Nucleation of Bainite at Small Angle Dislocation Network in Austenite and Its Effects on Mechanical Properties in Steels

Abstract

Bainite lath nucleated WithIN Grain (BWING) has been clarified to be greatly effective for the increase in both strength and toughness. The nucleation of BWING has been enhanced by the introduction of inclusion into austenite, which would act as nucleation site for BWING. However, the inclusion itself would be brittle and at the interface between inclusion and matrix, high density of dislocations are apt to be accumulated under deformation, inducing cleavage at the interface. As density of inclusion increases, steel would be strengthened, but brittle. Therefore, the introduction of dislocation network, that is, Small-Angle Dislocation Network (SADN) into austenite is tried in the present work. SADN would be stable in the high temperature region such as in austenite, because pinned dislocation existed in SADN would prevent the bow-out of dislocations through absorbing point defects. The formation of SADN in austenite, which act as nucleation site for BWING, was clarified experimentally. The bainite lath neighbor to the nucleated one might be formed under relaxing the stress field around the nucleated one and/or inclusion. It was analyzed that the average size of Aggregates of bainite Laths having nearly Parallel Slip systems between neighboring bainite laths (ALPS) corresponds to that of dimples on fracture surface. The process for ductile fracture is offered based on the crystallographic analysis using transmission electron microscopy.