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

Abstract

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.

Influence of Microstructure Constituents on Ductile to Brittle Transition Behavior in Multi-phase Steel Sheets

“…This behavior is a feature of two-step DBT. 14) In Fig. 3, the absorbed energy and the FAT curves of Steel B and C were similar to those of Steel A.…”

“…In our previous studies, [12][13][14] we have investigated the ductile to brittle transition (DBT) with decreasing temperature in ferrite + pearlite steel sheets using a Charpy impact test with sub-size specimens. The absorbed energy transition with decreasing temperature showed a clear middle shelf (MS) between upper shelf (US) and lower shelf (LS).…”

“…We have already indicated that the volume fraction of hard structure is an important factor in two-step DBT; thus, the transition curve of absorbed energy changes from one-to two-step with the volume fraction of pearlite increasing. 14) Similarly, other microstructural factors seem to affect the DBT behavior of multi-phase steel sheet. This two-step DBT behavior was observed in multi-phase steel sheets containing MA as well as ferrite + pearlite steel sheets.…”

“…In our previous study, 14) we evaluated the effect of volume fraction of pearlite on ferrite + pearlite microstructure. The increase in the pearlite volume fraction in the ferrite matrix altered the DBT behavior from one-step to two-step, and increased T UM ; however, T ML remained unaffected.…”