DYNAMIC DEFORMATION AND FRACTURE BEHAVIOR OF ULTRA-FINE-GRAINED DUAL PHASE STEELS FABRICATED BY EQUAL CHANNEL ANGULAR PRESSING

In this study, dynamic deformation and fracture behavior of ultra-fine-grained dual phase steels fabricated by equal channel angular pressing (ECAP) was compared with that of conventionally annealed ultra-fine-grained steels. In the ECAPed conventionally annealed specimen, pearlites were decomposed, and the ferrite grain size increased up to 0.5 μm. The intercritically annealed specimen consisted of dual phases of equiaxed ferrites and blocky martensites (volume fraction; 35%, size; 1 μm). The dynamic torsional test results indicated that maximum shear stress of the annealed specimens was lower than that of the as-pressed specimen, but fracture shear strain was higher. These results suggested that annealing or promoting dual phase microstructures of the ECAPed specimens could be a good way to increase the fracture resistance under dynamic loading, as it reduced or prevented the formation of adiabatic shear bands or voids. In particular, the ECAPed ultra-fine-grained dual phase steel can be more reliably used under dynamic conditions since it overcomes the shortcomings of reduced strain hardenability, ductility, and dynamic properties.