Microstructural Changes by Annealing and Mechanical Properties of Ultra Fine-Grained Low Carbon Steels Processed by ECAP
Takeo AKITA, Masahide GOTOH, Kazuo KITAGAWA, Masayuki SHOZU and Yukio HIROSE
Abstract:Microstructures and mechanical properties of ultra fine-grained low carbon steels fabricated by equal channel angular pressing (ECAP) were investigated. Several specimens statically annealed for 5 hours at temperatures of 773-873K were studied simultaneously. Electron back-scattering diffraction (EBSD) measurements were carried out for microstructure observation. Differential scanning calorimeter (DSC) curves were obtained for studying the thermal stability of specimens. The initial grain size of ~10ƒÊm in the ferrite-pearlite steel was refined to ~0.2 ƒÊm by 4 passes of ECAP. The tensile strength of the as-ECAPed specimen was around 2 times higher than that of the initial normalized specimen, although the ductility decreased by half after the process. At annealing temperature of 773-823K, tensile strength and yield stress decreased as the average grain size of each specimen increased. However the grain growth by annealing was not significant. At annealing temperature of 873K, the nonuniform grain growth occurred suddenly, and as a result the tensile strength decreased. On the Hall-Petch relation diagram, the gradient of the diagram over the average grain size of 1ƒÊm was slightly different from that of under 1ƒÊm. In addition, the nonuniform radical grain growth at the temperature of 873K was in accord with the appearance of the exothermic peak in the DSC curves. The accumulation of the local strain in the 823K-annealed specimen was investigated by the kernel average misorientation (KAM) approach using EBSD, and it is suggested that accumulation of the local strain in the 823K-annealed specimen is one of the causes of the nonuniform radical grain growth at the temperature of 873K. Key Words:Ultra fine grain, ECAP, EBSD, DSC, Low carbon steels, Grain growth