Deformation and fracture mechanisms in fine- and ultrafine-grained ferrite/martensite dual-phase steels and the effect of aging

摘要

Three ferrite/martensite dual-phase steels varying in the ferrite grain size (12.4, 2.4 and 1.2 ìm) but with the same martensite conten (~30 vol.%) were produced by large-strain warm deformation at different deformation temperatures, followed by intercritical annealing Their mechanical properties were compared, and the response of the ultrafine-grained steel (1.2 urn) to aging at 170 °C was investigated The deformation and fracture mechanisms were studied based on micro structure observations using scanning electron microscopy and electron backscatter diffraction. Grain refinement leads to an increase in both yield strength and tensile strength, whereas uniform elon gation and total elongation are less affected. This can be partly explained by the increase in the initial strain-hardening rate. Moreover the stress/strain partitioning characteristics between ferrite and martensite change due to grain refinement, leading to enhanced martens ite plasticity and better interface cohesion. Grain refinement further promotes ductile fracture mechanisms, which is a result of the improved fracture toughness of martensite. The aging treatment leads to a strong increase in yield strength and improves the uniforn and total elongation. These effects are attributed to dislocation locking due to the formation of Cottrell atmospheres and relaxation o internal stresses, as well as to the reduction in the interstitial carbon content in ferrite and tempering effects in martensite.