Microstructural Evolution in Adiabatic Shear Bands of Ultra-Fine-Grained Low-Carbon Steels Fabricated by Equal Channel Angular Pressing

摘要

The object of the present study is to investigate the microstructural development of the adiabatic shear band formed by dynamic torsional experiments of ultra-fine-grained low-carbon steels fabricated by equal channel angular pressing (ECAP). Dynamic torsional tests were conducted on four steel specimens, two of which were annealed after ECAP, using a torsional Kolsky bar, and then the test data were compared via microstructures, tensile properties, and fracture mode. The ECAP'ed specimen consisted of very fine, equiaxed grains of 0.2 mu m in size, which were slightly coarsened and had an equiaxed shape after annealing. Some adiabatic shear bands were observed at the gage center of the dynamically deformed torsional specimen, and their width was smaller in the ECAP'ed specimen than in the 1-hr annealed specimen, although they were not found in the 24-hr annealed specimen. Ultra-fine, equiaxed grains of 0.05~0.2mu m in size were formed inside the adiabatic shear band, and their boundaries had characteristics of high-angle grain boundaries. These phenomena were explained by dynamic recovery and recrystallization due to the highly localized plastic deformation and temperature rise occurred within the shear band. The temperature rise in the shear band formation process was inferred to be above 540°C from the observation of spheroidization behavior of cementites in pearlite grains.