role in the strain hardening behavior of lath martensitic steel.