Microstructure of Continuous Cooled Interstitial-Free Steel and the Determination of Dislocation Density

• Main Authors: Ya-Chu Yu, 虞雅筑
• Other Authors: Jer-Ren Yang
• Format: Others
• Language: en_US
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/4cnq2s

碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 106 === Ultra-low carbon IF steels (0.001-0.002 wt% C) were studied in the present research. Hollow cylinder specimens were used to obtain rapid cooling by the high resolution dilatometer. The transformation mechanism was analyzed by dilatometric measurement. The transformation temperature of massive ferrite and the critical cooling rate in corresponding steel were obtained. Optical microscopy (OM), electron backscattered diffraction (EBSD), and transmission electron microscope (TEM) were used to realize the effects of cooling rate and characterize grain morphologies of massive ferrite. The fraction of massive ferrite, containing lots amount of substructure, increases with the cooling rate. Dislocation density of massive ferrite was measured and is higher than that in polygonal ferrite. In the second part, the research focuses on the inelastic mean free path measurement by using electron energy-loss spectrometry (EELS). Log-ratio method is used widely in the thickness measurement. The inelastic mean free path was measured under both TEM and STEM mode. Different experimental conditions were operated. The mean free path decreases with the increasing of semi-collection angle in both TEM and STEM mode. The convergent angle effect is more significant under STEM mode, and the results show that lager convergent angle make the mean free path small. The effects of dislocation and amorphous layer have also been revealed.