Effect of Pearlite Volume Fraction on Two-step Ductile to Brittle Transition in Ferrite + Pearlite Structure Steel Sheets

摘要

Multi structural steels exhibit high strength and good formability; however, their performance depends on the volume fraction of the secondary phase. In our previous study, ferrite + pearlite structural steel sheet showed the characteristic two-step ductile-to-brittle transition (DBT) with decreasing temperature, and the absorbed energy curve exhibited a distinct middle shelf. In this study, we evaluated the effect of pearlite volume fraction (V_P) on the DBT behavior by the Charpy impact test with sub-size specimens. For specimens without pearlite, the absorbed energy directly dropped from the upper shelf to the lower shelf with decreasing temperature. For samples with 2-3% pearlite, the absorbed energy corresponding to the transition temperature range was dispersed between the two shelves, and the transition behavior seemed to be the typical DBT behavior. When V_P was increased to 21%, the absorbed energy just above the transition-finish temperature became stable at a middle level between the two shelves; thus, the existence of a distinct middle shelf was confirmed. Although the transition-start temperature increased with increasing V_P, V_P did not affect the transition-finish temperature and the absorbed energy at the middle shelf. These results were analyzed with a simple model based on the Yoffee diagram.