부산대학교 도서관

Here we utilized two cooling methods, forced air cooling and furnace cooling, to obtain two different triplex microstructureswhich were martensite-based and bainite-based, respectively. Compared with furnace cooled samples, the forced air cooledsamples presented about 5% higher ultimate tensile strength, 10% higher hardness and 43% lower Charpy impact energyvalue. Nano-indentation test was conducted to reveal the local mechanical properties. The nano-hardness of martensite,bainite and the mixture microstructure is 5.633 GPa, 4.200 GPa and 5.082 GPa, respectively. Mn, S, C and Si segregationat the grain boundaries in martensite are severely detrimental to the toughness. High rotation angle grain boundaries andprecipitates of rare earth element enhanced the toughness of bainite. Martensite with high density dislocations in fine lathsexhibited higher nano-hardness than bainite with lower density dislocations in wider sized laths. The interaction of the twophases with martensite twinning sub structure between bainite laths showed intermediate nano-hardness.