Ductile Fracture Behavior of Bainite-MA Dual-Phase Steels

Shimamura, Junji (JFE Steel Corporation) | Ohta, Shusaku (JFE Steel Corporation) | Yasuda, Kyono (JFE Steel Corporation) | Ishikawa, Nobuyuki (JFE Steel Corporation) | Yokota, Tomoyuki (JFE Steel Corporation) | Igi, Satoshi (JFE Steel Corporation)

OnePetro 

ABSTRACT

Dual-phase microstructure control is an essential measure for improving strain capacity of linepipe steels. High strength steels with Bainite (here, “bainite” has the same meaning as bainitic ferrite)-MA (Martensite-Austenite constituent) dual-phase microstructures have been developed and practically applied to the linepipes for the strain-based design to achieve higher strain capacity which has low Y/T (Yield/Tensile strength) ratio even after pipe coating. On the other hand, dual-phase steels tend to show lower Charpy energy in the upper shelf region than single-phase steel. In order to improve Charpy energy through microstructure control, detailed investigation on ductile fracture behavior in bainite-MA dual phase steel was conducted. It was found that void nucleation was enhanced in the bainite phase adjacent to the bainite/MA boundaries with coarser MA particles, resulting in lower Charpy energy. On the other hand, void nucleation was suppressed in the steels with finer MA. Therefore, Charpy energy can be improved by obtaining finer MA particles in bainite-MA dual-phase steels. From a finite element analysis of local plastic strain around the boundaries between bainitic ferrite and MA, it was concluded that the equivalent plastic strain decreases in the case of finer MA and longer distance between two MAs compared to coarser MA and shorter distance between two MAs. In this study, the effect of MA morphologies (volume fraction, size, number fraction) on the mechanical properties such as Y/T ratio and Charpy impact energy in the ductile fracture region will be discussed.

INTRODUCTION

In order to achieve safety and reliability of the pipeline installed in seismic or permafrost region, it is quite important to apply the linepipes with sufficient strain capacity against buckling and weld fracture caused by large deformation of pipe. Using the steels with lower Y/T (yield-to-tensile strength) ratio, higher strain hardenability or higher uniform elongation, is effective to improve strain capacity of linepipes and to prevent buckling or weld fracture (Denys et al., 2000; Wang et al., 2002,2007; Suzuki et al., 2002,2004; Ishikawa et al., 2004).