This is the second of a two-part series designed to elucidate the role of dislocation dynamics and particle-nucleated cleavage in the ductile-brittle transition temperatures (DBTT) of Fe-binary alloys. Detailed measurements of slow-bend and impact transitions demonstrate that additions of Ni slightly increase the cleavage fracture stress and effective fracture surface energy of iron while Si additions provide the opposite trend. With the establishment of 19 flow and fracture parameters, a semiempirical DBTT model evolves which compares favorably to observed transition temperatures under slow notch bend and impact conditions for Fe, Fe-Ni and Fe-Si. The model clarifies how reinforcing effects of solid solution softening and an increased cleavage fracture stress leads to a monotonically decreasing transition temperature with increasing Ni additions. In addition, it demonstrates how opposing effects of softening and a lowered cleavage fracture stress leads to a minimum in the transition temperature with increasing Si additions.