Effects of martensite morphology and volume fraction on quasi-static and dynamic deformation behavior of dual-phase steels

The effects of martensite morphology and volume fraction on the quasi-static and dynamic deformation behavior of dual-phase steels were investigated in this study. Quasi-static and dynamic torsional tests were conducted using a torsional Kolsky bar for four steel specimens, which had different martensite morphology and volume fraction, and then the test data were compared via microstructures, tensile properties, and fracture mode. In the intermediate quenched (IQ) steel specimens, very fine fibrous martensites were well distributed in the ferrite matrix, but bully martensites were mixed with ferrites in the step quenched (SQ) specimens. Quasi-static torsional properties were similar to tensile properties, and fracture occurred in a ductile mode in IQ specimens, whereas cleavage fracture was predominated in SQ specimens. Under a dynamic loading condition, the fracture mode of SQ specimens was changed from cleavage to ductile fracture, whereas IQ specimens had a ductile fracture mode, irrespective of loading rate. These phenomena were analyzed using a shear lag model, phase continuity, and the thermal softening effect of martensite.